JPS6155049B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high temperature measurement method and apparatus for measuring the temperature of a graphite tube of a graphite tube atomizer used in flameless atomic absorption spectrometry over a wide operating temperature range. In this case, a radiation detector exposed to radiation from the graphite tube is used to generate a temperature-dependent signal, and a variable gain amplifier is connected to the output of the radiation detector, and the output signal of the variable gain amplifier is is related to the temperature value of the graphite tube.

It is known to measure the temperature of the graphite tube of a graphite tube atomizer (atomizer) with a pyrometer type (e.g. "Analyti-cal").
Chemistry, Vol. 46 (1974), No. 8, pp. 1028 to 1031). A photo diode is provided for this purpose and is exposed to radiation from the graphite tube atomizer to produce a temperature measurement. This temperature measurement is compared to a temperature command value to control the heating power of the graphite tube. At this time, since the graphite tube is heated with the full heating power until the temperature command value is reached, the temperature command value is quickly reached. Once the temperature command value is reached, this temperature is maintained by conventional control actions.

In addition, the temperature of the object to be measured is measured from the total radiation within the range of 8μ to 14μ, thereby measuring the temperature between 100℃ and 2700℃.
Pyrometers have been proposed that cover the measurement range of °C. A measurement range of 100°C to 2700°C is necessary to monitor and control the temperature of the graphite tube of a graphite tube atomizer for flameless atomic absorption spectrometry in various operating modes. However, when performing such measurements, the emissivity of the graphite tube affects the measurement accuracy. This emissivity is different for each individual graphite tube and varies widely over the useful life of the graphite tube depending on the analytical conditions. Therefore, when using a conventional graphite tube atomizer, the measured value of the graphite tube temperature is subject to error.

Temperature measurement methods that are not affected by the emissivity of the object to be measured are also known. For example, the temperature of an object to be measured can also be measured with a thermocouple. However, thermocouples can only be used within a limited temperature range of approximately 600°C or less. It is not possible to measure temperature with thermocouples over the wide operating temperature range of graphite tubes in graphite tube atomizers reaching 2700°C to 2800°C. Thermocouples are destroyed at high temperatures.

High temperature measurements unrelated to the emissivity of the object to be measured are performed using a colorimetric pyrometer. A colorimetric pyrometer measures the radiation intensity of an object under test in two different wavelength ranges and correlates the two with each other. The ratio of the radiation intensities in the two wavelength ranges provides a temperature measurement that is independent of the emissivity of the object being measured. However, colorimetric pyrometers can only be used at very high temperatures in practice. Therefore, using an ordinary colorimetric pyrometer,
It is not possible to cover the operating temperature range of graphite tubes, which extends for example from 100°C to 2800°C.

The main purpose of the present invention is to be able to measure the temperature of graphite tubes used in flameless atomic absorption spectrometry over a wide temperature range of approximately 100°C to 2800°C, regardless of their emissivity, using a pyrometer type. An object of the present invention is to provide a method and apparatus.

The invention is based on the discovery that the emissivity of a graphite tube is independent of its temperature. According to the method of the present invention, a graphite tube containing a sample is first heated to a predetermined temperature, and this temperature is measured by a method or device that is unrelated to emissivity but can only be used within a narrow temperature range. . continue,
The precisely measured temperature is then used to calibrate a radiation detector that can be used over a wide temperature range from approximately 100°C to 2800°C, depending on the emissivity.

The apparatus of the invention also processes the output signals of the radiation detector and the calibration means to form an error signal for automatic calibration of the radiation detector circuit. This error signal corrects for variations in emissivity, allowing accurate measurements to be made over the entire temperature range.

Next, specific examples of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a block diagram illustrating a preferred embodiment of the invention in which an atomizer graphite tube 10 is simultaneously sampled by a thermocouple 12 and a high temperature radiation detector 14. Radiation detectors are capable of measuring temperatures over a very wide range, e.g. 100°C to 2800°C, but their accuracy varies from tube to tube or even within a single tube over its lifetime. affected by the varying emissivity of the graphite tube. Thermocouples, however, are not affected by the emissivity of graphite tubes, but have a very narrow range of use, usually below 600°C. Since the variation in emissivity is known to be independent of the temperature of the graphite tube, some type of measurement device, such as a thermocouple 12, is used to calibrate the radiation detector 14 and its wide temperature measurement range. The accuracy of the radiation detector 14 can be maintained over time.

