CA1234704A - Optical fiber thermometer - Google Patents
Optical fiber thermometerInfo
- Publication number
- CA1234704A CA1234704A CA000463651A CA463651A CA1234704A CA 1234704 A CA1234704 A CA 1234704A CA 000463651 A CA000463651 A CA 000463651A CA 463651 A CA463651 A CA 463651A CA 1234704 A CA1234704 A CA 1234704A
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- fiber
- measuring device
- recited
- temperature
- temperature measuring
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Abstract
Abstract of the Disclosure The invention utilizes a blackbody cavity formed on the tip of a light transmitting fiber. The blackbody cavity serves as a signal generator which emits blackbody radiation whose amplitude and frequency spectra are indicative of the blackbody cavity tempera-ture. Radiation from the blackbody cavity is transmitted via the light transmitting fiber to an optical receiving device which converts the received radiation into an electrical signal having a signal characteristic indica-tive of the magnitude of received radiation.
Description
~2~34~
OPT I CAL F I BIER TO ERMOMET ERR
Back round of the Invention g This invention relates generally to tempera-lure measurement apparatus, and more specifically to relatively high temperature measurements extending to approximately 2400C.
The accurate measurement of high temperatures has always been and is a major industrial problem Severe operating environments limit the use of standard temperature measuring techniques: conventional materials and design constraints limit both the maximum operating temperature and their use in chemically aggressive environments. The invention it directed toward a measuring technique which can improve upon existing techniques, such as thermocouples, resistance then-mometers and pyrometers, or provide a new measurement capability in regimes where conventional technique do not operate. The temperature measuring device may find application in a large number of environments, basically wherever it is desirable to take temperature measure-mints either for monitoring the temperature quantity per so, or for control purposes, such as, for example, in industrial facilities.
The invention utilizes a black body cavity formed on the tip of a light transmitting fiber. the black body cavity serves a a signal generator which emits black body radiation whose amplitude and frequency I
spectra are indicative of the black body cavity tempera-lure. Radiation from the black body cavity is transmitted via the light transmitting fiber to an optical receiving device which converts the received radiation into an electrical signal having a signal characteristic indict-live of the magnitude of received radiation. The basic technique set forth above has been described in US.
Patent No. 3,626,75~.
Summary of the Invention It is an object of the invention to improve upon the prior art and to provide a highly accurate temperature measuring device using a black body cavity member.
It it a further object of the invention to provide an accurate temperature measuring device for temperatures within the range of 500-2400C using a black body cavity radiator.
Another object of the invention is to provide - 20 a temperature measuring device capable of efficiently transmitting black body radiation in the band of 0.3~ m-Lomb over an optical link or fiber which does no emit or absorb radiation in the radiation band over a temperature range within 500-2400C.
US A further object of the invention is to provide a temperature measuring device which uses a high temperature optical fiber having an optically dense solid oxide tip to enable black body radiation measure-mints.
~Z347(~4 Yet another object of the invention is to pro-vise a temperature measuring device having suitable eke-mica and mechanical durability for use in oxidizing environments having high temperature ranges on the order of 500-2400C.
The invention may be characterized as a them-portray measuring device comprising an elongated optic eel fiber, a black body cavity, a high temperature fiber and a light detector. The black body cavity is post-toned at one end of the high temperature fiber and both the black body cavity and the high temperature fiber are able to withstand temperatures in the range of 500-2400C. The black body cavity emits radiation in the range of 003~m-l.0 m within said temperature range of ~15 500-2400C. The high temperature fiber transmits radiation in the temperature range of interest without substantial absorption or remoteness thereof. The light detector is coupled to receive light transmitted by the high temperature fiber and includes a narrow band filter for selecting a narrow optical Rand within said 0.3~Xm-lnOl~m range. The light detector provides an output signal which is proportional to the intensity of the received and filtered light. The temperature measuring device may also include an output device ; 25 responsive to the output signal from the light detector to provide an indication of the temperature measurement.
Several distinct advantages result from Utah-living the black body cavity radiator in accordance with the invention. When used as a temperature standard, a I
thermometer using a sapphire high temperature optical fiber will permit temperature measurements to approxima-tell 2000C- an increase of 500C above the maximum operating temperature of the present day thermocouple standard The invention may also be up to about 40 times more accurate than the existing standard. In contrast to the present thermocouple standards that are based on calibration and interpolation between fixed points, the optical fiber thermometer is based on fundamental radiation laws, and may be used to measure thermodynamic temperatures directly.
In industrial gay stream, for example, the optical fiber thermometer responds faster than convent tonal measurement techniques. The device also has lower heat transfer losses and therefore can measure temperatures at lower gas stream velocities. The inane-lion is thus useful for measurement and control of high temperatures in gas turbines and internal combustion engines, as well as in chemical processes.
Measurements made using the inventive device are free from the interference effects caused by strong nuclear and electromagnetic radiation fields. There-fore, the optical fiber thermometer is also applicable in power generation equipment, nuclear energy systems, weapons, space applications and similar areas. More-over, the temperature measuring device is chemically stable in many environments and can directly measure liquid and solid temperatures with superior spatial resolution and temporal response and low drift.
I
Accordingly, the device is applicable in the primary metal and chemical processing industries.
In accordance with the invention, a black body cavity is formed on the tip of a thin single crystal aluminum oxide (sapphire) or zirconium oxide (zircon) high temperature fiber, and the radiance emitted at a single wavelength from the cavity is used to measure its temperature. A high temperature fiber and an optional additional low -temperature fiber, transmit the signal to a conventional optical detector.
A thermometer in accordance with the invention consists of three basic elements, the signal generator, one or more transmitting optical fibers, and the optical detector. The signal generator at the tip of the high temperature fiber is selected to be an effective black-body radiator. A simple black body cavity can be created by sputtering a thin optically dense metallic or oxide coating on the surface of the fiber. The apparent emit-lance of the cavity is not a strong function of them-portray, even for relatively small length to diameter ratios. The equations describing the radiant emission from such a cavity are well known. Therefore, the device can be calibrated at a single temperature and the fundamental radiation laws can be used to extrapolate to - 25 other temperatures The second element of the thermometer consists of a high temperature optical fiber which transmits the signal to the detector. A low temperature fiber is also advantageously employed in many applications, especially I
where it it desired to locate the sensor relatively far away from the black body cavity. In most engineering applications, it it only required that the transmission losses be reproducible to 1% and not be a function of temperature. the high temperature fiber (aluminum oxide or zirconium oxide) are selected for their high temperature stability and ability to transmit optical wavelength from Amy to 1~0 em at high temperatures.
The coupling Lucy due to reflection and misalignment between the high and low temperature fibers are not a function of temperature. Transmission losses due to absorption in the low temperature fiber may be made negligible if the fiber is cooled during operation and if its length is not too long such as less than 100 m.
