CA2064540A1 - Method and apparatus for gas analysis - Google Patents
Method and apparatus for gas analysisInfo
- Publication number
- CA2064540A1 CA2064540A1 CA002064540A CA2064540A CA2064540A1 CA 2064540 A1 CA2064540 A1 CA 2064540A1 CA 002064540 A CA002064540 A CA 002064540A CA 2064540 A CA2064540 A CA 2064540A CA 2064540 A1 CA2064540 A1 CA 2064540A1
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- Prior art keywords
- pressure
- gas
- measurement
- substance
- taken
- Prior art date
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Links
- 238000000034 method Methods 0.000 title claims abstract description 32
- 238000004868 gas analysis Methods 0.000 title claims abstract description 9
- 239000007789 gas Substances 0.000 claims abstract description 74
- 238000005259 measurement Methods 0.000 claims abstract description 45
- 239000000126 substance Substances 0.000 claims abstract description 35
- 230000008033 biological extinction Effects 0.000 claims abstract description 10
- 238000005086 pumping Methods 0.000 claims description 5
- 238000010438 heat treatment Methods 0.000 claims description 4
- 239000000463 material Substances 0.000 claims description 3
- 230000000737 periodic effect Effects 0.000 claims description 2
- 230000005855 radiation Effects 0.000 claims description 2
- 238000007796 conventional method Methods 0.000 abstract 1
- 239000000203 mixture Substances 0.000 description 4
- 238000010521 absorption reaction Methods 0.000 description 3
- 239000012159 carrier gas Substances 0.000 description 3
- 230000001419 dependent effect Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 2
- 238000004566 IR spectroscopy Methods 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000009472 formulation Methods 0.000 description 1
- 230000007774 longterm Effects 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 239000002674 ointment Substances 0.000 description 1
- 230000002035 prolonged effect Effects 0.000 description 1
- 238000001179 sorption measurement Methods 0.000 description 1
- 230000003595 spectral effect Effects 0.000 description 1
- 238000001228 spectrum Methods 0.000 description 1
- 230000002123 temporal effect Effects 0.000 description 1
- 238000011144 upstream manufacturing Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
- G01N21/3518—Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
Landscapes
- Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Chemical & Material Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
- Sampling And Sample Adjustment (AREA)
- Investigating Or Analysing Materials By The Use Of Chemical Reactions (AREA)
Abstract
Abstract A Method and an Apparatus for Gas Analysis The present invention relates to a method and an apparatus for determining concentrations of substances in gases. The method operates by electromagnetic irradiation and by measuring the intensity of the rays transmitted, at a wavelength which is characteristic of the substance to be detected, wherein the gas containing the substance is measured at at least two different gas pressure values. In order to overcome the drawbacks of the conventional methods which are carried out with reference gases which have to be produced and supplied expressly, but in order nonetheless to be able to quickly and easily determine the concentration of a substance in a gas with a relatively high degree of accuracy, it is proposed according to the invention that an initial measurement be taken at an initial, basically constant pressure of the gas to be analysed, and for the gas pressure to then be changed, and for a second measurement to be taken at a second constant gas pressure with an otherwise identical measuring arrangement, and from the ratio of, or difference between, the intensities measured when the two measurements are taken, the concentration of the substanceto be detected is ascertained in the gas by computer or using a graph, on the basis of a theoretical connection between the extinction and concentration by volume of that substance.
Description
A Method and an Apparatus for Gas Analysis The present invention relates to a method and an apparatus for gas analysis.
In particular, th~ present invention relates to a method for determining concentrations of substances in gases by electromagnetic irradiation and by measuring the radiation intensity trar.smitted with a wavelength which is characteristic of the substance to be detected, wherein the gas containing the substance is measured is measured at at least two clifferent gas pressure values.
A method of this kind is disclosed in DE-PS 31 16 3~4.
An appropriate apparatus for non-dispersive photometric gas analysis has an electromagnetic emitter and an appropriate detector, between which the gas to be analysed is disposed in a measuring chamber, wherein the measuring chamber has an intake and an outlet for the gas to be analysed.
Practically every conceivable gaseous substance absorbs electromagnetic waves at specific wavelengths which, as a whole, are characteristic of the substance.
In many cases, it is sufficient for practical purposes to take measurements at one single fixed wavelength in order to be able to establish, on the basis of the absorption (extinction) at that wavelength, the size of concentration of a specific substance. The characteristic wavelengths are, for most substances which consist of multi-atomic molecules, disposed in the infra-red spectral region, wherein, however, basically the whole of' the electromagnetic spectrum of microwaves up to the Roentgen rays is available for appropriate measuring methods. However, owing to the great practical significance of infra-red spectroscopy, the following description will deal exclusively with the example of infra-red gas analysis, but this does not mean that the invention is limited thereto.
W:ith the known method in accordance with the German Patent Specification mentioned Ln the introductlon, calibration is performed using known concentrations and partial pressures of the substances whose concentration is to be determined during the analysis. ln addition, the pressure dependency of the extinction is measured. Then, the gas to be analysed is measured at any pressure, and, by assigning the measur:ing point to a system of curves of pressure-dependent extinction values, the partial pressure and thus also
In particular, th~ present invention relates to a method for determining concentrations of substances in gases by electromagnetic irradiation and by measuring the radiation intensity trar.smitted with a wavelength which is characteristic of the substance to be detected, wherein the gas containing the substance is measured is measured at at least two clifferent gas pressure values.
A method of this kind is disclosed in DE-PS 31 16 3~4.
An appropriate apparatus for non-dispersive photometric gas analysis has an electromagnetic emitter and an appropriate detector, between which the gas to be analysed is disposed in a measuring chamber, wherein the measuring chamber has an intake and an outlet for the gas to be analysed.
Practically every conceivable gaseous substance absorbs electromagnetic waves at specific wavelengths which, as a whole, are characteristic of the substance.
