Int J Thermophys (2017) 38:37 DOI 10.1007/s10765-016-2171-9 TEMPMEKO 2016 A Sub-millikelvin Calibration Facility in the Range 0 C to 30 C R. Bosma 1 A. Peruzzi 1 R. Van Breugel 1 C. Bruin-Barendregt 1 Received: 21 June 2016 / Accepted: 16 December 2016 / Published online: 5 January 2017 Springer Science+Business Media New York 2016 Abstract A new sub-millikelvin calibration facility for the range 0 Cto30 Cis described, that allows calibration of customer thermometers, other than standard platinum resistance thermometers, with an uncertainty lower than 1 millikelvin. The improvements with respect to the traditional calibration facility are reported with particular emphasis on the temperature control (better than 0.2 mk), resistance measurement and calibration procedure. The new facility was validated by using 6 standard platinum resistance thermometers and the calibration uncertainty in the range from 0 Cto30 C amounted to 0.31 mk 0.35 mk. To demonstrate the potentiality of this facility, two oceanographic thermometers, Sea-Bird Electronics SBE 3 and SBE 35, were calibrated with an expanded uncertainty of 0.8 mk (k = 2). Keywords Calibration Deep ocean thermometer Sub-millikelvin 1 Introduction In the range from 0 Cto30 C, the realization and dissemination of the ITS-90 temperature scale are performed using two fixed points, water and gallium, and a standard platinum resistance thermometer, SPRT [1]. The scale between the fixed points is based on the interpolating function for the SPRT, W r (T 90 ), and the deviation function, W (T 90 ) W r (T 90 ). The uncertainty of the SPRT is then well below the millikelvin Selected Papers of the 13th International Symposium on Temperature, Humidity, Moisture and Thermal Measurements in Industry and Science. B R. Bosma rbosma@vsl.nl 1 VSL, Dutch Metrology Institute, Delft, The Netherlands
37 Page 2 of 8 Int J Thermophys (2017) 38:37 Fig. 1 Deep ocean thermometers Sea-Bird, Model SBE 35 (a) and Model SBE 3 (b) level and at the fixed points even near tenth of microkelvin. For the dissemination of the ITS-90 scale, other thermometers, such as thermistors, are calibrated by comparison with the SPRT s in liquid baths [2], but the best uncertainties attainable for these services are at the moment typically several millikelvin. In the framework of the European Metrology Research Programme, project MeteoMet 2, VSL needed to calibrate two deep sea ocean thermometers, the Sea- Bird Electronics SBE 3 and SBE 35 (see Fig. 1) in the range from 0 Cto30 C. The uncertainty of the calibration needed to be well within 1 mk to meet the requirements of the oceanografic community. A paper on the Effect of pressure on deep-ocean thermometers is under review in the same journal. While the SBE 35 design allows it to be calibrated at the fixed points, the SBE 3 has only a very thin thermometer stem of 0.8 mm diameter and 60 mm length, which does not suit the fixed points. As both thermometers are thermistor-based instruments, there is not a reference function similar as for a standard platinum resistance thermometer, SPRT. In this paper, the modifications are described made to a conventional calibration bath in order to obtain the sub-millikelvin uncertainty level. Special attention is given to the temperature control of the bath, the temperature stability and uniformity and the non-uniqueness contribution. The new set-up was validated using multiple SPRT s and as an example of sub-millikelvin calibration, two deep ocean thermometers, a SBE 3 and a SBE35, were calibrated. 2 Realization of the Sub-millikelvin Set-up in the Range from 0 Cto 30 C The comparison calibrations in the range from 0 Cto30 C at VSL are realized using an ice point, an ethanol bath (below 10 C) and a water bath (above 10 C). The baths temperatures are controlled with controllers using Pt100 control sensors. The reference SPRT and the customer thermometer(s) are inserted in the wells of a comparison block, which can be submerged from a few centimetre to up to 45 cm (stem length of SPRT s)
Int J Thermophys (2017) 38:37 Page 3 of 8 37 Table 1 CMC claim in water bath at 10 C for Pt100 with the previous calibration measurement capability, CMC, and the CMC for a SPRT calibrated with the sub-millikelvin facility Facility Previous CMC Sub-millikelvin CMC Temperature / C 10 0u i /mk 28 Reference thermometers(s) Calibration 0.24 0.14 0.14 Selfheating 1.00 0.02 0.02 Bridge 0.10 0.05 0.05 Resistor 0.10 0.10 0.11 Customer thermometer Bridge 0.25 0.05 0.05 Resistor 0.12 0.10 0.11 Repeatability 0.13 0.10 0.10 Bath/comparator block 1.60 0.20 0.20 Non-uniqueness 0.00 0.16 Combined uncertainty 1.9 0.31 0.35 Expended uncertainty (k = 2) 3.9 0.62 0.71 in the liquid. The reference SPRT s are measured with a resistance bridge at 1 ma, without selfheat correction, but adding an uncertainty component in the calibration uncertainty (in most commercial calibrations the calibration time is found to be more important than the lowest calibration uncertainty). A typical uncertainty claim for a comparison calibration in this temperature range is 4 mk (see Table 1). To reduce the calibration uncertainty below 1 mk the following actions were needed: Improve the temperature control for stability and uniformity, Minimize the influence of immersion effect of reference and customer thermometers and uncertainty from bath validation, Reduce the uncertainty in the resistance measurement, Reduce the uncertainty component from the selfheat effect on reference thermometer. The Tamson TV7000SP water bath at VSL is normally used in the range from 10 C to 80 C, and the control stability, measured with a SPRT in the comparison block, is between 2 mk and 3 mk (see Fig. 2). To improve the control stability, a control set-up was used similar to the low-temperature cryostat of VSL [3]. A SPRT was used as control sensor, read by 71/2 digit digital multimeter, DMM, and a dc power supply as control actuator. A computer with a PID algorithm controlled the operation of DMM and power supply. No selfheat correction was used for the control sensor, because only the temperature stability is important and not the absolute temperature. As the control power was only 300 W, the total temperature range could not be covered with a cooler at a fixed temperature setpoint. An optimum was found with an offset of 3 C between the setpoint of the bath, t SP, and the temperature of the cooler, t PV,Cooler. Figure 3
37 Page 4 of 8 Int J Thermophys (2017) 38:37 Fig. 2 Stability in the comparison block, t 90 t SP, in Tamson water bath using the Hart Scientific 2100 temperature controller shows the stability of the bath temperature, t PV, and the stability of the temperature in the comparison block, t 90. The monitored stability of the bath, shown in Fig. 3a and obtained from the DMM reading, is limited by the settings of the DMM, which are a trade-off between sampling time and filtering. The effective stability in the comparison block is shown in Fig. 3d, which shows the improvement with respect to the old set-up, shown in Fig. 2. The influence of immersion was minimized by performing the measurements only at full immersion, which means between 40 cm and 45 cm, the bath validation uncertainty was also reduced by performing the measurement with 2 SPRT s at opposite positions in the comparison block and measured the SPRT s before and after the customer thermometer, providing in situ information about the stability and uniformity of the temperature in the comparison block. As the bath had two independent heaters, both the new controller and the default controller were used, using the default controller as booster (setpoint 0.5 C below t SP ). The uncertainty in the resistance ratio measurement was reduced from 1 10 6 to 5 10 8 by using an ASL-F18 instead of an ASL-700, and the uncertainty component for the selfheat was reduced by correcting for the selfheating effect. Consequence of using an ASL-F18, performing selfheat correction and using 2 SPRT s, was the longer calibration time: the measurement time increased from less than 5 min to more than 30 min. 3 Validation of the Facility Using Calibrated SPRT s For the validation of the facility, 6 thermometers (see Table 2) were used and all were calibrated at fixed points in the range Hg to Ga or Ar to Ga. Two thermometers, R3416 and R3194, were used as reference and measured the reference temperature, t 90, and the other four thermometers were regarded as units under test (UUT). The
Int J Thermophys (2017) 38:37 Page 5 of 8 37 Fig. 3 Stability, t PV t SP, in the bath (a) over the range from 0 Cto30 C together with the cooling bath temperature and offset, t 90 t PV,Cooler (b) and the control power, P, (c) and the temperature stability, t 90 t SP, in the comparison block (d) Table 2 Overview of the thermometers used for the validation of the new sub-millikelvin set-up with calibration range and uncertainty, u(t) Identification Manufacturer Type Serial number Calibration u(t) range mk R3416 Rosemount 162CE 3416 Hg Ga 0.25 R3194 Rosemount 162CE 3194 Hg Ga 0.20 LN4240 Leeds & Northrup 8163 1 724 240 Hg Ga 0.18 LN8869 Leeds & Northrup 8167-25 1 868 869 Hg Ga 0.18 R2649 Rosemount 162CE 2649 Ar Ga 0.42 T274686 Tinsley 5187 SB 274 686 Ar Ga 0.34
37 Page 6 of 8 Int J Thermophys (2017) 38:37 Table 3 Results of the validation of the sub-millikelvin set-up using the R3416 and R3194 as standard to obtain the reference temperature, t 90,and LN4240 as UUT. The uncertainty for the UUT is 0.18 mk t 90 u(t 90 ) t UUT t 90 E n R3416 & R3194 LN4240 C mk K K/K 5.9988 0.24 0.04 0.1 2.9989 0.25 0.12 0.2 0.1991 0.23 0.00 0.0 8.9987 0.24 0.06 0.1 11.9989 0.24 0.14 0.2 17.9990 0.24 0.24 0.4 20.9989 0.24 0.23 0.4 23.9988 0.27 0.13 0.2 26.9988 0.26 0.04 0.1 29.4991 0.42 0.31 0.5 14.9988 0.23 0.28 0.5 19.9987 0.28 0.12 0.2 24.9989 0.25 0.24 0.4 Fig. 4 Uncertainty contribution of the reference temperature including the standard deviation of the measurement and the comparator block stability and uniformity (a) and the E n -value for all UUT thermometers in the validation test (b) measured resistance, R UUT, was then transformed into temperature, t UUT,usingthe fixed point calibration coefficients, and the obtained temperature was compared with the temperature indication of the two reference thermometers. Table 3 shows the results for the measurement with LN4240 as UUT. The difference between the UUT and reference temperature, t UUT t 90, divided by the combined uncertainty of the UUT and the reference thermometers, E n, is smaller than 1 over the whole range. The uncertainty of the reference temperature, u(t 90 ), in Table 3 is a combination of the measurement standard deviation and temperature stability and uniformity in the comparison block, but is exclusive the calibration uncertainty of the reference standards. The contribution
Int J Thermophys (2017) 38:37 Page 7 of 8 37 Fig. 5 Sea-Bird SBE 3 (a) and SBE 35 (b) during the calibration in the water bath Fig. 6 Results of the Sea-Bird SBE 35 (a) and SBE 3 (b) calibration. The dotted lines show the uncertainty of the trend line is smaller than 0.2 mk over nearly the whole range. Only at 30 C, the uncertainty contribution is higher. Figure 4b shows that the E n -value for all UUT thermometers is smaller than 1 and that the facility is capable of calibrating thermometers with an uncertainty smaller than 1 mk (see Table 1).
37 Page 8 of 8 Int J Thermophys (2017) 38:37 4 Calibration of Deep Ocean Thermometers The two Sea-Bird Electronics thermometers were calibrated by comparison with the same two thermometers used during the validation, R3416 and R3194. Due to the short length of the SBE 3 sensor, this thermometer was completely submerged in the water bath (Fig. 5a) and the head of the SBE 35 was above the water (Fig. 5b), similar to the two SPRT s. The calibration was performed from 0 Cupto30 C in steps of 3 C(seeFig.6). The results for the SBE 35, shown in Fig. 6a, also include the fixed point measurements, which agree very well with the comparison measurements. The uncertainty of the linear interpolation, representing the reproducibility of the sensor, is 0.19 mk for the SBE 35 and 0.24 mk for SBE 3, and the total calibration uncertainty for the thermometers becomes 0.39 mk and 0.42 mk, respectively. 5 Conclusions Using a high-accuracy temperature controller, a facility was built for calibrations with sub-millikelvin uncertainty in a conventional calibration bath. The CMC for the facility is smaller than 0.7 mk (k = 2), which was confirmed through the validation using 6 SPRT s. The facility was used to calibrate two oceanographic thermometers, a Sea-Bird Electronics, SBE 3 and SBE 35, which were calibrated with a expanded uncertainty of 0.8 mk (k =2). Acknowledgements This work was performed in the frame of the European Metrology Research Programme (EMRP) Joint Research Project ENV58 MeteoMet2. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. References 1. BIPM, techniques for approximating the international temperature scale of 1990, Paris, July (1990) 2. J. Bojkovski, V. Batagelj, J. Drnovšek, V. Žužek, XIX IMEKO World Congress, Fundamental and Applied Metrology, September 6-11, Lisbon, Portugal, pp. 1581 1584 (2009) 3. R. Bosma, R. Van Breugel, C. Bruin-Barendregt, A. Peruzzi, AIP Conf. Proc 1552, 480 (2013). doi:10. 1063/1.4819588