The third element of the thermometer is the detector system which can be constructed from a wide range of conventional components The principle come pennants are a light gathering lent, a narrow band filter, and a photomultip~ier or silicon detector Either one or more narrow band filters can be used A single narrow band filter with the band centered at wavelengths ; between 0.3 Jut and 1.0 Lump may be used during normal high temperature (500-2400C) operation. The wavelengths between Amy and 1.0 em are chosen obtain a maximum sensitivity and to minimize high temperature optical absorption in the high temperature fiber The device can be elf calibrated by measuring two wavelengths at two different temperatures. Standard photometric pray-Tess can maintain a 1% level of uncertainty, while ~2~7(~
advanced techniques can measure light intensity to better than 0.1%; the respective uncertainties in temperature measurement are 0.05% and 00005~.
Brief Desert lion of the Drawn I_ L
The invention is described by reverence to the following detailed description taken in conjunction with the accompanying drawings wherein:
Fig 1 is a schematic diagram of the complete optical fiber thermometer;
FIG. pa is a partial cross-sectional view of a simple black body cavity formed by a single optically dense thin film sputtered on a high temperature optical fiber;
FIG. 2b is a cross-sectional view of a black-body cavity formed by a solid optically dense tip on the end of a high temperature optical fiber;
FIG. 2c is a cross-sectional view of a black-body cavity formed by a solid optically den e tip and an optically dense thin film;
FIG. Ed is a cross-sectional view of the bl2ckbody cavity of FIG. 1 overreacted with a protective layer, FIG. ye is a cross-sectional view of the black body cavity of FIG. 2c overreacted with a protective layer; and FIG 3 is a chart showing an analysis of the major error of an optical fiber thermometer temperature measurement.
~;23'~7~'~
Detailed Description of the Preferred Embodiment kiwi As illustrated in FIG. 1, the optical fiber thermometer 10 in accordance with the invention compare-per a signal generator in the form of a black body covet positioned at the tip of a high temperature optical fiber 14. The high temperature fiber 14 is shown coupled to a low temperature optical fiber 16 which in turn is coupled to a detector 18. Detector 18 may come prose a lens 20, narr~wband optical filter 22, optical sensor 24 and amplifier 26.
The optical sensor 24 may be selected from numerous known devices so as to have a high sensitivity to the particular wavelength and temperature range lo desired to be measured. The detector 18 may Allah comprise a beam splitter 30, mirror 31, additional narrow band optical filter 32, additional optical sensor 34. The additional filter and sensor are used for calibration purposes or to provide additional them-portray ranges of high sensitivity. A neutral density filter may alto be incorporated into the optical path ahead of sensors 24 or 34~
Once the transmission detector efficiency of the thermometer is known, the device can be self-calibrated by measuring the radiance at two wavelength sat any two unknown but different temperatures. For example, at 1000C, using 0.6~(m and 0.7~m wavelengths -and maintaining a 0~1% photometric error, the accuracy of the self-calibration will be 5 x 10-2%.
I I
The outputs of the optical tensors 24 and 34 are fed to an output device 36 which provides a visual readout of the temperature dependent upon the magnitude of the sensor output signals. The output device may be a digital voltmeter for example.
The signal generator at the tip of the high temperature fiber is fabricated to be an effective black body radiator. Typically, cavities with length to diameter (L/D) ratios of 1:1 to 20:1 are preferred. The equations describing the radiant emission from such a cavity are well known. The Plank equation describing the spectral distribution of radiance for an ideal black body it used in conjunction with the Pouffe equation describing the apparent remittance of a cavity of finite dimensions and isotropic ally diffuse reflecting walls in order to determine the radiance at the exit of the cavity. Tile flux entering the fiber is obtained by integrating the internally reflected flux from the critical angle to the axis of the fiber. The total radiance entering the fiber is simply aeOCl Lowe) = [W/m] (1) 0 Leap ~C2/ T-)]
where a = area of the cavity exit, my eon = apparent remittance of the cavity Of = first radial on constant, aye x 10~16 W o my C2 = second radiation constant 1.438786 x 10 2 m K
= wavelength in vacuum, m.
4~753'~
is selected to provide a maximum sense-tivity to temperature changes with a measurable radiance and low absorption in the high temperature fiber at eye-voted temperatures. To obtain maximum sensitivity it is desired to use the short wavelength side of the black-body radiation curve. From 600C to 1300C, a 0.1 m-wide band centered between 0.6~ Mom is desirable.
In a preferred example at 1000C and 0.6~Lm, a 0.25-mm-diameter cavity will emit 10-7 watts of optical power in a 0.1~ m bandwidth and a 1% change in temperature will cause a 20% change in radiance. In another pro-furred example above 1300C~ a mud band centered at 0.4 Mom will provide similar sensitivity and radiance Black body Cavity Materials and Design The black body cavity 12 is composed by sputa toning one or more opaque films onto the fiber 14 to form black body cavity dimensions with an L/D ratio from 1:1 to 20:1. In FIG. pa, a thin, optically dense (opaque) film 40 is sputtered directly on the high them-portray fiber 14. The end of the fiber is formed at an angle of 20-70 from the axis to increase light scat-toning of the light within the cavity. Sputtered film thicknesses preferably include 0.2~ m to Lomb. The optically dense film 40 can be a noble metal film, such a platinum or iridium, depending on the maximum open-cling temperature of the thermometer For example, a platinum film will operate to temperature of 1760F, I I
and an iridium film will operate Jo temperatures of 2370F.
The optically dense film 40 may alternatively comprise an optically dense oxide film to form the black body cavity on the tip of the high temperature fiber. As in the case of the noble metal films, the oxide film must be compatible with the high temperature fiber and stable at high temperatures. Aluminum oxide doped with several percent of chromium (from .1 to 20 and preferably about 10~) is an example of such an optically dense oxide film for use with sapphire high temperature fibers.
In another preferred embodiment of the invent lion as shown in FIG. 2b, the black body cavity is formed from a solid optically dense tip 42 coupled to the end of the fiber 140 In this case, the emissivity of the cavity approaches one and becomes independent of temper azures when the absorption length is large with regard to the wavelength, but smaller than the cavity length.
Preferred cavity lengths are from 0.25 mm to 25 mm, inclusive The optical absorption coefficient of the oxide can be adjusted through impurity doping, and a chromium doped aluminum oxide it a preferred oxide tip for a sapphire oxide fiber. The oxide tip may be fabric acted separately and then bonded to the end of the high temperature fiber. Chromium may be doped in the amount of .1-20~ and preferably about 10% by weight.
In a most preferred embodiment shown in FIG. 2c, the solid optically den oxide tip 42 has a ~1234 thin film 44 sputtered thereon to form aft effective black body cavity on the end of the high temperature fiber 14. The sputtered film 44 may be a noble metal, for example platinum or iridium, which it optically dense.
The stability of the thin film 40 of the black body cavity it improved by overreacting the optic gaily dense film with a protective film 46 as shown in FIGS. 1 and Ed. In the case of a sapphire fiber, the protective film is aluminum oxide; for a zircon fiber, a zircon or yittria or calcia stabilized zircon) thin film is applied to reduce the rate of degradation of the original film. In both instances, the films are sputtered from lam to Ox thick. As shown in FIG. ye, a similarly fabricated film 46 may be applied to the embodiment of FIG. 2c to provide a protective layer for the optically dense film 44.