In many cases, it is sufficient for practical purposes to take measurements at one single fixed wavelength in order to be able to establish, on the basis of the absorption (extinction) at that wavelength, the size of concentration of a specific substance. The characteristic wavelengths are, for most substances which consist of multi-atomic molecules, disposed in the infra-red spectral region, wherein, however, basically the whole of' the electromagnetic spectrum of microwaves up to the Roentgen rays is available for appropriate measuring methods. However, owing to the great practical significance of infra-red spectroscopy, the following description will deal exclusively with the example of infra-red gas analysis, but this does not mean that the invention is limited thereto.
W:ith the known method in accordance with the German Patent Specification mentioned Ln the introductlon, calibration is performed using known concentrations and partial pressures of the substances whose concentration is to be determined during the analysis. ln addition, the pressure dependency of the extinction is measured. Then, the gas to be analysed is measured at any pressure, and, by assigning the measur:ing point to a system of curves of pressure-dependent extinction values, the partial pressure and thus also
2 ~ 5 ~ ~
indirectly the concentration of the substance in the gas to be analysed is established.
This method admittedly operates with great accuracy because by detecting the pressure dependency the so-called pressure widening effect is also taken into consideration and can be measured accordingly with narrow bands. However, this method is extremely tedious because a plurality of calibration measurements is needed which - at least when there are long intervals between the measurements -have to be repeated with each new measurement, in order to compensate for errorscaused by changes occurring with the passage of time (dirt, temperature fluctuations etc.).
With other known methods, a reference measurement is taken with a pure gas or the carrier gas which is possibly not absorbed in the wavelength range concerned, for which purpose either the measuring chamber is thoroughly rinsed out and/or completely changed over. Therein, the measuring chamber can also be changed by deflecting the measuring ray with mirrors, or the like.
These known methods are also relatively expensive and harbour further sources of error since the rinsing operation and that of completely changing the gas areno-t one hundred per cent possible, and when the measuring chamber is changed over there is no guarantee that measuring conditions will be completely identical with the actual measurement of concentration. Another problem with using reference gases is also the fact that appropriate amounts of reference gas must be supplied, wherein the gas must also be kept in closed containers because otherwise it is not possible to know the exact composition of the reference gas.In addition, the reference gas must be produced and/or ordered expressly. The space requirements also mean that these reference gases are often under pressureand this is problematical because quite heavy fractions of the gases can condense out and thus alter the composition of the remainder of -the gaseous components.
In the face of this prior art, the aim of the present invention is to create a nletilod and an apparatus of the kind mentioned in the introduction which make it possible for the concentration of a substance in a gas to be determined very qulckly and easily with a re:Latively high degree of accuracy.
With respect to the method, this problem is solved in that a first measurement is taken at an in:itial, substantially constant pressure of` the gas to be analysed, the pressure of the gas is then altered, and a second measurement is taken at a second constant gas pressure with an otherwise identical measuring arrangement7 and from the ratio of, or difference between, the intensities when the two measurements are taken, the concentration of the substance in question in the gas is ascertained by computer or by using a graph, on the basis of a theoretical connection between extinction and concentration by volume.
Compared with the known method, the basic difference between it and the method according to the invention is that the reference measurement does not measure a pure gas or a pure carrier gas or a gas with a known concentration of the substance whlch is to be measured. Instead, the gas to be analysed is itself used for the reference measurement, but at a dif'ferent known pressure, and a specific theoretical connection is assumed to exist between the extinction i.e.
between the natural logarithm of the ratio of the measured intensity to the irradiated electromagnetic intensity with the measured wavelength, and between the concentration by volume of the absorbing substance in the chamber which is penetrated by rays.
With a preferred embodiment of the method according to the invention, the pressure therein when the first measurement is taken is greater than the pressure when the second measurement is taken, and during the time between the two measurements the pressure is reduced from the first value to the second pressureby pumping off or siphoning off the gas to be analysed. This method is advantageous because by pumping or (with excess pressure) siphoning off the gas present in the measuring chamber, there cannot be any change in the concentration of the substance which is to be detected because no additional gas reaches the measuring chamber which may differ in its composition.
For practical reasons, the pressure when the first measurement is taken should either be atmospheric pressure, or, if the gas to be analysed is under greater pressure, or if an appropriate pressure pump is available, the pressure should be greater than atmospheric pressure, whilst the pressure is reduced to the second pressure vaLue either by siphoning of'r the excess pressure to ambient pressure or by pwDping it off to attain a value which is less tharl atmospheric pressure.
The pressure dif`ference between the two measurelDerlts taken should be as greatas possible, and, if possible, the pressure when the second measurement is taken 4 2 ~
should be reduced to such an extent that it is at most half the pressure existing when the first measurement was taken.
The selection of pressure i9 also clearly dependent on the concentration of the substance to be measured, on its specific absorption and on the length of the measured distance. In other words, if the concentration of` the substance is very low and/or if the measured distance is very short and/or if the adsorption o the substance in question is relatively small, then the first measurement shouldbe taken with as great an excess pressure as possible, so that extinction is able to be measured at all, o~ so that it can be measured with sufficient accuracy.
If, on the other hand, the concentration of the substance which is to be measured is high and/or if the measured distance is long and/or if the specific absorption of the substance in question is very high, then the measurement can be taken at considerably lower pressure. If so desired, or in order to operate in a simplerway due to the prevailing conditions, both the first and second measurements canbe taken at a pressure which is less than atmospheric pressure.
With regard to ascertaining the concentration, in the simplest of cases it is possible to use the so-called Lambert-Beer's Law, on which a simple exponential decrease of the measured intensity of the concentration is based.
However, for practical purposes, empirical operations have proved more expedient.
