THERMOPHYSICAL MEASUREMENTS

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1 THERMOPHYSICAL MEASUREMENTS THE STANDARD FUNCTION FOR A PLATINUM RESISTANCE THERMOMETER ABOVE THE SETTING POINT OF SILVER A. I. Pokhodun, M. S. Matveev, and N. P. Moiseeva UDC 536.5: A method and apparatus are described for measuring the characteristics W(T) at ~ for a hightemperature platinum resistance thermometer. A special design has been used containing a radiating cavity of black-body type localized around the sensitive element. The measurements were made with a spectropyrometric comparator by brightness comparison at the Ag, Au, and Cu reference points, and also at temperatures representing multiple and fractional brightnesses. The results are compared with the W(T) obtained by interpolating measurements made by the contact method at the Zn, Al, and Cu reference points. The differences in the W(T) constituted the temperature equivalent of 0.16, -0.14, and 0.04 at the reference points correspondingly for Ag, Au, and Cu. The over-all measurement error is estimated as O. 19 ~ An interpolation formula is proposed that is based on the standard function Wref(T90) given in the definition of ITS-90. Recently, research has been done to refme the platinum resistance thermometer characteristic W(T) =R (T)/R (273,16), in which R(T) is the resistance at temperature T and R(273.16) is the resistance at the triple point of water, where use has been made of optical temperature-measurement methods [1-3], which give more reliable W(T) at medium temperatures and have been used in developing the international temperature scale ITS-90. In this research, high temperature platinum resistance thermometers HTT have been compared with pyrometers in national standards. However, various difficulties arose in the range above the setting point of silver. Here we examine the scope for determining W(T) by means of a pyrometric spectrocomparator and a special HTT in which the sensitive element coincides in space with the radiating cavity. Thermometer Design. This is shown in Fig. la. The sensing element is a spiral of platinum wire 0.4 mm in diameter in bifilar mounting on a two-start thread on a silica cylinder having a diameter of 5 mm for the internal hole. The internal cavity in the cylinder is closed at one end, while at the opposite end it is joined to the coaxial outer cylinder, which is the protective tube for the thermometer. Two platinum leads are welded to each end of the spiral, which provide for connecting the secondary instrument. The resistance at the triple point of water is about 0.6 ft. The thermometer is filled with dry air, and its length without the leads is 625 ram, outside diameter on the stop side 12 mm. The black-body model is placed within the cavity, which contains a cylindrical tube made of nickel foil 0.1 mm thick, together with a graphite bottom and graphite stop (Fig. lb). The last contains a conical hole with an outside diameter of 2 ram. The length of the model is 29 mm, and the outside diameter is 5 mm. The model provides a source of radiation whose characteristics enable one to use Planck's law to determine the temperature. A uniform spatial temperature pattern is provided by a tubular heater having four windings, of which a section is shown with the resistance thermometer inside it in Fig. 2. The main winding I is made of nichrome strip 6 mm wide and 0.5 mm thick and works with two additional nichrome windings II and III made of wire with diameter 1.5 mm and designed to produce a pattern with a low temperature gradient; the auxiliary heater IV reduces the heat loss along the body of the HTT. The currents in winding I-III are controlled automatically by electronic regulators by means of thermocouples mounted in the heater. To reduce the convection and improve the uniformity, the tube contains inserts made of heat-resistant steel and graphite. Translated from Izmeritel'naya Tekhnika, No. 9, pp , September, /93/3609,' Plenum Publishing Corporation 1017

