Abstract. 1. Introduction

Transcription

1 The importance of establishing a very-high temperature radiation thermometry measurement capability at the National Metrology Institute of South Africa (NMISA) Speaker: Dr Efrem K Ejigu National Metrology Institute of South Africa (NMISA) Private Bag X34, Lynnwood Ridge, Pretoria, 0040, South Africa eejigu@nmisa.org Phone: Fax: Abstract ITS90 scale above Silver (Ag) point is realized using standard linear radiation thermometer by measurement at a reference point which is Ag, Gold (Au) or Copper (Cu) point. The fixed point measurement at Cu point coupled with relative responsivity, non-linearity and range ratio measurement is used in the scale realization above Cu point through extrapolation based on Planck Law. The uncertainty of the scale above Cu point increases as a square of the ratio between measured and reference temperature at very high temperatures. Since extrapolation above the copper point involves significant source of uncertainty it makes measurement in the high temperature range less accurate and less reproducible. At present radiation thermometry deals with blackbody sources with working temperature up to 3500 o C. In order to decrease the uncertainty new types of fixed points are being investigated and found to be effective. The assigning of fixed point temperature to some of the suitable high temperature fixed point cells is completed. This development leads to the era of low uncertainty measurement at a very high temperature range. In this paper the importance of developing such measurement capability at NIMSA is demonstrated. 1. Introduction Radiation thermometer is used to realize ITS90 above Ag point through extrapolation [1] based on Planck law of radiation in contrast to platinum resistance thermometers which uses interpolation to realize ITS90 below Ag point. The ITS90 scale realized by platinum resistance thermometers involves number of fixed points and this helps the scale uncertainty to be smaller. The improvement in the reproducibility of metal-carbon (M-C) eutectic fixed point black body source shows potential in using them as one of the defining temperature points above Cu point [2]. Radiation thermometers can be used to realize a scale above Cu point through interpolations (involving M-C eutectic fixed point cells) thereby reducing the uncertainty increase experienced when realized through extrapolation [3]. So far number of metal carbon eutectic fixed point thermodynamic temperature has been determined and assigned, which is more than enough to calibrate a radiation thermometer at a very low uncertainty in temperature range until 3000 o C.

2 A low uncertainty measurement capability in the temperature region above Cu point is very important in number of industrial applications. South Africa has heavy industries that require accurate measurement at very high operating temperatures. The country has a lot of potential to attract technology based investment that requires accurate high temperature measurement and developing the capability is critical. The industries include metal smelting, nuclear energy reactors, and solar energy monitoring. The fixed point calibration methodology which is used to realize the scale is discussed in the calibration methodology section. In the discussion section the fixed point measurement result of linear pyrometer LP4 is used to demonstrate the different fixed point calibration methodologies together with the uncertainty of scale realization. The uncertainty of measurement improvement at a very high temperature is compiled in section High temperature uncertainty improvement. Finally concluding remarks were provided in the conclusion section. 2. Calibration methodology A radiation thermometer is an opto-electronic device which is used to measure the thermal energy of a radiation emitting object. The temperature of the emitting object can be determined based on Planck law of radiation. A basic radiation thermometer has optical and electrical components. The optical components are for the focussing of the emitted radiation on to the detector while the electronic part is for converting the optical signal into electrical signal and to input power. There are different kinds of radiation thermometers depending on application. Most radiation thermometer labs of an NMI has high accuracy linear pyrometer that can operate at different temperature region. These kind of pyrometers need to be calibrated at fixed points so that they can be used as a standard to provide traceability above Al point. There are different methods of calibration namely n=0 (no-fixed point required), n=1(one fixed point required), n=2 (two fixed point required), and n 3 (three or above fixed point required) 3,4. In all the methods except for n 3 and n=0 method a relative spectral responsivity measurement is required. In the n=0 method an absolute spectral responsivity measurement is required. 3. Discussion 3.1. Measurement NMISA standard for temperature points above Al point, LP4, was measured at number of fixed points At NMIJ. The fixed points were Al, Ag, and Cu. The LP4 detector was characterized by a double monocromater and relative responsivity was determined. The wavelength of the filter (900nm) was also characterized so that the shift of the central wavelength can be determined. The measurements done at NMIJ were meant to calibrate the LP4. Using the results of the measurement the effect of using fixed points above Cu in decreasing uncertainty will be demonstrated. The consultative committee for temperature (CCT) is at a position to extend IT90 beyond Cu point to ITS-XX once the redefinition of the Kelvin is completed. The assigning of fixed point temperature points for some eutectic fixed