These reproduce the actual trend of the transmitted intensity in dependency on the concentration more accurately than the Lambert-Beer's Law. With the preferred embodiment of the invention, with regard to the connection between theadmitted intensity and concentration, an equation of the formula I/Io = a exp (b x) + (1 - a) exp (c x) i5 taken, wherein a, b and c are constants dependent on the substance and x is the concentrat:ion by volume of' the substance.
By measuring two dif`ferent pressure va:L~Ies, it is possible to find out the concentration x of the substance using the above equation, because owing to the two known pressure values, the ratio, at least, of` the concentration by volume is also known, whilst the general equation of state is used as a basis for idealgases (possibly also for real gases). When the carrier gas is air, and at room temperature and atmospheric pressure it can as a good approximation be assumed that the concentration by volume is proportional to the pressure (ideal gas).
According to a further embodiment of the invention, the measurements are taken at different pressures in a periodic cycle, so that statistical information can also be given.
In addition, use of the light intensity variation principle is expedient, at least in cases where the measurement is taken in the infra-red range and at roomtemperature, because at room temperature a considerable infra~red background hasalready been found which is eliminated by measuring variations oY light intensity.
It is also expedient, if, in addition to the pressure, the temperature is also detected in the measuring chamber, and a pressure correction may be made by computerised means if the temperatures between the two measurements taken at different pressures clearly differ from each o-ther. Simply by creating an excess pressure, or by pumping out anci obtaining an underpressure, it is possible for a considerable change in temperature to take place in the gas in the measuring chamber, and this temperature change must be corrected by computer on the basis of the above-mentioned gas law. Otherwise, the clear connection between the pressure and concentration is only applicable when the temperature is constant.
However, as an al-ternative, it is also possible to wait to carry out the measurement until temperature equilibrium is obtained. This happens quite quickly in measuring chamberc of smaller volume. A temperature sensor is also expedient in these cases since it indicates temperature equilibrium. In order to reach temperature equilibrium more quickly, the gas can also be preheated to a constant value when it enters the trough, and the trough may itself also be kept at a constant temperature.
With respect to the apparatus mentioned in the introduct:ion, the problem forming the bas:is of the invention is solvecl in that the outlet- or intake side of themeasurirlg chamber ls conllected to a pump, the corresponcling other side i.e.
e:Lther the intake slcle or the outlet side, or both, being able to be closed bya valve, the emltter having a stab:ilised power supply, ancl the measuring chamber, emitter and detector being arranged in such a way that their positions relative to each other and also the path of the rays are spatially constant during one complete measuring cycle.
In the negative formulation, it could also be said that with the apparatus according to the invention there are no means for deflecting the infra-red ray, no means for changing over the measuring chamber and also no means for changing the gas to be analysed for a reference gas or a calibration gas. However, thereare means permitting the gas to be analysed to be kept at at least two differentconstant pressures in the measuring chamber.
It is also expedient if a control device is provicled which has a time measuringmeans, this device controlling the individual component parts of the system in the desired temporal sequence, so that first of all the gas to be analysed is introduced into the measuring chamber, so that an initial measurement is then taken an a constant pressure, the measuring operation then ending and the pressure being changed, with the temperature and pressure possibly being checked, a second measurement then being taken at a second pressure which is constant, whereupon the entire measuring cycle can be repeated. For this purpose, the control device controls the pumps and valves which are present and also the detecting means for the measuring system or the detector, wherein the control operations take place either in dependency on time alone, but possibly also in dependency on the pressure obtained and/or on the temperature as an additional criterium, whereby the period of time, in particular, which is allowed for reducing the pressure, may be extended until the desired pressure and/or the desired temperature have been reached.
One embodiment of the invention is preferred wherein heating means are provided at the intake to the trough and/or on the trough itself. The outlet can, of course, also be provided with corresponding heating means. These kinds of heating means and appropriate thermostatic controls can be used to keep the trough at an ever constant temperature, so that the gas inside the trough also very quickly adopts that temperature independently of the prevailing pressure.
This is so particularly if the gas intake is heated and has a relatively narrow through-flow cross-section. It is expedient therein if the intake and also the side wall of the trough are made of a material, usually metal, which has good heat conducting properties. It is to be understood that the faces of the troughare provided with windows which a:Llow rays to pass through them. In order to reach telupernture equilibrium fast, it is also expedient iE the transverse measurements of the trough are kept correspondingly small, so that thecorresponding small volume of gas is quickly tempered.
2 ~
F`urther advantages, features and possible applications of the present inventionwill emerge from the following description which relates to the drawings showingpreferred embodiments of the invention, and wherein:
Figure 1 is a drawing of the apparatus according to the invention, Figure 2 i9 the illust.ration of the course of a measuring cycle, and Figure 3 is the relative trend of` tlle intensity of the rays in dependency on .the concentration of the component parts being absorbed in the gas being investigated, and in dependency on the pressure.
Figure 1 shows the emitter 1 which emits rays across the measuring chamber 3 which is itself trough-shaped. The rays are scanned by the detector 2. A
chopper 12 and a filter 13 are also shown, wherein the filter 13 only allows rays which are modulated by the chopper frequency to pass therethrough. As a result,the entire infra-red background is eliminated, and the detector Z only receives the light which has been emitted by the infra-red emitter 1 and which has been modulated by the chopper. The measuring chamber, or trough 3, has an intake 4 which can be closed by a magnetic valve 7, and it has an outlet 5 which communicates with a vacuum pump 6. Disposed in the space enclosed between the valve 7 and the pump 6 is a pressure sensor 10, to be more precise a drain connection 5, and also a temperature sensor 11 which is in direct contact with the gas in the trough 3, but which is protected by means, not shown, from directinfra-red rays from the emitter 1. The temperature sensor 11 can selectively also be arranged in the intake connection 4 or in the drain connection 5.