High Temperature Optical Fiber The high temperature optical fiber 14 it an important element of the thermometer design which accomplishes several function. The primary function of the fiber is to collect and transmit the light from the black body cavity 12 to the light detector 18. This transmission may take place directly or via the low.
temperature optical film 16. Accordingly, the high temperature optical fiber 14 must be stable at high temperatures in order to maintain mechanical integrity and an optically smooth surface In most applications, the fiber must be chemically stable in the prank of a ~23~7~
high temperature oxidizing environment. Commonly used optical glass (silica) fibers are not suitable since they start to melt at relatively low temperature, e.g., 1000-1200C. Moreover, the surrounding plastic protect live coatings in standard voice/data optical fibers havoc low melting point of about 200~C~ A further prohibit-in characteristic of glass fibers is that they begin to absorb light at about kiwi In accordance with the invention, the fiber 14 must neither absorb nor no emit light within the 0.3~m to lam band at the operating temperature, nor should such light be scattered off its surface. The high them-portray fiber of the invention mutt be able to transmit light within 0.3~m to Lomb bandwidth above 500C.
Ceramic fibers, in particular single crystal aluminum oxide fibers, have been found to have the desired prop-reties and it thus suitable for use in the invention.
The high temperature fiber 14 it grown or worked to a cylindrical shape with typical length to diameter ratio of loo to 1000:1, with an optically smooth surface. The actual length to diameter ratios are determined by the heat transfer to the thermometer and the required accuracy. Generally, as the rate of heat transfer from the environment to the thermometer decreases, larger length to diameter ratios are required. More accurate devices also require larger length to diameter ratios.
The practical diameter range of the high temperature fiber 14 is 00025 mm to 2.5 mm, with the ~13 ~Z3~7~'~
preferred range from 0.125 mm to 1.25 mm. Generally, the diameter of the fiber will be decreased when greater spatial revolution or superior thermal shock resistance is required. The optical signal is usually attenuated in the optical detector 24 for fibers with diameters larger than 17 25 mm, and thus, larger fibers are used primarily for increased mechanical strength.
Low Temperature optical Fiber The primary design requirement of the low them-portray fiber 16 it low optical loss over the few to several hundred feet required to transmit the signal to a remote detector. Such low temperature fibers are readily available and known to those skilled in the art.
In accordance with the invention, it is preferable to use a low temperature fiber 16 with a different diameter than the high temperature optical fiber 14 is order to reduce coupling losses at the interface between the two fibers. The low temperature fiber 16 can have a diameter either larger or smaller than the high temperature fibs 14.
The low temperature fixer may be cooled during operation to reduce transmission losses. Further, by maintaining a fixed air gap between the high and low ... . .. ..
temperature fibers, transmission losses from the crystal/air interface can be reduced within I Losses in the low temperature portion of the high temperature fiber crystal are also negligible Signal attenuation is less the 1% in 30 cm even in imperfect sapphire crystals 1 25 mm in diameter, grown by the edge-defined ~3'~7~
growth method and containing varying amounts of optical catering defects within the crystal and its surface.
At temperatures above 1100C, scattering, absorption, and remission at internal defects and impurities and surface imperfection are potential problems, and high quality cryYtal3 must be used At prevent, selected flame polished fibers are of sufficient quality that, in Tut from 1100 to 1700C in gas turbine combustor exhaust gases at 0.~7 to 0.47 Mach (240-345 m/sec), the total scattered, absorbed and rheumatoid radiant intent sty measured in a 1.2S mm diameter, cm long uncoated fiber was less than 2% of the radiance from an identical fiber with the black body sputtered on its end. No evidence of fundamental lattice absorption has been observed and therefore it can be anticipated that with superior crystals, the amount of scattering and absorb-lion will be further reduced. The majority of the scattering appeared to be due to surface imperfections.
Exposure of the fiber to a hot gas stream for several hours at these temperatures has not caused significant surface degradation However, it can he anticipated that, as the melting temperature of sapphire (2050C3 is approached, the surface topography will change due to surface diffusion or contamination and the practical maximum temperature and lifetime of the device will depend on the response of the surface to the environ-mint. The lifetime can be improved through designs that limit the length of fiber exposed to the high them-portray environment and protect the fiber from direct impact damage or contamination.
-15~
~Z39~7~4 There are two feature of the temperature device which are distinct improvements over existing devices. First, the sensitivity is quite high. For example, using a silicon detector and amplifier with a 105/1 dynamic range and a AVOW equivalent input noise within a 0-1 kHz bandwidth, a 10~3C change can be measured at 1000C with a signal-to-noise ratio of 10:1.
because of the small thermal mass of the optical fiber tip, only a 10-8 eel heat input is required to change the temperature of the cavity by 10-3C. Second, the useful frequency bandwidth is much larger than come parable metallic thermocouples. The increased bandwidth is due to an improved thermal response of the fiber as well as the high signal-to-noise ratio. In the optical fiber thermometer, the thermal waves are attenuated at the surface, the high frequency gain decreases only 3 dub per octave, and a two- to eight-fold increase in the transfer function is obtained. The high ~ignal-to-noise ratio permits small cavity temperature oscillations to be detected and further increases the bandwidth relative to that obtained with a fin wire thermocouple. The actual practical bandwidth it determined by the tempera-lure fluctuation power spectral density functions and the spatial resolution considerations of a specific applique-anion. As an example, in a gas turbine combustor, the optical fiber thermometer will provide useful information in a 0-100 kHz bandwidth, which is 100-fold improvement over present fine wire thermocouple technique.
~23~Q~
Experiments Initial experiments were conducted from 700C
to 1000C in a commercial black body furnace. The fur nice temperature was measured with a calibrated thermos couple. Experiments were conducted with uncoated and coated 1.25 mm, 30 cm-long sapphire fibers The uncoated fiber way inverted directly into the black body furnace to evaluate furnace calibration, transmission and photometric errors. In a separate test, a loom thick platinum tip with an L/D ratio of 2/1 was sputa toned on the other fiber and this fiber was inserted to the end of the conical black body cavity. The sapphire fibers were coupled across a fixed air gap to a 0.6 mm diameter, 10 m-long, low temperature glass optical fiber. The detector system consisted of a lens, neutral filter with optical densities of 0.3 to 2.0, beam splitters, 0.1 m-wide filters centered at Amy and 0.7~m, and two photo multipliers. The photo multiplier output current was measured across a 50 k3aresistor with a voltmeter.
The data are presented in the following Table I. In Table I en lo defined by the convolution integral of the narrow band filter transfer function and the spectral radiance computed from equation 1, and the relative theoretical value of the flux at this wave-length is computed from equation I For this demonstra-lion, the 850C measurement was used as the calibration -point. Inspection of the data shows that over a 300/1 range ox radiance, the uncoated fiber data display ~23 I
standard deviations of 0~054~ and 0.048% from the values expected from equation 1, while the coated fiber data display standard deviations of 0.050% and 0.072~ The similarity of the standard deviations of the uncoated and coated fiber data shows that the errors are prim manly due to the furnace calibration or photometric errors rather than errors in the radiance from the tip of the sapphire fiber and transmission through the fibers. However, even this simple test demonstrates that unusual accuracy can easily be obtained with an elementary device.
