Standard Platinum Resistance Thermometers Research Articles

Analysis of the instability of the characteristics of a standard platinum resistance thermometer in the interval between verifications and a method for reducing the measurement error

Analysis of the instability of the characteristics of a standard platinum resistance thermometer in the interval between verifications and a method for reducing the measurement error

A new interpolation equation in the ITS-90 subrange from the triple point of water to the freezing point of indium

For all the subranges above 0.01 °C, other than the interpolation between the triple point of water (TPW) to the melting point of gallium, temperatures on the international temperature scale of 1990 (ITS-90) are specified without the melting point of gallium. This study suggests a new interpolation equation in the subrange from the TPW to the freezing point of indium which is a polynomial of fractional third order with two coefficients to be determined by the measuring values of standard platinum resistance thermometers (SPRTs)’ resistance ratios at the melting point of gallium (W Ga) and at the freezing point of indium (W In). A sample set of 30 SPRTs from various countries and laboratories is used to quantify the comparisons between this new interpolation and the ITS-90. Analyses show that the reproducibility is significantly improved compared with the ITS-90 in the same subrange. The peak value of overlapping subrange inconsistencies (SRI) from TPW to the melting point of gallium is reduced from 0.91 mK to 0.21 mK. Both the mean and standard deviation of the SRI peak values for the new equation decrease by approximately a factor of four compared to those of the ITS-90 (mean decreasing form 0.20 mK to 0.05 mK and standard deviation decreasing form 0.32 mK to 0.07 mK). Type 3 non-uniqueness (NU3) determinations of the same subrange have also been looked into with a data set of four SPRTs compared in thermostatic baths using a copper block and the results show that NU3 scales down from a range −0.15 mK–0.25 mK of the ITS-90 to −0.10 mK–0.15 mK of the new interpolation equation. Propagation of Uncertainties investigation shows that the new equation inflates merely 7% the measurement uncertainties of fixed points in a relatively short interval from 30 °C to 60 °C.

SIM.T-K9.3: Standard platinum resistance thermometer ITS-90 realizations from the Hg triple point to the Zn freezing point

Main textThis report shows the results of a bilateral comparison of temperature standards between Instituto Nacional de Metrologia, Qualidade e Tecnologia (Inmetro, Brazil), and Trinidad and Tobago Bureau of Standards (TTBS, Trinidad and Tobago), carried out between January and July of 2020. The measurands of this comparison were the W(t90) ratios of two Standard Platinum Resistance Thermometers (SPRTs), used as transfer standards, measured at the zinc freezing point (Zn FP, 419.527 °C), tin freezing point (Sn FP, 231.928 °C), gallium melting point (Ga MP, 29.7646 °C), triple point of water (TPW, 0.01 °C) and mercury triple point (Hg TP, -38.8344 °C).To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Comparison report of the ITS-90 realization at the freezing point of silver between NIM and CMS

Main textThe APMP key comparison APMP.T-K4.1 was initiated on the request of the Center for Measurement Standards, Taiwan (CMS) to link their standards to the Consultative Committee for Thermometry (CCT) Key Comparison (KC) and facilitate the CMS CMC submission. National Institute of Metrology, China (NIM) provided the linkage to the CCT-K4 at the temperature of the freezing point of silver. In the APMP.T-K4.1, one high temperature standard platinum resistance thermometer (HTSPRT) was chosen as the transfer instrument and was calibrated at the freezing point of silver. The comparison was carried out in a participant-pilot-participant sequence where NIM served as the pilot. The linkage was based on the temperature of CMS reference Ag fixed point relative to the key comparison reference value (KCRV) of CCT-K4 at the Ag fixed point via the difference in temperature between the NIM reference Ag fixed point and NIM reference Ag fixed point (links to the KCRV of the CCT-K4). The temperature difference between the standards of CMS and the KCRV of the CCT-K4 was within the evaluated comparison uncertainties of the ATPM.T-K4.1. This report provides detailed information on the comparison results, linkage mechanism, and the Degree of Equivalence of the CMS relative to the institutes having participated in the CCT-K4.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