The detector 2 is connected to an electronic measuring instrument 9 which records the intensity values of the infra-red rays detected by the detector 2. The entire system is controlled by a control device 8 which also consists of a time measuring means.
The control means of the apparatus shown in Figure 1 are best clescribed with the aid of the flow d:Lagr~m in ~igure 2. In that diagram, the horizontal axis represents the tlme t and the vertical a~cis represents the pressure p in the trough 3. ~asically, ~our phases I to IV can be distinguished. At the start of the measuring cycle, the valve 7 is opened to comm~micate the intalce connection 4 ot the trough 3 with a source of the gas which is to be invest:igated. If no measurement has been talcen with the same gas, the pump 6 can first of all be used to rinse out the trough 3 at the start of the first measuring operation. After this possible rinsing operation, the pump 6 is switched off.
As soon as the pressure in the trough 3 has reached a desired value, or as soon as pressure equilibrium with the gas source has been reached, the valve 7 is closed (if the source pressure is constant, it can first of all remain open).
In Figure 2, Phase I characterises the operations whereby the trough is filled and whereby the constant pressure value pl is reached. The measuring phase thenbegins. The control device 8 causes the electronic measuring instrument 9 to record the values which are detected by the detector 2. Therein, the control device, or electronic measuring instrument. also simultaneously records the pressure detected by the sensor 10 and possibly also the temperature detected by the sensor 11.
This phase is characterised as phase II in Figure 2.
If the valve 7 was not previously closed, the magnetic valve 7 is closed by the control device 8 at the end of phase II, at the latest. Then, in phase III, thepump 6 is activated, and the gas is pumped out of the trough 3 until a pressure p2 is reached. In the flow diagram shown in Figure 2, the pressure value p2 is approximately half the pressure pl, but it can also be much less. When pressurep2 is reached at the end of phase III, the pump is switched off. If the pump is designed in such a way that backflow of the gas is not excluded, a valve should be arranged in the connection 5 upstream of the pump 6, this valve possibly being able to be closed by the control device 8 at the end of phase III.
If the pumping is carried out slowly, no significant temperature change should be recorded. If, however, the temperature in the trough 3 has changed, then phase III can optionally be prolonged with the pump switched off until temperature equilibrium is reached. Alternatively, however, the temperature recorded by the sensor 11 can also be detected by the control device, and allowance be made for it by computer by correction of the pressure measured by the sensor 10.
The gas can also be tempered right at the intAke, or even by the wall of the trough, iY this is thermostatically preheated to a fixed value. Temperature equilibrium is reached above all relatively quickly if the wall of the trough and/or of the intake or outlet is made of a material which has good heat conducting properties, and i~ the arrangement as a whole is relatively small in si~e. with the trough thus having smaller transverse dimens-ons.
As soon as phase III has ended, and as soon as a constant pressure p2 has been obtained, the control device 8 then causes the electronic measuring instrument 9 to record the intensity values of the infra-red rays detected by the detector.This phase is characterised by IV in Figure 2. On condition that the two measurements are taken at the same temperature and with the same given pressure and temperature ratios, the equation of state for ideal gases can be used, and the intensity pattern in dependency on the concentration and pressure can be reproduced with very close approximation, by the following e~uation:
= f(p c) = f( p ~3 = al exp(-a2 pO ~3 ~ al) exp(-a3 Pp c) wherein c is the (relative) concentration of the substance at pressure p ~, p is the current pressure, and al, a2 and a3 are constants.
The trend of this curve is illustrated in Figure 3, wherein the concentration is obviously to be taken as being constant for a given gas which does not change.
Corresponding curves for various concentrations differ only by a ~ixed factor which straightens or distends the curve in a hori~ontal extent.
Since the constants of the above ~unction can be ascertained by one single calibration measurement, the curve, and thus also the sought concentration of the components of the gas which are to be detected, is obviously established by measuring two points at different pressures. In practice, the concentration is found out by the above equation using numerical approximation methods.
By way of the pr-esent invention, a particularly simple but relatively accurate photometric gas analysis method has been created, wherein the corresponding apparatus excels by being particularly simple in its construction, because the infra-red measured ray does not have to be deflected, and neither does the measuring trough have to be changed, nor does the gas have to be changed with standard gas or calibration gas.
I'herefore, a series of error sources is also eliminated at the same time -errors which may be caused, in particular, by long-term changes to the measuringbackground -, such as windows which gradually become dirty with troughs which are changed over, gradually changing temperatures and changes with the passage of time with the emitters and detectors. These error sources cannot be o~ any importance with the measuring operations according to the invention which are carried out quickly in succession.
indirectly the concentration of the substance in the gas to be analysed is established.
This method admittedly operates with great accuracy because by detecting the pressure dependency the so-called pressure widening effect is also taken into consideration and can be measured accordingly with narrow bands. However, this method is extremely tedious because a plurality of calibration measurements is needed which - at least when there are long intervals between the measurements -have to be repeated with each new measurement, in order to compensate for errorscaused by changes occurring with the passage of time (dirt, temperature fluctuations etc.).
With other known methods, a reference measurement is taken with a pure gas or the carrier gas which is possibly not absorbed in the wavelength range concerned, for which purpose either the measuring chamber is thoroughly rinsed out and/or completely changed over. Therein, the measuring chamber can also be changed by deflecting the measuring ray with mirrors, or the like.
These known methods are also relatively expensive and harbour further sources of error since the rinsing operation and that of completely changing the gas areno-t one hundred per cent possible, and when the measuring chamber is changed over there is no guarantee that measuring conditions will be completely identical with the actual measurement of concentration. Another problem with using reference gases is also the fact that appropriate amounts of reference gas must be supplied, wherein the gas must also be kept in closed containers because otherwise it is not possible to know the exact composition of the reference gas.In addition, the reference gas must be produced and/or ordered expressly. The space requirements also mean that these reference gases are often under pressureand this is problematical because quite heavy fractions of the gases can condense out and thus alter the composition of the remainder of -the gaseous components.