The black body cavity configuration is used to obtain an apparent emissivity that it constant over the operating range of the thermometer. The theoretical errors encountered in thermometers using L/D ratios of 2 and 20 and several wavelengths are presented in FIG. 3 where it is observed that photometric accuracy is a relative accuracy for an interpolative device which ha an L/D ratio of 2 and can be calibrated at a single known temperature in a melting point furnace or a gall-brazed large black body furnace The device is seen to be approximately four times more accurate than an IFS
calibrated ANSI Type S thermocouple standard above 1000C~ The 0.1% photometric errors and radiance errors due to the apparent emissivity of a cavity with a 20:1 L/D ratio are those for a laboratory standard thermometer.
The terms and expressions which have been 5 employed in the foregoing specification are used therein 123~7~
as term of description and not of limitation, and there is no intention, in the use of such terms and expire-sons, of excluding equivalents of the features shown and described or portion thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow ~12~47~ 'I
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OPT I CAL F I BIER TO ERMOMET ERR
Back round of the Invention g This invention relates generally to tempera-lure measurement apparatus, and more specifically to relatively high temperature measurements extending to approximately 2400C.
The accurate measurement of high temperatures has always been and is a major industrial problem Severe operating environments limit the use of standard temperature measuring techniques: conventional materials and design constraints limit both the maximum operating temperature and their use in chemically aggressive environments. The invention it directed toward a measuring technique which can improve upon existing techniques, such as thermocouples, resistance then-mometers and pyrometers, or provide a new measurement capability in regimes where conventional technique do not operate. The temperature measuring device may find application in a large number of environments, basically wherever it is desirable to take temperature measure-mints either for monitoring the temperature quantity per so, or for control purposes, such as, for example, in industrial facilities.
The invention utilizes a black body cavity formed on the tip of a light transmitting fiber. the black body cavity serves a a signal generator which emits black body radiation whose amplitude and frequency I
spectra are indicative of the black body cavity tempera-lure. Radiation from the black body cavity is transmitted via the light transmitting fiber to an optical receiving device which converts the received radiation into an electrical signal having a signal characteristic indict-live of the magnitude of received radiation. The basic technique set forth above has been described in US.
Patent No. 3,626,75~.
Summary of the Invention It is an object of the invention to improve upon the prior art and to provide a highly accurate temperature measuring device using a black body cavity member.
It it a further object of the invention to provide an accurate temperature measuring device for temperatures within the range of 500-2400C using a black body cavity radiator.
Another object of the invention is to provide - 20 a temperature measuring device capable of efficiently transmitting black body radiation in the band of 0.3~ m-Lomb over an optical link or fiber which does no emit or absorb radiation in the radiation band over a temperature range within 500-2400C.
US A further object of the invention is to provide a temperature measuring device which uses a high temperature optical fiber having an optically dense solid oxide tip to enable black body radiation measure-mints.
~Z347(~4 Yet another object of the invention is to pro-vise a temperature measuring device having suitable eke-mica and mechanical durability for use in oxidizing environments having high temperature ranges on the order of 500-2400C.
The invention may be characterized as a them-portray measuring device comprising an elongated optic eel fiber, a black body cavity, a high temperature fiber and a light detector. The black body cavity is post-toned at one end of the high temperature fiber and both the black body cavity and the high temperature fiber are able to withstand temperatures in the range of 500-2400C. The black body cavity emits radiation in the range of 003~m-l.0 m within said temperature range of ~15 500-2400C. The high temperature fiber transmits radiation in the temperature range of interest without substantial absorption or remoteness thereof. The light detector is coupled to receive light transmitted by the high temperature fiber and includes a narrow band filter for selecting a narrow optical Rand within said 0.3~Xm-lnOl~m range. The light detector provides an output signal which is proportional to the intensity of the received and filtered light. The temperature measuring device may also include an output device ; 25 responsive to the output signal from the light detector to provide an indication of the temperature measurement.
Several distinct advantages result from Utah-living the black body cavity radiator in accordance with the invention. When used as a temperature standard, a I
thermometer using a sapphire high temperature optical fiber will permit temperature measurements to approxima-tell 2000C- an increase of 500C above the maximum operating temperature of the present day thermocouple standard The invention may also be up to about 40 times more accurate than the existing standard. In contrast to the present thermocouple standards that are based on calibration and interpolation between fixed points, the optical fiber thermometer is based on fundamental radiation laws, and may be used to measure thermodynamic temperatures directly.
In industrial gay stream, for example, the optical fiber thermometer responds faster than convent tonal measurement techniques. The device also has lower heat transfer losses and therefore can measure temperatures at lower gas stream velocities. The inane-lion is thus useful for measurement and control of high temperatures in gas turbines and internal combustion engines, as well as in chemical processes.
Measurements made using the inventive device are free from the interference effects caused by strong nuclear and electromagnetic radiation fields. There-fore, the optical fiber thermometer is also applicable in power generation equipment, nuclear energy systems, weapons, space applications and similar areas. More-over, the temperature measuring device is chemically stable in many environments and can directly measure liquid and solid temperatures with superior spatial resolution and temporal response and low drift.
I
Accordingly, the device is applicable in the primary metal and chemical processing industries.
In accordance with the invention, a black body cavity is formed on the tip of a thin single crystal aluminum oxide (sapphire) or zirconium oxide (zircon) high temperature fiber, and the radiance emitted at a single wavelength from the cavity is used to measure its temperature. A high temperature fiber and an optional additional low -temperature fiber, transmit the signal to a conventional optical detector.
A thermometer in accordance with the invention consists of three basic elements, the signal generator, one or more transmitting optical fibers, and the optical detector. The signal generator at the tip of the high temperature fiber is selected to be an effective black-body radiator. A simple black body cavity can be created by sputtering a thin optically dense metallic or oxide coating on the surface of the fiber. The apparent emit-lance of the cavity is not a strong function of them-portray, even for relatively small length to diameter ratios. The equations describing the radiant emission from such a cavity are well known. Therefore, the device can be calibrated at a single temperature and the fundamental radiation laws can be used to extrapolate to - 25 other temperatures The second element of the thermometer consists of a high temperature optical fiber which transmits the signal to the detector. A low temperature fiber is also advantageously employed in many applications, especially I
where it it desired to locate the sensor relatively far away from the black body cavity. In most engineering applications, it it only required that the transmission losses be reproducible to 1% and not be a function of temperature. the high temperature fiber (aluminum oxide or zirconium oxide) are selected for their high temperature stability and ability to transmit optical wavelength from Amy to 1~0 em at high temperatures.
The coupling Lucy due to reflection and misalignment between the high and low temperature fibers are not a function of temperature. Transmission losses due to absorption in the low temperature fiber may be made negligible if the fiber is cooled during operation and if its length is not too long such as less than 100 m.