2 I 2 3 # 5 6, 7 8! 4, 5 a Fig. 1. Platinum resistance thermometer with black cavity: 1) stop; 2) quartz spiral; 3) platinum resistor; 4) nickel tube; 5) graphite bottom; 6) quartz body; 7) head; 8) leads. i 2 g 4, S r- /-- 9 :'.::..:3:::.' :'.',.: :::.:.,'::'::..'.;li~".i'~-".::'...-, P, ;.3 :.7; " " ~-.X\\\\\\\\\'.;r ' ~ ~ ,...,._. _- _.~ _ ~. -,:., x... L -, -, - >. ',. ~. y x j...-v :.-:,'.-"..':".--.-".,:... :.- :-::,..-'.'..'...'::.:.'.:.;..:...'.v.-,'.-~ ",-~.." l...'" ~ ~1- :::".:..'...ii: ( '..::i.:.l.,: ~..(.~..~i..:~.~7~.):~:!:~.~:~.:~i~:~:.`~:~.~.~!~:~.~):.~ ~'.2.7..~ [? - ::..".:.:...:. '/x.,- > / / i ~'_<.z.~,2l<;..,.-~,:. c.~t,.-j_z..: -~, -" 9.; / /,"," ~.z.~:j~z,,~f~..~..22~ Fig. 2. Tubular oven with platinum resistance thermometer inside: 1) quartz tube; 2) heater winding IV; 3) heater winding III; 4) aluminum tube; 5) heater winding I; 6) heat-resistant steel insert; 7) control thermocouple; 8) platinum thermometer; 9) heat winding II. The regulators were adjusted to produce a uniform pattern. The temperature differences were +0.35~ symmetrically about the center of the cavity and sensing element over a length of 100 mm at the average temperature of 1038~ SP-4K Pyrometric Speetrocomparator. This operates by modulating two optical beams and determining the point of equality for the spectral brightnesses at a given wavelength. Figure 3 shows the comparator scheme. There are two symmetrical optical channels. The sources 1 are imaged by the lenses 2 in the planes of the stops 3, which block out the radiation from the peripheral radiating areas. The radiation isolated by the lenses 4 and prism 5 are directed to mirror 6 in the string modulator 7, from which the mirror 8 and lens 9 provide projection on the entrance slit of the double monochromator 10 in turn with a modulation frequency of 1070 Hz. The microscope 11 examines the image positions. The modulator replaces one flux by the other in such a way that if the brigtnesses are equal, the total flux at the detector 12 remains constant, while if they are unequal, there is alternating component having the modulation frequency. Equal brightness is detected by the null detector 13 following the synchronous detector 14 and tuned amplifier 15 from the minimum in the component at the modulation frequency governed by generator 16. The detector is an FI~U-84-6 photomultiplier. The measurements were made at nm. The threshold at that wavelength with a relative aperture in the optical system of 1:30 is ~ in the range ~ The imaged area of the radiating surface is a rectangle 0.7 mm wide and 1 mm long. The comparator has a slight channel asymmetry (temperature equivalent of ~ for sources of the same type), so the measurements are usually made by substitution. One channel operates constantly in a fixed configuration on a fixed part of the strip in a reference temperature lamp, which acts as the tare and as the temporary measure of the temperature unit previously transferred from a standard source. The tare lamp is an SI vacuum tungsten strip one. The container is fitted with a cooler through which thermostatic distilled water flows. The cooler temperature is monitored by a built-in ST4-16A thermistor, and the temperature does not vary by more than 0.3~ during the measurements. Thermostatic control and monitoring are also used with the resistance coils and normal cell used in the absolute current and voltage measurements. 1018

3 TABLE 1 W(T) at reference points HTTNo. ] Tzn TAI TCu , , ~ , , TABLE 2 HTT No. R(273.16), ~, TZn after calibrationatpoint TAI TCu R(273.16), ~, iafter pyromeee~ measurement 0, ~ ,59(Y008 (1, , , , TABLE 3 T, ~ W(T) 956,67 4, ,28 4, ,20 4,60630 Temperature Lamp Calibration. The following technique is used to do this, as the lamp is used to determine the temperature of the platinum thermometer sensing element. Initially, the temperature lamp was calibrated at the setting points of metals: Ag, Au, and Cu. The standard sources were black bodies made as graphite cavities immersed in the molten metals within graphite crucibles. The crucibles were made of graphite containing not more than 0.03% ash. The metal masses were: Ag 100 g, Au 180 g, and Cu 85 g. The impurity contents in the metals were not more than 0.002%. We used the brightness doubling (division) method, which is also often called the light flux addition method, to obtain the temperature lamp currents at other temperatures. Within this calibration, one uses an auxiliary lamp replacing the standard source and a mirror system enabling one to alter the ray paths through the semitransparent mirrors by means of slides and obtain brightnesses that are multiples of 2 +l, from which one calculates the corresponding temperatures via Planck's law. In this way, we provided additional points for the temperature strip whose temperatures corresponded to two and four times the brightness in the solidification of silver, or to a quarter, half, and twice the brightness of gold, or a quarter and half the brightness of copper. Thermometer Stabilization and Calibration. Two high-temperature thermometers with special design were made to determine W(T) at the Mendeleev All-Union Metrology Research Institute. The thermometers were given cyclic stabilization at l l00~ before the calibration at the reference points. Thermometer No. 4 was annealed in a vertical oven initially at 600 and 800~ for 15 h and was then given cyclic annealing (length of a single cycle 5 h) at ll00~ for 50 h. At the end of the stabilization, the change in resistance at the triple point of water in a single cycle was 0.1 mk. Thermometer No. 5 was annealed in cycles of 5 h each at ll00~ for 50 h. A defect was observed at the point of emergence of the platinum leads from the protective tube. After repair, the thermometer was annealed for a further 24 h at 1100~ The resistance instability at the triple point of water during a single annealing cycle did not exceed + 1 mk. The thermometers were calibrated at the reference points for Zn, A1, and Cu. The resistance change for No. 5 at the triple point of water during the calibration and measurements with the pyrometer did not exceed the temperature equivalent of 3 mk. 1019