In the face of this prior art, the aim of the present invention is to create a nletilod and an apparatus of the kind mentioned in the introduction which make it possible for the concentration of a substance in a gas to be determined very qulckly and easily with a re:Latively high degree of accuracy.
With respect to the method, this problem is solved in that a first measurement is taken at an in:itial, substantially constant pressure of` the gas to be analysed, the pressure of the gas is then altered, and a second measurement is taken at a second constant gas pressure with an otherwise identical measuring arrangement7 and from the ratio of, or difference between, the intensities when the two measurements are taken, the concentration of the substance in question in the gas is ascertained by computer or by using a graph, on the basis of a theoretical connection between extinction and concentration by volume.
Compared with the known method, the basic difference between it and the method according to the invention is that the reference measurement does not measure a pure gas or a pure carrier gas or a gas with a known concentration of the substance whlch is to be measured. Instead, the gas to be analysed is itself used for the reference measurement, but at a dif'ferent known pressure, and a specific theoretical connection is assumed to exist between the extinction i.e.
between the natural logarithm of the ratio of the measured intensity to the irradiated electromagnetic intensity with the measured wavelength, and between the concentration by volume of the absorbing substance in the chamber which is penetrated by rays.
With a preferred embodiment of the method according to the invention, the pressure therein when the first measurement is taken is greater than the pressure when the second measurement is taken, and during the time between the two measurements the pressure is reduced from the first value to the second pressureby pumping off or siphoning off the gas to be analysed. This method is advantageous because by pumping or (with excess pressure) siphoning off the gas present in the measuring chamber, there cannot be any change in the concentration of the substance which is to be detected because no additional gas reaches the measuring chamber which may differ in its composition.
For practical reasons, the pressure when the first measurement is taken should either be atmospheric pressure, or, if the gas to be analysed is under greater pressure, or if an appropriate pressure pump is available, the pressure should be greater than atmospheric pressure, whilst the pressure is reduced to the second pressure vaLue either by siphoning of'r the excess pressure to ambient pressure or by pwDping it off to attain a value which is less tharl atmospheric pressure.
The pressure dif`ference between the two measurelDerlts taken should be as greatas possible, and, if possible, the pressure when the second measurement is taken 4 2 ~
should be reduced to such an extent that it is at most half the pressure existing when the first measurement was taken.
The selection of pressure i9 also clearly dependent on the concentration of the substance to be measured, on its specific absorption and on the length of the measured distance. In other words, if the concentration of` the substance is very low and/or if the measured distance is very short and/or if the adsorption o the substance in question is relatively small, then the first measurement shouldbe taken with as great an excess pressure as possible, so that extinction is able to be measured at all, o~ so that it can be measured with sufficient accuracy.
If, on the other hand, the concentration of the substance which is to be measured is high and/or if the measured distance is long and/or if the specific absorption of the substance in question is very high, then the measurement can be taken at considerably lower pressure. If so desired, or in order to operate in a simplerway due to the prevailing conditions, both the first and second measurements canbe taken at a pressure which is less than atmospheric pressure.
With regard to ascertaining the concentration, in the simplest of cases it is possible to use the so-called Lambert-Beer's Law, on which a simple exponential decrease of the measured intensity of the concentration is based.
However, for practical purposes, empirical operations have proved more expedient.
These reproduce the actual trend of the transmitted intensity in dependency on the concentration more accurately than the Lambert-Beer's Law. With the preferred embodiment of the invention, with regard to the connection between theadmitted intensity and concentration, an equation of the formula I/Io = a exp (b x) + (1 - a) exp (c x) i5 taken, wherein a, b and c are constants dependent on the substance and x is the concentrat:ion by volume of' the substance.
By measuring two dif`ferent pressure va:L~Ies, it is possible to find out the concentration x of the substance using the above equation, because owing to the two known pressure values, the ratio, at least, of` the concentration by volume is also known, whilst the general equation of state is used as a basis for idealgases (possibly also for real gases). When the carrier gas is air, and at room temperature and atmospheric pressure it can as a good approximation be assumed that the concentration by volume is proportional to the pressure (ideal gas).
According to a further embodiment of the invention, the measurements are taken at different pressures in a periodic cycle, so that statistical information can also be given.
In addition, use of the light intensity variation principle is expedient, at least in cases where the measurement is taken in the infra-red range and at roomtemperature, because at room temperature a considerable infra~red background hasalready been found which is eliminated by measuring variations oY light intensity.
It is also expedient, if, in addition to the pressure, the temperature is also detected in the measuring chamber, and a pressure correction may be made by computerised means if the temperatures between the two measurements taken at different pressures clearly differ from each o-ther. Simply by creating an excess pressure, or by pumping out anci obtaining an underpressure, it is possible for a considerable change in temperature to take place in the gas in the measuring chamber, and this temperature change must be corrected by computer on the basis of the above-mentioned gas law. Otherwise, the clear connection between the pressure and concentration is only applicable when the temperature is constant.
However, as an al-ternative, it is also possible to wait to carry out the measurement until temperature equilibrium is obtained. This happens quite quickly in measuring chamberc of smaller volume. A temperature sensor is also expedient in these cases since it indicates temperature equilibrium. In order to reach temperature equilibrium more quickly, the gas can also be preheated to a constant value when it enters the trough, and the trough may itself also be kept at a constant temperature.
With respect to the apparatus mentioned in the introduct:ion, the problem forming the bas:is of the invention is solvecl in that the outlet- or intake side of themeasurirlg chamber ls conllected to a pump, the corresponcling other side i.e.
e:Lther the intake slcle or the outlet side, or both, being able to be closed bya valve, the emltter having a stab:ilised power supply, ancl the measuring chamber, emitter and detector being arranged in such a way that their positions relative to each other and also the path of the rays are spatially constant during one complete measuring cycle.