The third element of the thermometer is the detector system which can be constructed from a wide range of conventional components The principle come pennants are a light gathering lent, a narrow band filter, and a photomultip~ier or silicon detector Either one or more narrow band filters can be used A single narrow band filter with the band centered at wavelengths ; between 0.3 Jut and 1.0 Lump may be used during normal high temperature (500-2400C) operation. The wavelengths between Amy and 1.0 em are chosen obtain a maximum sensitivity and to minimize high temperature optical absorption in the high temperature fiber The device can be elf calibrated by measuring two wavelengths at two different temperatures. Standard photometric pray-Tess can maintain a 1% level of uncertainty, while ~2~7(~
advanced techniques can measure light intensity to better than 0.1%; the respective uncertainties in temperature measurement are 0.05% and 00005~.
Brief Desert lion of the Drawn I_ L
The invention is described by reverence to the following detailed description taken in conjunction with the accompanying drawings wherein:
Fig 1 is a schematic diagram of the complete optical fiber thermometer;
FIG. pa is a partial cross-sectional view of a simple black body cavity formed by a single optically dense thin film sputtered on a high temperature optical fiber;
FIG. 2b is a cross-sectional view of a black-body cavity formed by a solid optically dense tip on the end of a high temperature optical fiber;
FIG. 2c is a cross-sectional view of a black-body cavity formed by a solid optically den e tip and an optically dense thin film;
FIG. Ed is a cross-sectional view of the bl2ckbody cavity of FIG. 1 overreacted with a protective layer, FIG. ye is a cross-sectional view of the black body cavity of FIG. 2c overreacted with a protective layer; and FIG 3 is a chart showing an analysis of the major error of an optical fiber thermometer temperature measurement.
~;23'~7~'~
Detailed Description of the Preferred Embodiment kiwi As illustrated in FIG. 1, the optical fiber thermometer 10 in accordance with the invention compare-per a signal generator in the form of a black body covet positioned at the tip of a high temperature optical fiber 14. The high temperature fiber 14 is shown coupled to a low temperature optical fiber 16 which in turn is coupled to a detector 18. Detector 18 may come prose a lens 20, narr~wband optical filter 22, optical sensor 24 and amplifier 26.
The optical sensor 24 may be selected from numerous known devices so as to have a high sensitivity to the particular wavelength and temperature range lo desired to be measured. The detector 18 may Allah comprise a beam splitter 30, mirror 31, additional narrow band optical filter 32, additional optical sensor 34. The additional filter and sensor are used for calibration purposes or to provide additional them-portray ranges of high sensitivity. A neutral density filter may alto be incorporated into the optical path ahead of sensors 24 or 34~
Once the transmission detector efficiency of the thermometer is known, the device can be self-calibrated by measuring the radiance at two wavelength sat any two unknown but different temperatures. For example, at 1000C, using 0.6~(m and 0.7~m wavelengths -and maintaining a 0~1% photometric error, the accuracy of the self-calibration will be 5 x 10-2%.
I I
The outputs of the optical tensors 24 and 34 are fed to an output device 36 which provides a visual readout of the temperature dependent upon the magnitude of the sensor output signals. The output device may be a digital voltmeter for example.
The signal generator at the tip of the high temperature fiber is fabricated to be an effective black body radiator. Typically, cavities with length to diameter (L/D) ratios of 1:1 to 20:1 are preferred. The equations describing the radiant emission from such a cavity are well known. The Plank equation describing the spectral distribution of radiance for an ideal black body it used in conjunction with the Pouffe equation describing the apparent remittance of a cavity of finite dimensions and isotropic ally diffuse reflecting walls in order to determine the radiance at the exit of the cavity. Tile flux entering the fiber is obtained by integrating the internally reflected flux from the critical angle to the axis of the fiber. The total radiance entering the fiber is simply aeOCl Lowe) = [W/m] (1) 0 Leap ~C2/ T-)]
where a = area of the cavity exit, my eon = apparent remittance of the cavity Of = first radial on constant, aye x 10~16 W o my C2 = second radiation constant 1.438786 x 10 2 m K
= wavelength in vacuum, m.
4~753'~
is selected to provide a maximum sense-tivity to temperature changes with a measurable radiance and low absorption in the high temperature fiber at eye-voted temperatures. To obtain maximum sensitivity it is desired to use the short wavelength side of the black-body radiation curve. From 600C to 1300C, a 0.1 m-wide band centered between 0.6~ Mom is desirable.
In a preferred example at 1000C and 0.6~Lm, a 0.25-mm-diameter cavity will emit 10-7 watts of optical power in a 0.1~ m bandwidth and a 1% change in temperature will cause a 20% change in radiance. In another pro-furred example above 1300C~ a mud band centered at 0.4 Mom will provide similar sensitivity and radiance Black body Cavity Materials and Design The black body cavity 12 is composed by sputa toning one or more opaque films onto the fiber 14 to form black body cavity dimensions with an L/D ratio from 1:1 to 20:1. In FIG. pa, a thin, optically dense (opaque) film 40 is sputtered directly on the high them-portray fiber 14. The end of the fiber is formed at an angle of 20-70 from the axis to increase light scat-toning of the light within the cavity. Sputtered film thicknesses preferably include 0.2~ m to Lomb. The optically dense film 40 can be a noble metal film, such a platinum or iridium, depending on the maximum open-cling temperature of the thermometer For example, a platinum film will operate to temperature of 1760F, I I
and an iridium film will operate Jo temperatures of 2370F.
The optically dense film 40 may alternatively comprise an optically dense oxide film to form the black body cavity on the tip of the high temperature fiber. As in the case of the noble metal films, the oxide film must be compatible with the high temperature fiber and stable at high temperatures. Aluminum oxide doped with several percent of chromium (from .1 to 20 and preferably about 10~) is an example of such an optically dense oxide film for use with sapphire high temperature fibers.
In another preferred embodiment of the invent lion as shown in FIG. 2b, the black body cavity is formed from a solid optically dense tip 42 coupled to the end of the fiber 140 In this case, the emissivity of the cavity approaches one and becomes independent of temper azures when the absorption length is large with regard to the wavelength, but smaller than the cavity length.
Preferred cavity lengths are from 0.25 mm to 25 mm, inclusive The optical absorption coefficient of the oxide can be adjusted through impurity doping, and a chromium doped aluminum oxide it a preferred oxide tip for a sapphire oxide fiber. The oxide tip may be fabric acted separately and then bonded to the end of the high temperature fiber. Chromium may be doped in the amount of .1-20~ and preferably about 10% by weight.
In a most preferred embodiment shown in FIG. 2c, the solid optically den oxide tip 42 has a ~1234 thin film 44 sputtered thereon to form aft effective black body cavity on the end of the high temperature fiber 14. The sputtered film 44 may be a noble metal, for example platinum or iridium, which it optically dense.
The stability of the thin film 40 of the black body cavity it improved by overreacting the optic gaily dense film with a protective film 46 as shown in FIGS. 1 and Ed. In the case of a sapphire fiber, the protective film is aluminum oxide; for a zircon fiber, a zircon or yittria or calcia stabilized zircon) thin film is applied to reduce the rate of degradation of the original film. In both instances, the films are sputtered from lam to Ox thick. As shown in FIG. ye, a similarly fabricated film 46 may be applied to the embodiment of FIG. 2c to provide a protective layer for the optically dense film 44.