3 points has already been completed. This brings a major change in pyrometer calibration which used to be affected by exponential uncertainty increase for the scale realized above Cu point due to the extrapolation involved. The measurement results were used to calibrate the LP4 by using different methods namely multiple-point, one-point and two-point. The calibration results from the different methods are compared and analysed to show the effect of using the new fixed points above Cu points in the decrease of the uncertainty of the scale realized Multiple point calibration (n=3) Pyrometers that are used as standards below copper point are normally calibrated by using range of fixed points below Cu point. Three fixed point measurements will be enough to calibrate such pyrometers. At temperatures below Cu point the well-known multiple point calibration is usually applied in order to determine the relationship between the Black Body radiator temperature and the pyrometer signal. The well-known Sakuma-Hatorri equation can be used to relate the thermometer signal to the Black Body temperature through a Planck law approximation as shown below in equation (1-2) [5,6]. E = S( ). L b (, T). d 1 E = C exp(c 2 /(A.T+B)) 1 2 Were: E Thermometer signal S Relative responsivity A Thermometer coefficient related to central wavelength B - Thermometer coefficient related to band width of the filter C Thermometer coefficient related to the transmittance of the filter - wavelength T- Reference thermodynamic temperature c 2 - Second radiation constant A Pyrometer can be characterized by the three coefficients A, B, and C as shown above in equation (2). The coefficients can be determined by fitting the three fixed point calibration data points using least fit square method [5]. NMISA Linear pyrometer standard was measured at NMIJ at three different fixed points namely Al, Ag and Cu and the following result was obtained

4 Table 1. Fixed point measurement at three fixed points Al, Ag and Cu. Fixed point Temperature (K) Signal (Photocurrent, A) Al E-11 Ag E-09 Cu E-09 Using the above measurement data in table (1) and after fitting the data points the following coefficient values were obtained. Table 2. Pyrometer coefficient calculated using three point interpolations for temperature range of 600 C to 1085 C. A (m) B (m.k) C (V) 9.049E E E-04 Uncertainty of Scale realization The uncertainty at each fixed point measurement can be calculated from uncertainty components that can be determined from the fixed point realization and size of source effect (SSE) measurement. Table 3. Calculated uncertainty for each fixed point measured Fixed point U, K=2( C) Al 0.1 Ag 0.1 Cu 0.1 The intermediate temperatures uncertainty can be calculated from the scale realized using the three fixed point measurement for instance scale uncertainty from the three fixed point measurements, non-linearity, output stability and noise.

5 U,K=2 ( C) Table 4. Uncertainty of the scale realized through three point interpolation 0,40 0,35 n = 3 Temperature ( C) U,K=2 ( C 0, , , ,323 0, , , ,78 0, , ,62 0, ,3736 0,25 0,20 0,15 0, Temperature ( C) Figure 1. Uncertainty of the scale realized using multiple-point calibration method Single point calibration (n=1) Pyrometers that are used as standard above copper point are calibrated by using measurement at Cu point and relative spectral responsivity measurement. At temperatures above Cu point a one fixed point measurement at Cu point coupled with relative spectral responsivity measurement is usually implemented in order to determine relationship between the Black Body radiator temperature and the pyrometer signal. The scale realization above copper point is through extrapolation which makes the uncertainty increase for temperatures above Cu point. In order to realize a scale using one fixed point & responsivity measurement the pyrometer signal at certain temperatures above Cu point need to be calculated as in the following equation (3-5) [7]. E = f S( ) L(, T)d 3 f = E ref S( ) L(,T ref )d 4 L(, T 2,3 ) = c e ( C 2 T2,3 ) 1 5 Were: c 1 first radiation constant T ref reference fixed point