In the negative formulation, it could also be said that with the apparatus according to the invention there are no means for deflecting the infra-red ray, no means for changing over the measuring chamber and also no means for changing the gas to be analysed for a reference gas or a calibration gas. However, thereare means permitting the gas to be analysed to be kept at at least two differentconstant pressures in the measuring chamber.
It is also expedient if a control device is provicled which has a time measuringmeans, this device controlling the individual component parts of the system in the desired temporal sequence, so that first of all the gas to be analysed is introduced into the measuring chamber, so that an initial measurement is then taken an a constant pressure, the measuring operation then ending and the pressure being changed, with the temperature and pressure possibly being checked, a second measurement then being taken at a second pressure which is constant, whereupon the entire measuring cycle can be repeated. For this purpose, the control device controls the pumps and valves which are present and also the detecting means for the measuring system or the detector, wherein the control operations take place either in dependency on time alone, but possibly also in dependency on the pressure obtained and/or on the temperature as an additional criterium, whereby the period of time, in particular, which is allowed for reducing the pressure, may be extended until the desired pressure and/or the desired temperature have been reached.
One embodiment of the invention is preferred wherein heating means are provided at the intake to the trough and/or on the trough itself. The outlet can, of course, also be provided with corresponding heating means. These kinds of heating means and appropriate thermostatic controls can be used to keep the trough at an ever constant temperature, so that the gas inside the trough also very quickly adopts that temperature independently of the prevailing pressure.
This is so particularly if the gas intake is heated and has a relatively narrow through-flow cross-section. It is expedient therein if the intake and also the side wall of the trough are made of a material, usually metal, which has good heat conducting properties. It is to be understood that the faces of the troughare provided with windows which a:Llow rays to pass through them. In order to reach telupernture equilibrium fast, it is also expedient iE the transverse measurements of the trough are kept correspondingly small, so that thecorresponding small volume of gas is quickly tempered.
2 ~
F`urther advantages, features and possible applications of the present inventionwill emerge from the following description which relates to the drawings showingpreferred embodiments of the invention, and wherein:
Figure 1 is a drawing of the apparatus according to the invention, Figure 2 i9 the illust.ration of the course of a measuring cycle, and Figure 3 is the relative trend of` tlle intensity of the rays in dependency on .the concentration of the component parts being absorbed in the gas being investigated, and in dependency on the pressure.
Figure 1 shows the emitter 1 which emits rays across the measuring chamber 3 which is itself trough-shaped. The rays are scanned by the detector 2. A
chopper 12 and a filter 13 are also shown, wherein the filter 13 only allows rays which are modulated by the chopper frequency to pass therethrough. As a result,the entire infra-red background is eliminated, and the detector Z only receives the light which has been emitted by the infra-red emitter 1 and which has been modulated by the chopper. The measuring chamber, or trough 3, has an intake 4 which can be closed by a magnetic valve 7, and it has an outlet 5 which communicates with a vacuum pump 6. Disposed in the space enclosed between the valve 7 and the pump 6 is a pressure sensor 10, to be more precise a drain connection 5, and also a temperature sensor 11 which is in direct contact with the gas in the trough 3, but which is protected by means, not shown, from directinfra-red rays from the emitter 1. The temperature sensor 11 can selectively also be arranged in the intake connection 4 or in the drain connection 5.
The detector 2 is connected to an electronic measuring instrument 9 which records the intensity values of the infra-red rays detected by the detector 2. The entire system is controlled by a control device 8 which also consists of a time measuring means.
The control means of the apparatus shown in Figure 1 are best clescribed with the aid of the flow d:Lagr~m in ~igure 2. In that diagram, the horizontal axis represents the tlme t and the vertical a~cis represents the pressure p in the trough 3. ~asically, ~our phases I to IV can be distinguished. At the start of the measuring cycle, the valve 7 is opened to comm~micate the intalce connection 4 ot the trough 3 with a source of the gas which is to be invest:igated. If no measurement has been talcen with the same gas, the pump 6 can first of all be used to rinse out the trough 3 at the start of the first measuring operation. After this possible rinsing operation, the pump 6 is switched off.
As soon as the pressure in the trough 3 has reached a desired value, or as soon as pressure equilibrium with the gas source has been reached, the valve 7 is closed (if the source pressure is constant, it can first of all remain open).
In Figure 2, Phase I characterises the operations whereby the trough is filled and whereby the constant pressure value pl is reached. The measuring phase thenbegins. The control device 8 causes the electronic measuring instrument 9 to record the values which are detected by the detector 2. Therein, the control device, or electronic measuring instrument. also simultaneously records the pressure detected by the sensor 10 and possibly also the temperature detected by the sensor 11.
This phase is characterised as phase II in Figure 2.
If the valve 7 was not previously closed, the magnetic valve 7 is closed by the control device 8 at the end of phase II, at the latest. Then, in phase III, thepump 6 is activated, and the gas is pumped out of the trough 3 until a pressure p2 is reached. In the flow diagram shown in Figure 2, the pressure value p2 is approximately half the pressure pl, but it can also be much less. When pressurep2 is reached at the end of phase III, the pump is switched off. If the pump is designed in such a way that backflow of the gas is not excluded, a valve should be arranged in the connection 5 upstream of the pump 6, this valve possibly being able to be closed by the control device 8 at the end of phase III.
If the pumping is carried out slowly, no significant temperature change should be recorded. If, however, the temperature in the trough 3 has changed, then phase III can optionally be prolonged with the pump switched off until temperature equilibrium is reached. Alternatively, however, the temperature recorded by the sensor 11 can also be detected by the control device, and allowance be made for it by computer by correction of the pressure measured by the sensor 10.