High Temperature Optical Fiber The high temperature optical fiber 14 it an important element of the thermometer design which accomplishes several function. The primary function of the fiber is to collect and transmit the light from the black body cavity 12 to the light detector 18. This transmission may take place directly or via the low.
temperature optical film 16. Accordingly, the high temperature optical fiber 14 must be stable at high temperatures in order to maintain mechanical integrity and an optically smooth surface In most applications, the fiber must be chemically stable in the prank of a ~23~7~
high temperature oxidizing environment. Commonly used optical glass (silica) fibers are not suitable since they start to melt at relatively low temperature, e.g., 1000-1200C. Moreover, the surrounding plastic protect live coatings in standard voice/data optical fibers havoc low melting point of about 200~C~ A further prohibit-in characteristic of glass fibers is that they begin to absorb light at about kiwi In accordance with the invention, the fiber 14 must neither absorb nor no emit light within the 0.3~m to lam band at the operating temperature, nor should such light be scattered off its surface. The high them-portray fiber of the invention mutt be able to transmit light within 0.3~m to Lomb bandwidth above 500C.
Ceramic fibers, in particular single crystal aluminum oxide fibers, have been found to have the desired prop-reties and it thus suitable for use in the invention.
The high temperature fiber 14 it grown or worked to a cylindrical shape with typical length to diameter ratio of loo to 1000:1, with an optically smooth surface. The actual length to diameter ratios are determined by the heat transfer to the thermometer and the required accuracy. Generally, as the rate of heat transfer from the environment to the thermometer decreases, larger length to diameter ratios are required. More accurate devices also require larger length to diameter ratios.
The practical diameter range of the high temperature fiber 14 is 00025 mm to 2.5 mm, with the ~13 ~Z3~7~'~
preferred range from 0.125 mm to 1.25 mm. Generally, the diameter of the fiber will be decreased when greater spatial revolution or superior thermal shock resistance is required. The optical signal is usually attenuated in the optical detector 24 for fibers with diameters larger than 17 25 mm, and thus, larger fibers are used primarily for increased mechanical strength.
Low Temperature optical Fiber The primary design requirement of the low them-portray fiber 16 it low optical loss over the few to several hundred feet required to transmit the signal to a remote detector. Such low temperature fibers are readily available and known to those skilled in the art.
In accordance with the invention, it is preferable to use a low temperature fiber 16 with a different diameter than the high temperature optical fiber 14 is order to reduce coupling losses at the interface between the two fibers. The low temperature fiber 16 can have a diameter either larger or smaller than the high temperature fibs 14.
The low temperature fixer may be cooled during operation to reduce transmission losses. Further, by maintaining a fixed air gap between the high and low ... . .. ..
temperature fibers, transmission losses from the crystal/air interface can be reduced within I Losses in the low temperature portion of the high temperature fiber crystal are also negligible Signal attenuation is less the 1% in 30 cm even in imperfect sapphire crystals 1 25 mm in diameter, grown by the edge-defined ~3'~7~
growth method and containing varying amounts of optical catering defects within the crystal and its surface.
At temperatures above 1100C, scattering, absorption, and remission at internal defects and impurities and surface imperfection are potential problems, and high quality cryYtal3 must be used At prevent, selected flame polished fibers are of sufficient quality that, in Tut from 1100 to 1700C in gas turbine combustor exhaust gases at 0.~7 to 0.47 Mach (240-345 m/sec), the total scattered, absorbed and rheumatoid radiant intent sty measured in a 1.2S mm diameter, cm long uncoated fiber was less than 2% of the radiance from an identical fiber with the black body sputtered on its end. No evidence of fundamental lattice absorption has been observed and therefore it can be anticipated that with superior crystals, the amount of scattering and absorb-lion will be further reduced. The majority of the scattering appeared to be due to surface imperfections.
Exposure of the fiber to a hot gas stream for several hours at these temperatures has not caused significant surface degradation However, it can he anticipated that, as the melting temperature of sapphire (2050C3 is approached, the surface topography will change due to surface diffusion or contamination and the practical maximum temperature and lifetime of the device will depend on the response of the surface to the environ-mint. The lifetime can be improved through designs that limit the length of fiber exposed to the high them-portray environment and protect the fiber from direct impact damage or contamination.
-15~
~Z39~7~4 There are two feature of the temperature device which are distinct improvements over existing devices. First, the sensitivity is quite high. For example, using a silicon detector and amplifier with a 105/1 dynamic range and a AVOW equivalent input noise within a 0-1 kHz bandwidth, a 10~3C change can be measured at 1000C with a signal-to-noise ratio of 10:1.
because of the small thermal mass of the optical fiber tip, only a 10-8 eel heat input is required to change the temperature of the cavity by 10-3C. Second, the useful frequency bandwidth is much larger than come parable metallic thermocouples. The increased bandwidth is due to an improved thermal response of the fiber as well as the high signal-to-noise ratio. In the optical fiber thermometer, the thermal waves are attenuated at the surface, the high frequency gain decreases only 3 dub per octave, and a two- to eight-fold increase in the transfer function is obtained. The high ~ignal-to-noise ratio permits small cavity temperature oscillations to be detected and further increases the bandwidth relative to that obtained with a fin wire thermocouple. The actual practical bandwidth it determined by the tempera-lure fluctuation power spectral density functions and the spatial resolution considerations of a specific applique-anion. As an example, in a gas turbine combustor, the optical fiber thermometer will provide useful information in a 0-100 kHz bandwidth, which is 100-fold improvement over present fine wire thermocouple technique.
~23~Q~
Experiments Initial experiments were conducted from 700C
to 1000C in a commercial black body furnace. The fur nice temperature was measured with a calibrated thermos couple. Experiments were conducted with uncoated and coated 1.25 mm, 30 cm-long sapphire fibers The uncoated fiber way inverted directly into the black body furnace to evaluate furnace calibration, transmission and photometric errors. In a separate test, a loom thick platinum tip with an L/D ratio of 2/1 was sputa toned on the other fiber and this fiber was inserted to the end of the conical black body cavity. The sapphire fibers were coupled across a fixed air gap to a 0.6 mm diameter, 10 m-long, low temperature glass optical fiber. The detector system consisted of a lens, neutral filter with optical densities of 0.3 to 2.0, beam splitters, 0.1 m-wide filters centered at Amy and 0.7~m, and two photo multipliers. The photo multiplier output current was measured across a 50 k3aresistor with a voltmeter.
The data are presented in the following Table I. In Table I en lo defined by the convolution integral of the narrow band filter transfer function and the spectral radiance computed from equation 1, and the relative theoretical value of the flux at this wave-length is computed from equation I For this demonstra-lion, the 850C measurement was used as the calibration -point. Inspection of the data shows that over a 300/1 range ox radiance, the uncoated fiber data display ~23 I
standard deviations of 0~054~ and 0.048% from the values expected from equation 1, while the coated fiber data display standard deviations of 0.050% and 0.072~ The similarity of the standard deviations of the uncoated and coated fiber data shows that the errors are prim manly due to the furnace calibration or photometric errors rather than errors in the radiance from the tip of the sapphire fiber and transmission through the fibers. However, even this simple test demonstrates that unusual accuracy can easily be obtained with an elementary device.
The black body cavity configuration is used to obtain an apparent emissivity that it constant over the operating range of the thermometer. The theoretical errors encountered in thermometers using L/D ratios of 2 and 20 and several wavelengths are presented in FIG. 3 where it is observed that photometric accuracy is a relative accuracy for an interpolative device which ha an L/D ratio of 2 and can be calibrated at a single known temperature in a melting point furnace or a gall-brazed large black body furnace The device is seen to be approximately four times more accurate than an IFS
calibrated ANSI Type S thermocouple standard above 1000C~ The 0.1% photometric errors and radiance errors due to the apparent emissivity of a cavity with a 20:1 L/D ratio are those for a laboratory standard thermometer.
The terms and expressions which have been 5 employed in the foregoing specification are used therein 123~7~
as term of description and not of limitation, and there is no intention, in the use of such terms and expire-sons, of excluding equivalents of the features shown and described or portion thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow ~12~47~ 'I
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Claims (44)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A temperature measuring device for use in regions of high temperature comprising:
(a) a heat resistant optical fiber having a first end for insertion into said region, said end including a black body cavity for emitting radiation whose intensity varies as a function of the temperature within said region, said blackbody cavity compri-sing an optically dense film comprising a noble metal on the outer surface of said fiber, and said fiber being capable of transmitting said radiation without substantial absorption thereof; and (b) means coupled to said fiber for detecting said radiation said means including narrow-band filter means for selecting radiation having a range of preselected wavelengths at which relatively small changes in temperature within said region result in relatively large changes in the intensity of radiation emitted by said blackbody cavity.
(a) a heat resistant optical fiber having a first end for insertion into said region, said end including a black body cavity for emitting radiation whose intensity varies as a function of the temperature within said region, said blackbody cavity compri-sing an optically dense film comprising a noble metal on the outer surface of said fiber, and said fiber being capable of transmitting said radiation without substantial absorption thereof; and (b) means coupled to said fiber for detecting said radiation said means including narrow-band filter means for selecting radiation having a range of preselected wavelengths at which relatively small changes in temperature within said region result in relatively large changes in the intensity of radiation emitted by said blackbody cavity.
2. The temperature measuring device of claim 1, further including a low temperature fiber coupled between said high temperature fiber and said detecting means.
3. The temperature measuring device of claim 2 wherein said high temperature fiber is coupled to said low temperature fiber across an air gap.
4. The temperature measuring device of claim 1 in which said regions of high temperature are defined as those regions in which the temperature is greater than 500°C.
5. A temperature measuring device us recited in claim 1 wherein said noble metal is selected from the group of platinum and iridium.
6. A temperature measuring device as recited in claim 5 wherein said high temperature fiber is a sapphire fiber and said device further comprises an alumina protective film overlaying said optically dense film.
7. A temperature measuring device as recited in claim 5 wherein said high temperature fiber is a zirconia fiber and said device further comprises a zirconia protective film overlaying said optically dense film.
8. A temperature measuring device as recited in claim 6 wherein said optically dense film has a thickness on the order of 0.2 µm-10 µm.
9. A temperature measuring device as recited in claim 7 wherein said optically dense film has a thickness on the order of 0.2 µm-10 µm.
10. A temperature measuring device as recited in claim 1 wherein said optically dense film comprises a chemically doped metallic oxide film.
11. A temperature measuring device as recited in claim 10 wherein said chemically doped metallic oxide film is chromium doped single crystal aluminum oxide.
12. temperature measuring device as recited in claim 1 wherein the length to diameter ratio of said blackbody cavity is in the range of 1:1 to 20:1.
13. A temperature measuring device as recited in claim 8 wherein the length to diameter ratio of said blackbody cavity is in the range of 1:1 to 20:1.
14. A temperature measuring device as recited in claim 9 wherein the length to diameter ratio of said blackbody cavity is in the range of 1:1 to 20:1.
15. A temperature measuring device as recited in claim 1 wherein said blackbody cavity has a surface inclined in the range of 20-70° from the longitudinal axis of said high temperature fiber.
16. A temperature measuring device as recited in claim 1 wherein said blackbody cavity is formed of an optically dense solid oxide material.
17. A temperature measuring device as recited in claim 2 wherein said blackbody cavity is formed of an optically dense solid oxide material.
18. A temperature measuring device as recited in claim 16 wherein said solid oxide material is chromium doped aluminum oxide.
19. A temperature measuring device as recited in claim 18 wherein said chromium is doped in the amount between .1-20% by weight.
20. A temperature measuring device as recited in claim 18 further comprising an optically dense film overlaying said solid oxide material.
21. A temperature measuring device as recited in claim 20 further comprising a protective layer over-laying said optically dense film.
22. A temperature measuring device as recited in claim 18 wherein said high temperature fiber is a sapphire fiber and said device further comprises an alumina protective film overlaying said optically dense film.
23. A temperature measuring device as recited in claim 18 wherein said high temperature fiber is a zirconia fiber and said device further comprises a zir-conia protective film overlaying said optically dense film.
24. A temperature measuring device as recited in claim 16 wherein said solid oxide material has the same diameter as said high temperature fiber and said blackbody cavity has a length to diameter ratio in the range of 1:1 to 20:1.
25. A temperature measuring device as recited in claim 3 wherein said low temperature fiber has a dif-ferent diameter than said high temperature fiber.
26. A temperature sensor comprising:
(a) an elongated high temperature optical fiber;
(b) a blackbody cavity positioned at one end of said high temperature fiber, said blackbody cavity and said high tempera-ture fiber able to withstand temperatures in the range of 500-2400°C;
(c) said blackbody cavity emitting radiation in the range of 0.3 µm-1.0 µm within said temperature range of 500-2400°C;
(d) said high temperature fiber transmitting radiation in said temperature range with-out substantial absorption thereof; and (e) a light detector coupled to receive light transmitted by said high temperature fiber and including a narrowband filter for selecting a narrow optical band within said 0.3 µm-1.0 µm range, said light detector providing an output signal proportional to the intensity of said received light.
(a) an elongated high temperature optical fiber;
(b) a blackbody cavity positioned at one end of said high temperature fiber, said blackbody cavity and said high tempera-ture fiber able to withstand temperatures in the range of 500-2400°C;
(c) said blackbody cavity emitting radiation in the range of 0.3 µm-1.0 µm within said temperature range of 500-2400°C;
(d) said high temperature fiber transmitting radiation in said temperature range with-out substantial absorption thereof; and (e) a light detector coupled to receive light transmitted by said high temperature fiber and including a narrowband filter for selecting a narrow optical band within said 0.3 µm-1.0 µm range, said light detector providing an output signal proportional to the intensity of said received light.
27. A temperature measuring device as recited in claim 26 further comprising a low temperature fiber coupled between said high temperature fiber and said light detector.