A novel radiometric calibration of an infrared thermometer

A new temperature calibration scheme was proposed and implemented on the National Physical Laboratory infrared radiometer Absolute Measurements of Blackbody Emitted Radiance (AMBER) operating in the wavelength range from 9 µm to 11 µm and equipped with a cryogenically cooled mercury cadmium telluride detector. The scheme involves measurements of the relative spectral responsivity of the filter radiometer including the spectral transmittance of the window and lens. It requires calibration at two reference temperatures, but does not involve measurement of the zero-radiance signal. The scheme uses a Ga-point blackbody as one reference point, and a variable temperature blackbody source at close to −30 °C with its temperature measured with a standard platinum resistance thermometer as the other. The measured temperature was compared using the variable temperature blackbody as the source against the temperature measured by the standard platinum resistance thermometer and agreement within the uncertainties was confirmed at intermediate temperatures. The scheme has the advantage that it makes full use of the small uncertainty realised by the fixed-point blackbody. The scheme was successfully implemented in an international comparison of sea surface temperature measuring radiometers where AMBER served to provide the reference.

Development and characterization of a high-emissivity reference blackbody radiation source with V-grooves and large aperture at low-temperature

A high-emissivity blackbody radiation source can improve the measurement level of infrared radiation temperature, which is critical for applications in high-value industries and human health monitoring. In this study, a new high-emissivity blackbody source with V-grooves and large aperture was redesigned and developed, which can be used as a radiation temperature reference source for the calibration of secondary blackbody sources and precision radiation thermometers in the temperature range from −10 ℃ to 100 ℃, including the temperature range of human body infrared radiation considered in the study of people exhibiting fever symptoms. The wavelengths of the infrared spectral radiation temperatures involved in the study were in the range of 8–14 μm. The blackbody source consisted of a blackbody cavity, a jet-stirred precision constant-temperature bath, a standard platinum resistance thermometer, a high-precision digital multimeter, a frost-proof cover, and an auxiliary test frame. The cavity was cylindrical and conical with a diameter of 60 mm, a conical top angle of 120°, and a total length of 300 mm. The V-grooves with an angle of 50° on the inner wall were coated with high-emissivity coatings. The cavity was then completely immersed in a thermostatic bath. The radiation temperature of the blackbody source was traced to the International Temperature Scale of 1990 using a standard platinum resistance thermometer, which was calibrated at fixed-points of the Ar, Hg, H2O, Sn, and Zn. The experimental results revealed that the normal effective emissivity of the blackbody measured by the thermal cavity reflectometer method was 0.999 751, which is consistent with the theoretical calculation results based on the Monte Carlo method using optical ray tracing technology. In addition, the accuracy of the radiation temperature was verified by comparing it with a reference blackbody source from the National Institute of Metrology (NIM), China. The temperature stability of the blackbody source did not exceed 0.005 ℃ within 30 min, the axial temperature uniformity of the blackbody cavity did not exceed 0.048 ℃, and the temperature uniformity at the bottom of the cavity was better than 0.053 ℃. The combined standard uncertainty of the radiation temperature of the blackbody radiation source was evaluated from 0.010 ℃ to 0.032 ℃. The novel design with 50° V-grooves allowed for the blackbody source to provide the combination of a large aperture and high emissivity, which improved radiance temperature measurements. The reliability of the new design was experimentally verified. This design provides a novel method and experimental basis for the future design of compact light-weight blackbody sources, which is useful for the development of compact spaceborne blackbody sources.

Least squares approach to standard platinum resistance thermometer subrange inconsistency reduction with redundant gallium and indium fixed points

The International Temperature Scale of 1990 (ITS-90) defines several different and sometimes overlapping temperature subranges. Where overlap occurs, different definitions of the temperature t90 exist that have equal status but produce different results. This discrepancy is called type 1 non-uniqueness or subrange inconsistency (SRI).In this paper, the SRI of the water-aluminum/water-zinc (SRI(Al:Zn)) and of the water-zinc/water-tin (SRI(Zn:Sn)) subranges of the ITS-90 were investigated for three cases. In the first case, the calculations followed the ITS-90 prescribed procedure. In the second case, the SPRT was calibrated at all fixed points of the subrange and its deviation function was then determined using the least squares method. In the third case, the least squares method was weighted by the uncertainties at each of the fixed points. One benefit of the least squares approach over exact interpolation is the reduction in uncertainty propagated from the fixed points. The calculations were applied to a large ensemble of 30 different SPRTs from the database of the Laboratory of Metrology and Quality at the University of Ljubljana, Slovenia. The sample consisted mainly of SPRTs manufactured by Fluke, Rosemount and AccuMac.The difference in the mean and standard deviation of SRI(Al:Zn) for the three cases was small, amounting to less than 0.06 mK. On the other hand, the mean of SRI(Zn:Sn) decreased from 0.73 mK to 0.01 mK and the standard deviation decreased from 1.25 mK to 0.43 mK when the weighted least squares approach was applied. Furthermore, the total propagated uncertainty from the fixed points decreased in particular temperature ranges with weighted least squares, especially from 50 °C to 300 °C (by about 50% compared to the ITS-90 case) and to a lesser extent from 400 °C to 600 °C (by about 10% compared to the ITS-90 case). The contribution of the difference in the uncertainty propagation between pairs of subranges to SRI was estimated to be at least 50% in all cases. According to the presented results, it can be advantageous to calibrate the SPRT at all available fixed points in the selected temperature subrange and then determine its deviation function using the weighted least squares method.

Investigations of Type 3 non-uniqueness in standard platinum resistance thermometers between 83 K and 353 K

A key feature of the International Temperature Scale of 1990 (ITS-90) is Type 3 non-uniqueness which arises from differences between individual Standard Platinum Resistance Thermometers (SPRTs) over a given subrange. This can be significant in the context of the best reported uncertainties of SPRT calibrations, but it is extremely difficult to determine because it is easily obscured by the measurement uncertainties. Hence, there is little good information available, particularly over much of the temperature range commonly encountered by long-stem SPRTs, and the estimates of its magnitude are potentially overstated. This paper presents high precision comparisons at NPL and CEM of two cohorts of 8 and 6 long-stem SPRTs, by measuring the ratio of each SPRT against a common SPRT in stirred liquid baths at temperatures from 178 K to 303 K, and 274 K to 353 K, respectively. Measurements of a third cohort of 8 SPRTs were made at INTiBS in a stirred liquid bath from 198 K to 303 K and in a temperature-controlled cryostat from 84 K to 185 K. The measurements were augmented by calibrations of each SPRT at the triple points of argon and mercury, the melting point of gallium and the freezing point of indium, so extending the overall temperature range down to 84 K and up to 429 K. The three cohorts were linked by two SPRTs which were common to all of them. The comparison differences, which ideally should represent the Type3 non-uniqueness, are presented for the specified ITS-90 interpolations, and for some alternative interpolation schemes. Specifically, it is assessed for interpolations in which the mercury triple point is replaced with the triple point of Xe, CO2 or SF6, which are widely considered to be potential candidates for replacing mercury in a future amendment of the ITS-90. The further alternative of using the melting point of gallium is also investigated.

ITS-90 SPRT calibration from the Ar TP to the Zn FP

Main textThis is a report to the Consultative Committee for Thermometry (CCT) on the key comparison 9 of Standard Platinum Resistance Thermometer (SPRT) calibration on the International Temperature Scale of 1990 (ITS-90) [1] from 83.8058 K (the Ar triple point) to 692.677 K (the Zn freezing point). The comparison followed a collapsed star protocol, with each of the fourteen participating laboratories shipping two standard platinum resistance thermometers (SPRTs) as transfer standards to the pilot laboratory (National Institute of Standards and Technology, NIST) for a direct comparison of SPRT calibration at the National Metrology Institute (NMI) of origin to calibration at NIST. Measurements were taken at the participating laboratories before and after measurement at NIST to assess the effect of transportation on the transfer standards. The pooled results from all laboratories were combined to calculate a key comparison reference value (KCRV).This report includes the calculation of, and comparisons to, the KCRV as well as bilateral comparisons between labs. In addition, there is space devoted to analysis of the shifts in SPRT values following travel.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Comparison report between NIM and CMS of the ITS-90 realization at the triple point of argon

Main textThe APMP key comparison APMP.T-K3.6 was initiated on the request of the Center for Measurement Standards, Taiwan (CMS) to link their standards to the Consultative Committee for Thermometry (CCT) Key Comparison (KC) and facilitate the CMS CMC submission. National Institute of Metrology, China (NIM) provided the linkage to the CCT-K3 at the temperature of the triple point of argon. In the APMP.T-K3.6, one standard platinum resistance thermometer (SPRT) was chosen as the transfer instruments, calibrated at the triple point of argon. The comparison was carried out in a participant-pilot-participant sequence where NIM served as the pilot. The linkage was based on the temperature of CMS reference Ar fixed point relative to the average (ARV) of CCT-K3 at the Ar fixed point via the difference in temperature between the NIM reference Ar fixed point and NIM reference Ar fixed point (links to the ARVs of the CCT-K3). The temperature differences between the standards of CMS and the ARVs of the CCT-K3 were within the evaluated comparison uncertainties of the ATPM.T-K3.6. This report provides detailed information on the comparison results, linkage mechanism, and the Degree of Equivalence of the CMS relative to the institutes having participated in the CCT-K3.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Realization of the triple point of carbon dioxide in a transportable cell using long-stem SPRTs

The action of the Minamata Convention requests in the near term to develop alternative fixed points to replace the mercury triple point (Hg TP) from the set of the defining points of the International Temperature Scale (ITS) of 1990. Carbon dioxide (CO2) TP is a candidate that exhibits the advantages of high chemical and physical stability and low health risk, and the disadvantage of relatively high pressure at room temperature. We report our study on the realization of the CO2 TP using long-stem standard platinum resistance thermometers (SPRTs). We present a novel design for a CO2 TP cell with thermometer immersion depth extending from 110 –200 mm with no increase in sample gas pressure. The outer dimension of the new cell is suitable for a drop-in compatible replacement of a normal cell for the Hg TP. We realized 14 plateaus of the CO2 TP using two long-stem SPRTs over a span of 12 months. The mean temperature at the CO2 TP is 216.591 30 (36) K at the melt fraction F = 0.5 and 216.591 36 (37) K at F = 1.0. Within the given uncertainties, the uncertainty arising from the transformation to ITS-90 contributes a major part of 0.29 –0.30 mK. The uncertainty formed by the pressure-head effect and the axial heat leak through the thermometer well of the new cell, the impurities of the sample gas and the inaccuracy of the electronic instruments contribute a smaller part of 0.21 –0.22 mK. The plateaus consistently exhibit small slopes of heights of less than 0.21 mK and time spans from 147 –347 h between F = 0.3 and F = 0.75. The small slopes of the plateaus demonstrate the repeatability of 0.056 and 0.059 mK, respectively, for the melt fraction F = 0.5 and 1.0, and the long-term stability of 0.031 mK.

Non-uniqueness and propagating uncertainties of the temperature scale realized using the triple point of sulfur hexafluoride

The non-uniqueness of the temperature scale realized on the standard platinum resistance thermometer (SPRT) calibrated at the triple point of sulfur hexafluoride (TPSF6) has been evaluated, given that SF6 is a potential candidate for substitution of the triple point of mercury (TPHg) which is indispensable point for the ITS-90. Three SPRTs calibrated at the TPSF6 were compared with a reference thermometer from 14 K to 250 K using a precise measurement system for capsule type thermometers. The uncertainty of the temperature scale due to propagation of the uncertainties for calibrating the SPRTs was also evaluated. The results showed that the non-uniqueness of the temperature scale of the SPRTs calibrated at the TPSF6 was almost equivalent to that of the current ITS-90. This indicates that the TPSF6 possesses the capability for realization of a modified ITS-90 which is equivalent to the current ITS-90.

Effect of Radio Frequency Signals on Resistive Measurements of Temperature

In many situations, the measurement of temperature is done with resistive sensors. This is the case with standard platinum resistance thermometers (SPRTs), industrial level PRTs, resistance temperature detectors (RTDs) and thermistors. There are many possible sources of uncertainty in these measurements. One potential source of error that may be difficult to quantify is the error due to radio frequency (RF) signals. This paper presents data from experimentation done with radio signals close to a temperature measurement location. While the data presented in this paper were not taken in a controlled RF measurement laboratory, they are done in a practical situation. The factors such a type of antenna used, walls between the antenna and measuring sensor, distance between the measuring sensor, and the RF source and length of the temperature sensor leads are disclosed. Different factors are considered such as frequency of the RF signal, power of the RF signal, resistance of the temperature probe and length of the temperature probe leads. Amateur radio equipment is used for the RF signal generation in the high frequency (HF), very high frequency (VHF) and ultra high frequency (UHF) bands. The author uses his knowledge of both amateur radio equipment and temperature metrology to present and analyze the data.

ITS-90 reproducibility, xenon fixed point substitution and new interpolating equations between 13.8033 K and 273.16 K

The International Temperature Scale of 1990 (ITS-90) underpins much of modern thermometry, yet efforts to comprehensively characterize and improve its reproducibility have been limited. Here, a low-uncertainty data set drawn from 13 standard platinum resistance thermometers is used to quantify all three main manifestations of scale irreproducibility over the range 13.8033 K to 273.16 K. Four overlapping ITS-90 subranges are investigated, along with three prospective approaches to temperature scale improvement. The current version of the Guide to the Realization of the ITS-90 is found to overestimate scale realization uncertainties, and recommendations are made to revise the guidance provided therein. Replacing the ITS-90 deviation function of subrange 3 with a new version further improves the interpolation quality of this subrange by a factor of two, and calibrating thermometers at the triple point of xenon rather than the triple point of mercury cuts the realization uncertainty of subrange 4 in half. New linear subranges spanning from the triple point of water to the triple points of mercury or xenon could be added to the scale without compromising the performance of the existing subranges. Combining new interpolation equations with calibration at the triple point of xenon provides the basis for a much more reproducible temperature scale than the ITS-90, with magnification of fixed point uncertainties nearly eliminated for all of subranges 1–4 above the triple point of oxygen.

Realization and evaluation of the triple point of sulfur hexafluoride

In this study, we realized the triple point (TP) temperature of sulfur hexafluoride (SF6) at the National Institute of Metrology (NIM), China. To this end, we designed an iso-thermal and immersion type apparatus with an open cell with a height of 18 cm, diameter of 5 cm, and a capacity of approximately 275 ml. The cell was designed with four thermometer wells for the simultaneous temperature measurements with four standard platinum resistance thermometers (SPRTs). The cell is connected with two buffer tanks that allow a sufficient amount of SF6 to surround the thermometer wells during the solidification and melting processes to ensure stable temperature plateaus with long durations. An SF6 sample with a purity of 99.9997%, which is the highest purity currently available, was supplied and analyzed from the Korea Research Institute of Standards and Science. The SF6 TP temperature was determined using four long-stem SPRTs. The melting plateaus obtained had a duration longer than 15 h, and the TP temperature of SF6 was determined to be 223.556 03(54) K (k = 1). These results are consistent with those of previous studies and further indicate that the SF6 TP is a good candidate to replace mercury TP, which is one of the fixed points defined in the International Temperature Scale of 1990, for the calibration of SPRTs.

Survey of subrange inconsistency of long-stem standard platinum resistance thermometers

The subrange inconsistency (SRI) of a large ensemble of long-stem standard platinum resistance thermometers (SPRTs), representative of worldwide production, has been investigated for all pairs of ITS-90 overlapping subranges between 83.8058 K and 933.473 K. The results are reported in terms of various statistical parameters to facilitate their comparison with the results of different authors, although a statistical test, applied to one specific pair of subranges, supported a Gaussian distribution of the results and justified the subsequent use of the mean and standard deviation as statistical parameters. Depending on the pair of overlapping subranges, the mean SRI as calculated varied from −1.23 mK to +0.21 mK and the SRI standard deviation varied from 0.04 mK to 0.62 mK. These numbers generally increased with the upper temperature limit of the pair of subranges, especially where the lower subrange requires a point which is not included in the upper subrange. The contribution to SRI from the fixed-point uncertainty propagation (PoU) was evaluated. The results showed that, although the effect of PoU on SRI largely cancels out for points common to both subranges, PoU still amounts to 59% to 130% of the differences between overlapping pairs of subranges. This means that the differences as calculated are probably a substantial overestimate of the true SRI. It is suggested that this effect is taken into account in making recommendations for typical uncertainties due to non-uniqueness.

Understanding immersion in zinc fixed-point cells using factorial design of experiments

The objective of this study was to investigate the ‘lift-off effect’ in zinc open cells, where an anomalously large change in temperature is observed when the standard platinum resistance thermometer (SPRT) is lifted off the bottom of the re-entrant tube during an immersion profile measurement. The goal was to determine the most important influences and hence what measures could be taken to mitigate it. The influences investigated were the filling gas, re-entrant tube material, re-entrant tube exterior surface condition, and the SPRT design. For each of these influence variables, three options were chosen. Since testing all combinations of different configurations arising would be extremely time consuming, the authors employed the optimal ‘design of experiments’ method using orthogonal arrays described by the Taguchi method, which is commonly used in manufacturing to optimise processes. Nine test configurations were identified, giving the necessary information on the interrelation between all influence variables. According to these results, the ideal configuration that yields the optimum immersion profile is a borosilicate tube, with full sandblasting and using nitrogen as filling gas for the cell. In addition, one particular SPRT design was found to be the least sensitive to the lift-off effect.

New interpolating equations of ITS-90 for SPRTs and HTSPRTs for the temperature range 419.527 °C–961.78 °C

The subrange inconsistency (SRI) of the International Temperature Scale of 1990 (ITS-90) arises from the use of different interpolating equations for the same standard platinum resistance thermometers (SPRTs), called Type 1 non-uniqueness, and is a significant component of the uncertainty in the realization of the scale. In order to eliminate the SRI, new interpolating equations were proposed for the subranges 0.01 °C to 660.323 °C and 0.01 °C to 961.78 °C, in which the subrange to the zinc point (419.527 °C) was extended using two additional terms without any overlap. This study investigated 30 SPRTs calibrated over the temperature range of 0.01 °C–660.323 °C and 19 high-temperature SPRTs (HTSPRTs) calibrated over 0.01 °C–961.78 °C at the National Institute of Metrology (NIM). The feasibility of the proposed equations were evaluated by comparing them with the equations defined in ITS-90 and calculating the propagation uncertainties. The results show that the differences between the defined interpolating equations in ITS-90 and the proposed interpolating equations are generally within ± 0.4 mK for temperature range of 419.527 °C–961.78 °C, which is close to or less than the SRIs between the water-zinc and water-aluminium subranges. The propagated uncertainties of the proposed equations were within reasonable limits. It is concluded that these new interpolating equations can be taken as equivalent to the ITS-90 interpolating equations with the SRI eliminated.

ITS-90 non-uniqueness and evaluation of characteristics of new 1000 Ω type platinum resistance thermometers for low-temperature measurement

For use in a low-temperature region below the triple point of water (TPW), the National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) developed a precise comparison system of capsule-type standard platinum resistance thermometers (CSPRTs) for dissemination of the International Temperature Scale of 1990 (ITS-90). This study compared eight CSPRTs with a reference CSPRT using the comparison system to study non-uniqueness at low temperature. We also evaluated characteristics of a new type of low-temperature PRT with about 1000 Ω of resistance at TPW (PT-1000s), which was recently developed by Netsushin Co, Ltd. to obtain better temperature sensitivity of resistance at a low temperature. The PT-1000 sensor is made from a high-grade platinum wire, similarly to standard PRTs of the ITS-90. We calibrated four PT-1000s based on the ITS-90 and evaluated their characteristics by comparison with a reference CSPRT. Results indicate that the character of the temperature scale of PT-1000s is very close to that of CSPRTs down to 14 K.

Challenges for In-Flight Calibration of Thermal Infrared Instruments for Earth Observation

Satellite instruments operating in the thermal infrared wavelength range >3 µm provide information for applications such as land surface temperature (LST), sea surface temperatures (SST), land surface emissivity, land classification, soil composition, volcanology, fire radiative power, cloud masking, aerosols, and trace gases. All these instruments are dependent on blackbody (BB) calibration sources to provide the traceability of the radiometric calibration to SI (Système International d’Unités). A key issue for flight BB sources is to maintain the traceability of the radiometric calibration from ground to orbit. For example, the temperature of the BB is measured by a number of precision thermometers that are calibrated against a reference Standard Platinum Resistance Thermometer (SPRT) to provide the traceability to the International Temperature Scale of 1990 (ITS-90). However, once calibrated the thermometer system is subject to drifts caused by on-ground testing, the launch and space environments. At best the uncertainties due to thermometer ageing can only be estimated as there is no direct method for recalibrating. Comparisons with other satellite sensors are useful for placing an upper limit on calibration drifts but do not themselves provide a traceable link to the SI. In this paper, we describe we describe some of the technology developments, including phase change cells for use as reference standards, thermometer readout electronics and implementation of novel coatings, that are in progress to enhance the traceability of flight calibration systems in the thermal infrared.