Accordingly, a thermocouple 12 is mounted in a conventional manner on the surface of the graphite tube 10, and its output voltage is applied to an amplifier 16 where it is amplified. At the same time, the temperature of the tube 10 is measured by a radiation detector 14 and the output signal of the radiation detector 14 is applied to a variable gain amplifier 18 having a gain control input 20. The output signals from amplifiers 16 and 18 are applied with opposite polarity to difference point 22 and the resulting difference signal is applied via control switch 24 to the inverting input of integrating amplifier 26. Integrating amplifier 26 is a conventional operational amplifier with a capacitor 28 in its negative feedback circuit, and its non-inverting input is coupled to ground potential. amplifier 26
The output of is coupled back to the control input 20 of the radiation detection amplifier 18.

In operation, control switch 24 is first opened and graphite tube 10 is heated to an elevated temperature that can be accurately measured by thermocouple 12. Control switch 24 is then closed and thermocouple 1
A signal representative of the output difference between radiation detector 2 and radiation detector 14 is applied to an integrating amplifier 26, so that the output voltage of this amplifier 26 increases. This increased output voltage of amplifier 26 causes the gain of amplifier 18 to change until its output signal is equal to the output signal of amplifier 16, and when the output signals of the two amplifiers 18 and 16 are equal, the gain obtained from difference point 22 is increased. The difference signal applied to amplifier 26 becomes zero. At this point the output of amplifier 26 is constant, then switch 24 is opened and thermocouple 12 is connected to graphite tube 1.
0 and is connected in series between the output of amplifier 18 and tube temperature controller 32, and is closed. Thermocouple 12 and its associated amplifier 16 are thus disconnected from the circuit. Integrating amplifier 26 generates a constant voltage control signal for amplifier 18 and
8 produces an output signal representative of the temperature of the graphite tube, independent of the emissivity of the particular tube. The temperature of the graphite tube 10 can then be increased or decreased as desired and the detector continues to operate accurately over its wide temperature measurement range.

FIG. 2 shows three openings or apertures 36,
3 is a cross-sectional view of tube 10 surrounded by casing 34 having 38 and 40; FIG. An opening or aperture 38 is provided for introducing sample material into the graphite tube in a conventional manner. Aperture 36 is provided to allow observation of the tube surface by radiation detector 14, where the temperature can be measured in a pyrometer type. Thermocouple 12 is inserted into casing 34 through opening or aperture 40 and placed in contact with graphite tube 10 . Once the calibration process described above is complete and switch 24 is opened, thermocouple 12 must be removed from graphite tube 10 to protect it from destruction.

FIG. 3 is a schematic circuit diagram showing a preferred example of the circuit shown in FIG. 1. As shown in FIG. 3, the amplifier 18 shown within the broken line block is the operational amplifier 4.
2, and the inverting input terminal of the operational amplifier 42 is connected to a variable resistance element 44 of a light controlled resistance circuit (raysistor) 45. The output terminal of operational amplifier 42 is connected to the anode of diode 50, and the cathode of diode 50 is connected to amplifier 4.
2 is connected to the inverting input terminal of the amplifier 4.
2, forming a negative feedback loop. The cathode of diode 50 is also connected to output terminal 52 of amplifier 18 via resistor 48 . terminal 52
is also connected to the anode of diode 46, whose cathode is connected to the anode of diode 50, completing the feedback circuit. This type of negative feedback allows the desired linearization of the amplifier output signal and allows the amplifier 18
The output signal of can be a linear function of the temperature of the graphite tube 10.

The output terminal 52 of the amplifier 18 is connected through a resistor 54 to a tube temperature control circuit 32 (not shown in FIG. 3). Furthermore, the output terminal 52 has a resistor 58
is connected to the difference generation point 22 via a control switch 24, preferably in the form of a FET (field effect transistor), to the inverting input terminal of an amplifier 26. The output signal from operational amplifier 26 is applied to light source 60 of optically controlled resistor circuit 45 .