28. A temperature measuring device comprising:
(a) an elongated high temperature optical fiber;
(b) one end of said high temperature fiber having an optically dense film covering the outer surface thereof, thereby defining a region of the covered end of said fiber exhibiting blackbody radiation behaviour, said black-body region and said high temperature fiber able to measure temperatures in the range of 500° - 2400°C., said blackbody region having a length to diameter ratio in the range 1/1 to 20/1;
(c) said blackbody region emitting radiation primarily in the range of 0.3 m-1.0 m within said temperature range of 500°-2400°C.;
(d) said high temperature fiber having a dia-meter in the range of 0.025 mm to 2.5 mm and transmitting radiation in said tempera-ture range without substantial absorption thereof;
(e) a low temperature fiber having a different diameter than said high temperature fiber and coupled to the other end of said high temperature fiber;
(f) a light detector coupled to receive light transmitted by said low temperature fiber and including a narrow-band filter for selecting a narrow optical band wherein relatively small changes in temperature within said band result in relatively large changes in intensity emitted by said black-body region, said narrow optical band with-in said 0.3 m-1.0 m range, said light detector providing an output signal propor-tional to the intensity of said received light, and (g) means responsive to said output signal for providing an indication of said temperature measurement.
(a) an elongated high temperature optical fiber;
(b) one end of said high temperature fiber having an optically dense film covering the outer surface thereof, thereby defining a region of the covered end of said fiber exhibiting blackbody radiation behaviour, said black-body region and said high temperature fiber able to measure temperatures in the range of 500° - 2400°C., said blackbody region having a length to diameter ratio in the range 1/1 to 20/1;
(c) said blackbody region emitting radiation primarily in the range of 0.3 m-1.0 m within said temperature range of 500°-2400°C.;
(d) said high temperature fiber having a dia-meter in the range of 0.025 mm to 2.5 mm and transmitting radiation in said tempera-ture range without substantial absorption thereof;
(e) a low temperature fiber having a different diameter than said high temperature fiber and coupled to the other end of said high temperature fiber;
(f) a light detector coupled to receive light transmitted by said low temperature fiber and including a narrow-band filter for selecting a narrow optical band wherein relatively small changes in temperature within said band result in relatively large changes in intensity emitted by said black-body region, said narrow optical band with-in said 0.3 m-1.0 m range, said light detector providing an output signal propor-tional to the intensity of said received light, and (g) means responsive to said output signal for providing an indication of said temperature measurement.
29. A temperature measuring device as recited in claim 28 wherein said blackbody region has a surface inclined in the range of 20°-70° from the longitudinal axis of said high temperature fiber.
30. A temperature measuring device as recited in claim 28 further comprising a protective layer overlay-ing said optically dense film.
31. A temperature measuring device as recited in claim 28 wherein said film comprises a noble metal.
32. A temperature measuring device as recited in claim 31 wherein said noble metal is selected from the group of platinum and iridium.
33. A temperature measuring device as recited in claim 32 wherein said high temperature fiber is a sapphire fiber and said device further comprises an alumina protective film overlaying said optically dense film.
34. A temperature measuring device as recited in claim 33 wherein said optically dense film has a thickness on the order of 0.2 µm-10 µm.
35. A temperature measuring device as recited in claim 32 wherein said high temperature fiber is a zirconia fiber and said device further comprises a zirconia protective film overlaying said optically dense film.
36. A temperature measuring device as recited in claim 35 wherein said optically dense film has a thick-ness on the order of 0.2 µm-10 µm.
37. A temperature measuring device as recited in claim 28 wherein said optically dense film comprises a chemically doped metallic oxide film.
38. A temperature measuring device as recited in claim 37 wherein said chemically doped metallic oxide film is chromium doped single crystal aluminum oxide.
39. A temperature measuring device as recited in claim 28 wherein said low temperature fiber is coupled to said high temperature fiber via an air gap.
40. A temperature measuring device comprising:
(a) an elongated high temperature optical fiber;
(b) a blackbody region positioned at one end of said high temperature fiber and comprising an optically dense solid oxide material, said blackbody region and said high temperature fiber able to measure tempera-tures in the range of 500°-2400°C. said blackbody region having a length to dia-meter ratio in the range 1/1 to 20/1;
(c) said blackbody region emitting radiation primarily in the range of 0.3 µm-1.0 µm within said temperature range of 500°-2400°C.;
(d) said high temperature fiber having a dia-meter in the range of 0.025 mm to 2.5 mm and transmitting radiation in said tempera-ture range without substantial absorption thereof;
(e) a low temperature fiber having a different diameter than said high temperature fiber and coupled to the other end of said high temperature fiber;
(f) a light detector coupled to receive light transmitted by said low temperature fiber and including a narrow-band filter for selecting a narrow optical band wherein relatively small changes in temperature within said band result in relatively large changes in intensity emitted by said black-body region, said narrow optical band within said 0.3 µm-1.0 µm range, said light detec-tor providing an output signal proportional to the intensity of said received light, and (g) means responsive to said output signal for providing an indication of said temperature measurement.
(a) an elongated high temperature optical fiber;
(b) a blackbody region positioned at one end of said high temperature fiber and comprising an optically dense solid oxide material, said blackbody region and said high temperature fiber able to measure tempera-tures in the range of 500°-2400°C. said blackbody region having a length to dia-meter ratio in the range 1/1 to 20/1;
(c) said blackbody region emitting radiation primarily in the range of 0.3 µm-1.0 µm within said temperature range of 500°-2400°C.;
(d) said high temperature fiber having a dia-meter in the range of 0.025 mm to 2.5 mm and transmitting radiation in said tempera-ture range without substantial absorption thereof;
(e) a low temperature fiber having a different diameter than said high temperature fiber and coupled to the other end of said high temperature fiber;
(f) a light detector coupled to receive light transmitted by said low temperature fiber and including a narrow-band filter for selecting a narrow optical band wherein relatively small changes in temperature within said band result in relatively large changes in intensity emitted by said black-body region, said narrow optical band within said 0.3 µm-1.0 µm range, said light detec-tor providing an output signal proportional to the intensity of said received light, and (g) means responsive to said output signal for providing an indication of said temperature measurement.
41. A temperature measuring device as recited in claim 40 wherein said solid oxide material is aluminum oxide doped with chromium.
42. A temperature measuring device as recited in claim 41 wherein said chromium is doped in the amount between 0.1-20% by weight.
43. A temperature measuring device as recited in claim 41 wherein said high temperature fiber is a sapphire fiber and said device further comprises an alumina protective film overlaying said optically dense aluminum oxide doped with chromium.
44. A temperature measuring device as recited in claim 41 wherein said high temperature fiber is a zirconia fiber and said device further comprises a zirconia protective film overlaying said optically dense aluminum oxide doped with chromium.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000463651A CA1234704A (en) | 1984-09-19 | 1984-09-19 | Optical fiber thermometer |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000463651A CA1234704A (en) | 1984-09-19 | 1984-09-19 | Optical fiber thermometer |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1234704A true CA1234704A (en) | 1988-04-05 |
Family
ID=4128749
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000463651A Expired CA1234704A (en) | 1984-09-19 | 1984-09-19 | Optical fiber thermometer |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1234704A (en) |
-
1984
- 1984-09-19 CA CA000463651A patent/CA1234704A/en not_active Expired
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