6 Relative responsivity E ref Thermometer signal at T ref T 2,3 temperatures above reference temperature The signal at T2 and T3 were calculated from Cu fixed point data by using the above equation (3). In addition the responsivity measurement result shown in figure (2) is also used in calculating the signal at T2 and T3. 1,0E+00 1,0E-01 Relative responsivity 1,0E-02 1,0E-03 1,0E-04 1,0E-05 1,0E-06 1,0E-07 1,0E-08 1,0E-09 1,0E Wavelength (nm) Figure 2. Measured relative responsivity (logarithmic scale) within range of wavelength at certain interval The following table shows the calculated signal for the two temperature points T2 and T3 Table 5. Calculated pyrometer signal for temperatures T2 and T3 from responsivity measurement and using equations (3-5) Fixed point Temperature, K Signal (Photocurrent, A) Cu E-09 T E-08 T E-07 Using the above data together with relative spectral responsivity measurement data the following coefficient was obtained

7 Table 6. Calculated pyrometer coefficient from one fixed point & relative responsivity measurement for temperature range of 960 C to 3000 C. A (m) B (m.a) C (V) 9.085E E E-04 Uncertainty of Scale realization Uncertainty is again determined at the fixed point measurement as indicated in section (3.1.2). Uncertainty at relative responsivity measurement is determined by considering uncertainty components that are related to wavelength measurement and detector responsivity. Table 7. Uncertainty calculated for Cu fixed point and relative responsivity measurement. U, K=2( C) Cu 0.1 Relative Responsivity 0.4 Using the above uncertainty it is possible to realize a scale within a certain temperature range through extrapolation. The scale uncertainty is determined from the uncertainty at the fixed point used and the uncertainty in responsivity measurement, gain ratio, non-linearity, stability and noise.

8 U, K=2 ( o C) Table 8. Uncertainty of the Scale realized through one-point calibration method Temperature U,K=2 ( C 960 0, , ,62 0, , , , , , , , , , , , , , , , ,50 2,00 1,50 1,00 0,50 n = 1 0, Temperature ( o C) Figure 3. Uncertainty of the scale realized using one-point calibration method Two point calibration (n = 2) It is possible to determine the relationship using interpolation method if eutectic fixed points can be applied. Presently the development of very high temperature fixed points is at a mature stage where the assigning of temperature points for some eutectic fixed points is completed. Following this development the CCT is planning to extend the ITS90 to ITS-XX. In ITS-XX scale Eutectic points will play a major role in allowing the scale realization using interpolation. This leads to a decreased uncertainty when compared to the extrapolated uncertainty. From the eutectic points Co-C( 1324 C ), Pd-C(1492 C ), Ru-C (1953 C) and Re-C (2474 C ) have their fixed point temperature assigned. In this scheme a partial knowledge of the central wavelength of the relative responsivity of the thermometer can be used. The ratio of signal measurement at two fixed points will allow the determination of the accurate central wavelength of the relative responsivity. Once the accurate central wavelength is determined it will be possible to calculate a third signal (E3) at a certain temperature (T3). The coefficient of the thermometer can be determined by using the two measured signals and the calculated signal using interpolation method implementing Sakuma- Hattori equation.

9 U,K=2 ( C) Uncertainty of Scale realization Assume the following uncertainty at eutectic point fixed points. Table 9. Calculated uncertainty for Cu point and assumed uncertainty for eutectic points Re-C and Ru-C U, K=2( C) Cu 0.1 Re-C 0.3* Ru-C 0.2* Using Cu and one or two of the eutectic points it is possible to realize a scale for certain temperature range. For instance using uncertainty at Cu and Re-C measurement and uncertainty from spectral responsivity measurement it is possible to realize a scale through interpolation. The values in the following table are obtained through two point interpolation. Table 10. Uncertainty of the Scale realized through two point calibration method Temperature ( C) U,K=2 ( C , ,62 0, , , , , , , , , , , , ,70 0,60 0,50 0,40 0,30 0,20 0,10 n = 2 0, Temperature ( C) Figure 4. Uncertainty of the scale realized using two- point calibration method High temperature measurement uncertainty improvement Demonstration of the importance of establishing a very-high temperature radiation thermometry measurement capability at the National Metrology Institute of South Africa (NMISA) is done by comparing the scale uncertainty determined from one-point scheme and two-point fixed point calibration method. The comparison is shown in figure (5).

10 U,K=2 ( C) 2,50 2,00 n = 1 n = 2 1,50 1,00 0,50 0, Temperature ( C) Figure 5. Uncertainty comparison between two-point and one-point scale realization methods. In the figure it is shown that the uncertainty determined by using two-point calibration method does decrease the uncertainty at the highest temperature by more than 70 % when compared to the uncertainty obtained through one-point calibration method for the specific example. The demonstrated measurement uncertainty improvement at very high temperatures has high implication in power consumption, environmental effect etc. The investment that the institute make today in establishing such a lab might be high but over the years to come it will be paid off when the impact it has in the economy of the country is considered. NMISA will also have the capability to provide traceability for temperature range above Cu point from its primary lab. Commercial calibration labs and industry can get reduced uncertainty traceability at NMISA without sending their standard overseas. With the facility for high temperature measurement NMISA can provide pyrometer characterization services which includes detector non-linearity, detector spectral-responsivity, Range ratio, ambient temperature dependence, size of source effect and distance effect. 4. Conclusion A comparison between calibration uncertainties of scales realized using two different methods is used to demonstrate the significant importance of establishing a very high temperature radiation thermometer measurement capability that includes eutectic fixed point at NMISA. It is demonstrated that using one or two eutectic fixed points above copper point together with Cu fixed point dramatically decreases the uncertainty of the scale realized above copper point when compared to the scale realized using one-point method. Reduced uncertainty traceability can be provided to commercial calibration labs, NMIs and Industry from the primary lab. It is also discussed that the high temperature fixed point measurement facility can also be used to characterize pyrometers for different effects.

11 5. Reference 1. H. W. Yoon, The realization and the dissemination of the thermodynamic temperature scales, Metrologia, 43, 2006, D. Low and Y. Yamada, Reproducible metal-carbon eutectic fixed-points, Metrologia, 43, 2006, Y. Yamada, Uncertainty of Radiation Thermometers calibrated by Interpolation between Fixed Points, Proceeding of ICROS-SICE International joint conference, Fukuoka, Japan, August, 2009, P. Bloembergen, Y. Yamada, N. Yamamoto and J. Hartmann, Realizing the High- Temperature Part of a Future ITS with the aid of Eutectic Metal-Carbon Fixed points, in Temperature: Its Measurement and Control in Science and Industry, ed. by Dean C. Ripple et al., AIP, New York, 7, 2003, F. Sakuma and S. Hattori, Establishing a practical temperature standard by using a narrow-band radiation thermometer with a silicon detector, in Temperature: Its Measurement and Control in Science and Industry, ed. by J. F. Schooley, AIP, New York, 5, 1982, Y. Ymaguchi and Y. Yamada, Uncertainty due to non-linearity in radiation thermometers calibrated by multiple fixed points, in Temperature: Its Measurement and Control in Science and Industry, ed. by Christopher W. Meyer, AIP, New York, 1552, 2013, F. Sakuma and L. Ma, Calibration facilities for high temperature radiation thermometers, Proceedings of SICE-ICASE international joint conference, Bexco, Busan, South Korea, October, 2006,