The gas can also be tempered right at the intAke, or even by the wall of the trough, iY this is thermostatically preheated to a fixed value. Temperature equilibrium is reached above all relatively quickly if the wall of the trough and/or of the intake or outlet is made of a material which has good heat conducting properties, and i~ the arrangement as a whole is relatively small in si~e. with the trough thus having smaller transverse dimens-ons.
As soon as phase III has ended, and as soon as a constant pressure p2 has been obtained, the control device 8 then causes the electronic measuring instrument 9 to record the intensity values of the infra-red rays detected by the detector.This phase is characterised by IV in Figure 2. On condition that the two measurements are taken at the same temperature and with the same given pressure and temperature ratios, the equation of state for ideal gases can be used, and the intensity pattern in dependency on the concentration and pressure can be reproduced with very close approximation, by the following e~uation:
= f(p c) = f( p ~3 = al exp(-a2 pO ~3 ~ al) exp(-a3 Pp c) wherein c is the (relative) concentration of the substance at pressure p ~, p is the current pressure, and al, a2 and a3 are constants.
The trend of this curve is illustrated in Figure 3, wherein the concentration is obviously to be taken as being constant for a given gas which does not change.
Corresponding curves for various concentrations differ only by a ~ixed factor which straightens or distends the curve in a hori~ontal extent.
Since the constants of the above ~unction can be ascertained by one single calibration measurement, the curve, and thus also the sought concentration of the components of the gas which are to be detected, is obviously established by measuring two points at different pressures. In practice, the concentration is found out by the above equation using numerical approximation methods.
By way of the pr-esent invention, a particularly simple but relatively accurate photometric gas analysis method has been created, wherein the corresponding apparatus excels by being particularly simple in its construction, because the infra-red measured ray does not have to be deflected, and neither does the measuring trough have to be changed, nor does the gas have to be changed with standard gas or calibration gas.
I'herefore, a series of error sources is also eliminated at the same time -errors which may be caused, in particular, by long-term changes to the measuringbackground -, such as windows which gradually become dirty with troughs which are changed over, gradually changing temperatures and changes with the passage of time with the emitters and detectors. These error sources cannot be o~ any importance with the measuring operations according to the invention which are carried out quickly in succession.
Claims (14)
1. A method for determining concentrations of substances in gases by electromagnetic irradiation and by measuring the radiation intensity transmittedwith a wavelength which is characteristic of the substance to be detected, wherein the gas containing the substance is measured at at least two different gas pressure values, characterised in that a first measurement is taken at a substantially constant initial pressure of the gas, the gas pressure is then changed and a second measurement is taken at a second substantially constant gaspressure and with the identical measuring arrangement, and from the ratio of, or difference between, the measured intensities, the concentration of the substance to be detected is ascertained by computer or by using a graph in the gas, on the basis of a theoretical connection between the extinction and concentration by volume of` that substance.
2. A method according to Claim 1, characterised in that the gas pressure is lower when the second measurement is taken than when the first measurement is taken, and that during the time between the two measurements the gas pressureis reduced to the second pressure by pumping off or siphoning off the gas to be measured from the measuring chamber.
3. A method according to Claim 2, characterised in that the pressure at the time of the first measurement is greater than, or equal to, the external atmospheric pressure, and that the pressure at the time of the second measurement is less than, or equal to, the external atmospheric pressure.
4. A method according to Claim 2 or Claim 3, characterised in that the pressure when the second measurement is taken is less than half the pressure when the first measurement is taken.
5. A method according to one of Claims 1 to 4, characterised in that the theoretical connection between extinction and concentration by volume of the substance is in accordance with the Lambert-Beer Law.
6. A method according to one of Claims 1 to 4, characterised in that the theoretical connection between extinction and concentration by volume of the substance corresponds to an empirically calculated function.
7. A method according to one of Claims 1 to 6, characterised in that the measurements are taken in a periodic cycle.
8. A method according to one of Claims 1 to 7, characterised in that the principle of the measurement of light intensity variations is applied.
9. A method according to one of Claims 1 to 8, characterised in that in addition to the pressure the temperature is also detected in the measuring chamber and a correction to the pressure may possibly be made by computerised means if the temperature difference between the first and second measurements exceeds a predetermined threshold value.
10. An apparatus for non-dispersive photometric gas analysis, the apparatus having an electromagnetic emitter (1) and a corresponding detector (2), between which the gas to be analysed is disposed in a measuring chamber (3), wherein the measuring chamber (3) has an intake (4) and an outlet (5), characterised in that the outlet- or intake side of the measuring chamber (3) is connected to a pump (6), the corresponding other side (intake or outlet) can be closed by a valve (7), a pressure sensor (10) is provided to detect the pressure in the measuring chamber, the emitter (1) has a stabilised power supply, the measuring chamber (3), emitter (1) and detector (2) are arranged spatially relative to each other in such a way that their positions relative to each otherand the path of the rays are constant during one complete measuring cycle.
11. An apparatus according to Claim 12, characterised in that it has a control device (8) with a time measuring means (9), wherein the control device is connected to the valve, the pump and a detecting device for the detector measured values, in order for these to be controlled periodically in sequence in dependency on time during a measuring cycle.
12. An apparatus according to Claim 11, characterised in that the control device is connected to the pressure sensor (10), in order to start up a measuring process in dependency on the pressure reached.
13. An apparatus according to Claim 11 or Claim 12, characterised in that the control device (8) is connected to a temperature sensor (11) which is thermally in contact with the measuring chamber (3).