4 TABLE 4 T, ~C W(T) 956,84 4, , ,42 4, ,60630 fi 'H Ll 2 3 \ ' O Fig. 3. Scheme for pyrometric spectrocomparator. No. 4 showed an ongoing increase in resistance at the triple point of water during the calibration and pyrometry. The temperature equivalent of the resistance increase was 30 mk. Tables 1 and 2 give the calibration and stabilization results. W(T) Values. These were determined by synchronous measurement of the thermometer's resistance and the temperature of the sensitive element from the cavity radiation. The thermometer and the black body in the radiation cavity were placed in a horizontal oven in the region of least temperature gradient. The oven was brought to the steady state and the lamp current was varied smoothly until the brightnesses were equal for the lamp strip and the hole in the stop in the black body. The resistance and the current after stabilization were recorded with an automatic dc bridge. The measurements were made at three temperatures, with 15 observations at each. Table 3 gives measured W(T) derived from the pyrometer with thermometer No. 5. Two corrections were applied to the temperatures as measured. 1. Correction for nonideal black-body behavior in the crucibles containing silver, gold, and copper. This correction was defined by calculating the blackness of the radiation cavity on the basis of the geometry and reference data on the emissivity of the graphite. The calculated blackness was and varied with temperature, which reduced the brightness by the temperature equivalent of ~ 2. Correction for the lack of ideal behavior in the black body in the thermometer. The blackness of the thermometer cavity was measured by extracting the black body from the cavity and placing it in the cavity of the crucible containing silver. 1020

5 TABLE 5 957,06 961,94 99,8, ,04 I078,36!084,66 956,84 961,78 998, ,18 1:078, ,62 0,22 0,16 --0, ,14 --0,06 0,04 0, , '0,~005'03 --0, , , TABLE 6 Error sources T, cc Thermometer Inexact resistance Leakage current Temperature difference along length Inexact calibration Long-time instability Nonideal cavity black body Pyrometer Inexact value of wavelength Working spectrum width Inexact calibration of temperature lamp Nonideal crucible black bodies 0,0.06 0,006 0;005 0;005 0,I9 0,0l Overall (mean square) error 0,i9 We then measured the brightness of the hole in the stop at the solidification point of silver. The values were compared with those obtained in measurements directly in the graphite crucible. The correction was ~ Table 4 gives the corrected W(T) for thermometer No. 5 as determined with the pyrometer. These W(T) are evaluated for reliability by comparison with calibration results at the ITS-90 reference points, but the platinum thermometer was calibrated only at the copper solidification point in the range in which W(T) was measured. Also, the temperatures at which W(T) was measured did not coincide with the ITS-90 reference points. The W(T) closest to the solidification point of copper corresponds to ~ The following operations were performed to analyze the results. We used the three measured W(T) with an equation of second degree in fitting over the range ~ and then the curve was extrapolated to the copper solidification point of ~ which was correctly done because the temperature difference was not more than 7~ and W(T) is monotone throughout the temperature range. The resulting value was compared with the calibration result for thermometer No. 5 at the copper solidification point: Wine --Wpyr =4, ,623154= =~0, There was a difference here with the temperature equivalent of about 0.04~ between the calculated W(T) and the calibration result. To evaluate the scope for determining the temperature with the HTT in the range TAg-Tcu, we used the calibration data for the Zn, AI, and Cu points alone by the following method. The temperature was calculated from the interpolation equation proposed in the text to ITS-90: wry) = Wro~(T) + dw(v). The standard function Wref(T) above the silver point was obtained by extrapolating Wref(T70 ) given in the ITS-90 text, with the deviation function taking the form dw(t) =A [ W(T)--I ] +B[ W(T) -- ~112+C[W(T)--I I a, 1021

6 . in which A, B, and C are coefficients calculated from the calibration results for the Zn, A1, and Cu reference points. From these W(T) one derives the corresponding temperature. Table 5 gives temperatures calculated with the extrapolated value of the standard function and the proposed deviation function together with pyrometer measurements on calibrating thermometer No. 5 by contact and pyrometric methods. Table 6 gives the error components in measurements on W(T) and the over-all evaluation at the lo level. Conclusions. It is possible in principle to use high-temperature platinum resistance thermometers as interpolation instruments in the temperature range from the reference points for silver to the reference point for copper. This interpolation method can be used to plot W(T) for such a thermometer from the calibration results at the zinc, aluminum, and copper reference points. The data from these experiments on the whole fit within the expected ranges. The research should be continued to refine the corrections and to reduce the individual components in the measurement error. REFERENCES P. B. Coates, J. W. Andrews, and M. V. Chattle, Metrologia, 21, No. 4, 31 (1985). H. J. Jung, Xumo Li, and J. Fisher, Doc. CCT/ M. Batuello, F. Lanza, and T. Ricolfi, Metrologia, 27, No. 2, 75 (1990). 1022

UNIT 3 Part 1 Temperature Measurements

UNIT 3 Part 1 Temperature Measurements 1 US06CPHY06 Instrumentation and Sensors Temperature Scales 3 Temperature Scales 4 Temperature Measurement Methods Non Electrical Change in Physical State Physical