Amplifier 16 shown in FIG. 1 includes an operational amplifier 62, shown as a dashed block in FIG. 3, and preferably identical to operational amplifier 42 of amplifier 18 described above. The inverting input of operational amplifier 62 is connected to the thermocouple output via a series resistor 64 and includes a negative feedback loop including resistor 66 . The output signals from operational amplifier 62 and amplifier 16 are coupled to difference point 22 via resistor 68. Resistors 58 and 68 provide signal isolation.
The resistors should preferably be of the same high value.

With the above-described negative feedback circuit provided in amplifier 18, the output signal of amplifier 18 appearing at terminal 52 is a linear function of temperature, as shown by line 70 in FIG. The thermocouple signal after linear amplification in amplifier 16 is also a linear function of temperature, as shown by line 72 in FIG.
It is shown in In operation, the graphite tube 10, monitored by the thermocouple 12 and the radiation detector 14, is brought to a temperature within the measurement range of the thermocouple, which may be, for example, 600°C, as shown by the vertical dashed line 74 in FIG. heated. Due to the emissivity of graphite tube 10, the output signal represented by line 70 is typically lower than the thermocouple output signal represented by line 72. The voltage difference appearing at the difference point 22 in FIGS. 1 and 3 is applied to an integrating amplifier 26 via a switch 24. Amplifier 26 changes the illumination intensity of light source 60 of light control resistor circuit 45 until the voltage signals from amplifiers 16 and 18 are equal. This means that the size of the line 70 in FIG.
It is represented by increasing to the position indicated by 1. When the voltage appearing at difference point 22 becomes zero, the output signal from integrating amplifier 26 remains constant. FET switch 24 is then rendered nonconductive and thermocouple 12 is disconnected from graphite tube 10. The temperature of the graphite tube 10 is then under the control of the tube temperature control circuit 32. Tube temperature control circuit 32 receives from amplifier 18 a graphite tube temperature signal corrected for varying emission coefficients.
Since the emissivity variation is independent of graphite tube temperature, the high temperature radiation detector provides an accurate temperature indication for the particular graphite tube over the entire temperature range.

As mentioned earlier, pyrometer radiation detectors detect varying graphite tube radiation using a pyrometry device that is insensitive to emissivity variations but is invariably useful over a very narrow temperature range. Calibration adjustments can be made to the rate.

Figure 5 shows a pyrometer radiation detector which, as mentioned earlier, is sensitive to variations in the emissivity of the graphite tube, but which, when calibrated, has a very wide range of use from approximately 100°C to 2500°C. 1 is a diagram illustrating a second embodiment of the present invention using a colorimetric pyrometer for the calibration of FIG. In this specific example, the graphite tube 10
is sensed by the radiation detector 14 and also the fifth
It is sensed by a colorimetric pyrometer 76, shown as a dashed line block in the figure. The output signal from colorimetric pyrometer 76 is coupled via a series switch 77 to an analog value memory or sample holding circuit. The sample holding circuit has an operational amplifier 78 whose non-inverting input is connected to receive a signal from switch 77. The output of amplifier 78 is connected directly to the inverting input and at its output a signal representing the temperature value measured by colorimetric pyrometer 76 appears.

Thermal radiation emitted from graphite tube 10 is sensed by radiation detector 14 . The detector 14
The output of is amplified by variable gain amplifier 18 in the same manner as described above in connection with FIG. In the embodiment shown in FIG.
An amplifier 80 whose gain can be varied by 2
It is equipped with A detector 1 is connected to the output terminal of the amplifier 18.
A signal representing the actual temperature measurement made by 4 appears. The output of amplifier 18 is therefore applied to signal difference generation point 86 via series resistor 84. Similarly, a signal representing the temperature measurement of colorimetric pyrometer 76 appears at the output of amplifier 78 and is applied via resistor 88 to signal difference point 86. If no signal calibration is required, the values of resistors 84 and 88 are the same. Since the signals applied to difference point 86 are of opposite polarity, the signal at point 86 will represent the difference in values measured by colorimetric pyrometer 76 and radiation detector 14. A signal representing this difference value is applied to an integrating amplifier 92 via a series switch 90. The output signal of amplifier 92 is used to vary the resistance of porosiometer 82 to the point where the output signals from amplifiers 78 and 18 are equal and the signal appearing at difference point 86 is zero.

The output signal from amplifier 18 represents the true temperature of graphite tube 10 as measured by radiation detector 14 corrected for variations in graphite tube emissivity. The output signal from amplifier 18 is then applied to the graphite tube control circuit. The control circuit includes a preamplifier 94 and a tube control circuit 96 for adjusting the current through the heating winding of the graphite tube 10 to calibrate the radiation detector circuit or for atomic absorption. Determine and maintain the selected input temperature for graphite tube operation in spectroscopic testing.

In operation, the temperature at which the colorimetric pyrometer 76 is designed to operate accurately is determined by the calibrated temperature input circuit 9.
Double throw switch 100 is switched to cause the electrical signal generated by circuit 98 to be input to the input terminal of preamplifier 94 with the opposite polarity of the output signal from amplifier 18. The temperature of the graphite tube is then measured by radiation detector 14 and its output signal is generated by amplifier 18. When the temperature selected by circuit 98 is reached under indication by the radiation detector circuit, switch 9
0 is closed and integrating amplifier 92 generates a signal that changes the value of resistor 82 to the point where the outputs of amplifiers 18 and 78 are equal. Once the outputs of amplifiers 18 and 78 are equal, the radiation detector is corrected for emissivity error. Switch 90 can then be opened and switch 100 is switched from selection circuit 98 to operating temperature selection circuit 102. This circuit 102 has been preconditioned to produce an output signal corresponding to the desired temperature of the graphite tube 10 during subsequent testing.

A third embodiment of the invention is shown in FIG. 6, in which the voltage across the graphite tube 10 is measured by a current transformer 106 connected to the power supply conductor of the graphite tube 10. A wattmeter 1 generates an output signal voltage corresponding to the product of currents
04 is used. The remainder of the circuit of FIG. 6 is the same as that of FIG. 5, and corresponding elements are designated with the same reference numerals. Therefore, no further explanation of the circuit of FIG. 6 is necessary.

The operation of the circuit of FIG. 6 will now be described with particular reference to FIG. 7, which shows the results of extensive power versus temperature testing performed on a large number of different graphite tubes. Curve 106 is the power-temperature curve of the graphite tube when the resistance value is maximum, and curve 108
represents the result for the graphite tube when the resistance value is minimum. The properties of all graphite tubes of a particular type lie between these two boundaries. As shown by the vertical dashed line in Figure 7, the upper limit of the characteristic range is approximately 2000℃.
Note that the temperature is high. The temperature at which the curves in Figure 7 intersect is a unique function of the supplied power;
At this temperature, the power supplied to graphite tube 10 can be converted into a signal by amplifier 104 for radiation detector calibration.

Accurate measurements of tube temperature for radiation detector calibration can be performed by measuring tube resistance as shown in the embodiments of FIGS. 8 and 9. As seen in FIG. 9, a graphite tube heated at a constant voltage reaches a constant resistance value at a relatively high temperature, as shown by dashed line 112. This high resistance value can then be used for generating a signal for calibrating the radiation detector circuit.

In FIG. 8, the current flowing through the tube heating element is measured by current transformer 114 and divided by divider circuit 116.
is applied to This divider circuit 116 divides the tube voltage drop by the tube current to obtain a quotient signal that is proportional to the tube resistance. After accurately calibrating this value, the resistance value display 11
8 can be displayed. When the measured resistance value becomes constant, the value of variable resistor 120 in the feedback circuit of radiation detection amplifier 80 is manually adjusted to adjust the output signal of amplifier 80 to the specific temperature represented by line 112 in FIG. You can make it compatible.
If desired, the voltage drop in tube 10 is reduced by amplifier 12.
2 and applied to preamplifier 94 via selection switch 124, thereby allowing the temperature control loop to operate as a voltage control loop to maintain a constant voltage across tube 10.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention in which the radiation detection circuit is calibrated by a thermocouple circuit; FIG. 2 is a cross-sectional view of the graphite tube shown in FIG. 1; Figure 3 is a circuit diagram showing the circuit in Figure 1 in detail, and Figure 4 is a circuit diagram showing the circuit in Figure 1 in detail.
Figure 5 is a block diagram showing a second specific example in which the radiation detection circuit is calibrated with a colorimetric pyrometer; Figure 6 is a radiation detection circuit for explaining the operation of the circuit in Figure 3; A block diagram showing a third specific example in which the detector is calibrated by a power measurement circuit, FIG. 7 is a graph for explaining the operation of the circuit in FIG. 6, and FIG. A block diagram showing yet another specific example and FIG. 9 are graphs for explaining the operation of the circuit of FIG. 8. DESCRIPTION OF SYMBOLS 10...Graphite tube, 14...High temperature radiation detector, 20...Gain control input terminal, 22...Difference generation point, 2
4... Control switch, 32... Tube temperature control circuit, 34
...Casing, 44...Variable resistance element, 45...Photoresistance circuit, 36, 38, 40...Aperture, 60...
Light source, 76... Colorimetric pyrometer, 96... Control circuit, 98
...Calibration temperature input circuit, 106, 114...Current transformer, 104...Power meter, 116...Divider circuit.

Claims (1)

[Claims] 1. A high temperature measurement method for measuring the temperature of a graphite tube of a graphite tube atomizer used in flameless atomic absorption spectrometry over a wide operating temperature range, which A radiation detector is used to generate a temperature-dependent signal, and a variable gain amplifier is connected to the output of the radiation detector, the output signal of the variable gain amplifier being related to the temperature value of the graphite tube. , in the high temperature measurement method, heating a graphite tube to a temperature within a limited partial temperature range included in the broad operating temperature range of the graphite tube atomizer, and determining the emissivity and the emissivity of the graphite tube within the partial temperature range. measures the temperature of the graphite tube within said partial temperature range using a second temperature measurement means providing an unrelated temperature measurement, and further measures the temperature of the graphite tube within said partial temperature range to a radiation detector. and varying the amplification gain of the variable gain amplifier until the output voltage value of the variable gain amplifier corresponds to the temperature measurement measured by the second temperature measurement means, and at this time. and then measuring the temperature of the graphite tube over the wide operating temperature range with a radiation detector while the amplification gain is set to the value. High temperature measurement method. 2. A radiation detector that is a high temperature measuring device that measures the temperature of a graphite tube of a graphite tube atomizer used in flameless atomic absorption spectrometry over a wide operating temperature range, and is exposed to radiation from the graphite tube. in a pyrometry device in which a temperature-dependent signal is generated, second temperature measurement means are provided for measuring the temperature of the graphite tube, said second temperature measurement means being arranged to measure the temperature of the graphite tube within a limited temperature range. a detector signal amplifier is provided for making a temperature measurement independent of the emissivity of the graphite tube and for generating a calibration output signal representing the result of the temperature measurement, the detector signal amplifier including a variable gain element; a difference generating circuit coupled to the output side of the detector for amplifying the output signal of the detector and connected to the output side of the second temperature measuring means and the detector signal amplifier; , generating a difference signal proportional to the difference between the calibration output signal and the radiation detector output signal, an integrating amplifier coupled to the difference generating circuit, the integrating amplifier responsive to the difference signal; A high temperature measuring device, characterized in that the variable gain element is adjusted so that the difference signal becomes zero. 3. The high temperature measuring device according to claim 2, wherein the second temperature measuring means comprises a thermocouple. 4. The high temperature measuring device according to claim 2, wherein the second temperature measuring means comprises a colorimetric pyrometer. 5. The high temperature measuring device according to claim 2, wherein the second temperature measuring means is a thermocouple detachably arranged to sense the temperature of the graphite tube. 6. The variable gain element of the detector signal amplifier is an optically controlled variable resistance element connected in series between the radiation detector and the detector signal amplifier, and the light source of the resistance element is the output of the integrating amplifier. A high-temperature measurement device according to claim 5, which is controlled by. 7. The second temperature measuring means comprises a power measuring circuit connected to receive signals representative of the voltage and current applied to the graphite tube, and the power measuring circuit measures the measured power. A pyrometry device according to claim 2, which generates a voltage output signal for display. 8. The high temperature measuring device according to claim 2, further comprising graphite tube control means for adjusting the power applied to the graphite tube in accordance with an input control signal. 9. The pyrometry device of claim 8, wherein the input control signal is controlled by the output of the detector signal amplifier. 10. The pyrometry device of claim 8, wherein the input control signal is the difference between the signal output of the detector signal amplifier and the signal output of a manually controlled temperature selection circuit.