14. An apparatus according to one of Claims 10 to 13, characterised in that means are provided for heating the trough (3) and/or the intake (4), and that troughs (3) and preferably also the intake (4) are made of a material whichhas good heat conducting properties.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE4112356A DE4112356A1 (en) | 1991-04-16 | 1991-04-16 | METHOD AND DEVICE FOR GAS ANALYSIS |
| DEP4112356.5 | 1991-04-16 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2064540A1 true CA2064540A1 (en) | 1992-10-17 |
Family
ID=6429678
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002064540A Abandoned CA2064540A1 (en) | 1991-04-16 | 1992-03-31 | Method and apparatus for gas analysis |
Country Status (13)
| Country | Link |
|---|---|
| EP (1) | EP0509249B1 (en) |
| JP (1) | JPH05249040A (en) |
| KR (1) | KR920020204A (en) |
| AT (1) | ATE194425T1 (en) |
| CA (1) | CA2064540A1 (en) |
| CS (1) | CS113292A3 (en) |
| DE (2) | DE4112356A1 (en) |
| HU (1) | HUT66169A (en) |
| PL (1) | PL294253A1 (en) |
| RO (1) | RO109899B1 (en) |
| RU (1) | RU2095789C1 (en) |
| TW (1) | TW222687B (en) |
| YU (1) | YU38192A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6341521B1 (en) * | 1998-10-16 | 2002-01-29 | L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude | Process and device for measuring the amount of impurities in a gas sample to be analyzed |
Families Citing this family (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4856864A (en) * | 1985-07-11 | 1989-08-15 | Raychem Corporation | Optical fiber distribution network including non-destructive taps and method of using same |
| DE19649343A1 (en) * | 1996-11-28 | 1998-06-10 | Fraunhofer Ges Forschung | Photometric system for gas sample examination |
| DE60142345D1 (en) * | 2000-09-25 | 2010-07-22 | Otsuka Pharma Co Ltd | METHOD FOR THE ANALYSIS OF HUMAN BREATHING AIR BY MEANS OF AN ISOTOPENIC GAS ANALYZER |
| JP3744903B2 (en) | 2003-01-20 | 2006-02-15 | セイコーエプソン株式会社 | Infrared absorption measurement method, infrared absorption measurement device, and semiconductor device manufacturing method |
| JP3705270B2 (en) | 2003-01-20 | 2005-10-12 | セイコーエプソン株式会社 | Infrared absorption measurement method, infrared absorption measurement device, and semiconductor device manufacturing method |
| FI125907B (en) * | 2013-09-24 | 2016-03-31 | Vaisala Oyj | Method and apparatus for measuring the concentration of gases dissolved in liquids |
| EP3055670A1 (en) * | 2013-10-11 | 2016-08-17 | MKS Instruments, Inc. | Systems and methods for pressure differential molecular spectroscopy of compressible fluids |
| JP6581439B2 (en) * | 2015-08-26 | 2019-09-25 | 旭化成エレクトロニクス株式会社 | Gas sensor calibration apparatus, gas sensor calibration method, and gas sensor |
| CN114112956A (en) * | 2021-11-23 | 2022-03-01 | 青岛崂应海纳光电环保集团有限公司 | Gas detection method and device |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB849983A (en) * | 1956-08-18 | 1960-09-28 | Hartmann & Braun Ag | Method and apparatus for the analysis by radiation of mixtures of gases and/or vapours |
| US4163899A (en) * | 1977-11-30 | 1979-08-07 | Andros, Inc. | Method and apparatus for gas analysis |
| DE3116344A1 (en) * | 1981-04-24 | 1982-11-18 | Kernforschungszentrum Karlsruhe Gmbh, 7500 Karlsruhe | Method of increasing the measurement accuracy of a gas analyser |
| GB2110818B (en) * | 1981-11-14 | 1985-05-15 | Ferranti Ltd | Non-dispersive gas analyser |
-
1991
- 1991-04-16 DE DE4112356A patent/DE4112356A1/en not_active Ceased
-
1992
- 1992-03-15 AT AT92104444T patent/ATE194425T1/en not_active IP Right Cessation
- 1992-03-15 EP EP92104444A patent/EP0509249B1/en not_active Expired - Lifetime
- 1992-03-15 DE DE59209844T patent/DE59209844D1/en not_active Expired - Lifetime
- 1992-03-31 CA CA002064540A patent/CA2064540A1/en not_active Abandoned
- 1992-04-08 JP JP4087064A patent/JPH05249040A/en active Pending
- 1992-04-13 YU YU38192A patent/YU38192A/en unknown
- 1992-04-14 CS CS921132A patent/CS113292A3/en unknown
- 1992-04-14 HU HU9201264A patent/HUT66169A/en unknown
- 1992-04-14 RO RO92-200518A patent/RO109899B1/en unknown
- 1992-04-15 RU SU925011416A patent/RU2095789C1/en active
- 1992-04-15 KR KR1019920006282A patent/KR920020204A/en not_active Application Discontinuation
- 1992-04-16 PL PL29425392A patent/PL294253A1/en unknown
- 1992-10-13 TW TW081108176A patent/TW222687B/zh active
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6341521B1 (en) * | 1998-10-16 | 2002-01-29 | L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude | Process and device for measuring the amount of impurities in a gas sample to be analyzed |
Also Published As
| Publication number | Publication date |
|---|---|
| EP0509249A3 (en) | 1993-12-08 |
| CS113292A3 (en) | 1992-12-16 |
| HU9201264D0 (en) | 1992-07-28 |
| TW222687B (en) | 1994-04-21 |
| RO109899B1 (en) | 1995-06-30 |
| ATE194425T1 (en) | 2000-07-15 |
| PL294253A1 (en) | 1993-05-17 |
| HUT66169A (en) | 1994-09-28 |
| EP0509249A2 (en) | 1992-10-21 |
| RU2095789C1 (en) | 1997-11-10 |
| JPH05249040A (en) | 1993-09-28 |
| DE59209844D1 (en) | 2000-08-10 |
| KR920020204A (en) | 1992-11-20 |
| EP0509249B1 (en) | 2000-07-05 |
| DE4112356A1 (en) | 1992-10-22 |
| YU38192A (en) | 1994-06-10 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |