I. Yang, C. H. Song, Y.-G. Kim & K. S. Gam

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Gam International Journal of Thermophysics Journal of Thermophysical Properties

1 Cryostat for Fixed-Point Calibration of Capsule-Type SPRTs I. Yang, C. H. Song, Y.-G. Kim & K. S. Gam International Journal of Thermophysics Journal of Thermophysical Properties and Thermophysics and Its Applications ISSN X Volume 32 Combined Int J Thermophys (2011) 32: DOI /s

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3 Int J Thermophys (2011) 32: DOI /s Cryostat for Fixed-Point Calibration of Capsule-Type SPRTs I. Yang C. H. Song Y.-G. Kim K. S. Gam Received: 24 March 2010 / Accepted: 12 July 2011 / Published online: 28 July 2011 Springer Science+Business Media, LLC 2011 Abstract A cryostat for fixed-point calibration of capsule-type SPRTs (standard platinum resistance thermometers) was developed. Using this system, cryogenic fixed points defined on the International Temperature Scale of 1990 (ITS-90) were realized. The cryogenic cells were argon, oxygen, neon, and two equilibrium-hydrogen (e-h 2 ) cells, made by INRiM, Italy. The uncertainty of the realization of each fixed point was estimated to range from 0.53 mk to 0.43 mk (k = 2). The realizations of the triple point of e-h2 using two sealed cells coincided within 0.1 mk. Therefore, we are able to calibrate capsule-type SPRTs down to K within an uncertainty of 1 mk (k = 2) by this system. A closed-cycle helium gas refrigerator was used for the cryostat. Each sealed cell was designed so that it could accommodate three sealed cells in the thermometer wells made within the cell. Therefore, the cryostat was designed to accommodate only one sealed cell at a time. The base temperature of this liquid-free cryostat, when one sealed cell and three capsule-type SPRTs were attached for calibration, was 17 K. For the realization of the triple point of e-h 2,weused liquid helium for additional cooling. Adiabatic melting of the triple point was realized by controlling the inner-most radiation shield at a temperature very close to that of the triple point, and by applying a heat pulse by a heater directly wound to the cell. The amount of the heater power and the waiting time for the thermal equilibrium after each heat pulse were chosen in a way that the adiabatic melting could be finished within 6 h for each cell. The triple point of each cryogenic fixed point was deduced from the equilibrium temperatures between the heat pulses and subsequent extrapolation to the liquidus point. For the oxygen cell, temperatures of two solid solid transitions I. Yang (B) C. H. Song Y.-G. Kim K. S. Gam Division of Physical Metrology, Korea Research Institute of Standards and Science, Daejeon , Korea iyang@kriss.re.kr

4 2352 Int J Thermophys (2011) 32: (α β and β γ transitions) were also measured, and the results were consistent with values reported in the literature within the designated uncertainty. Keywords Cryostat Low-temperature thermometry Standard platinum resistance thermometer 1 Introduction Below the triple point of mercury ( K), the fixed points defined in the International Temperature Scale of 1990 (ITS-90) are, from high to low, the triple points of argon, oxygen, neon, and equilibrium-hydrogen (e-h 2 ) [1]. In the range of K to K, the standard platinum resistance thermometers (SPRTs) calibrated at the above fixed points and the triple points of water and mercury play the role of standard interpolating thermometers. We built a cryostat system to calibrate capsule-type thermometers at the above cryogenic triple points. The lower temperature limit of the calibration was K. For the range below this temperature, a gas thermometer or a vapor-pressure thermometer must be constructed and used to realize the temperature scale. Considering the low demand from the industry, we aim to provide customer calibration services from K to K for capsule-type SPRTs, as this temperature range requires only triple-point measurements for calibration of the thermometers. In Sect. 2 of this paper, we describe detailed information on the instruments that we built or used to realize cryogenic triple points using the sealed cells. In Sect. 3,we show experimental results of the realization of the triple points. In Sect. 4, we assess the uncertainty of the realization of triple points. Finally in Sect. 5, we conclude this paper. 2 Instruments 2.1 Sealed Cell The sealed cells used in this work were manufactured by INRiM, Italy in Three capsule-type thermometers of diameter less than 5.4 mm can be calibrated with these cells. We have one cell for each fixed point except for the e-h 2 point, for which we have two. We realized all the cryogenic triple-point temperatures using these sealed cells, and also compared the results from the two e-h 2 cells. In the case of the e-h 2 cells, Gd 2 O 3 catalyst was present for rapid spin conversion. 2.2 Cryostat For the experiment, a cryostat was manufactured and used. A closed-cycle helium gas refrigerator was used to provide cooling power to the cryostat. A schematic diagram of the cryostat is shown in Fig. 1. When one sealed cell and three capsule-type SPRTs were installed in the cryostat, the lowest achievable temperature of the cryostat

5 Int J Thermophys (2011) 32: Fig. 1 Cryostat for fixed-point calibration was 17 K. Therefore, for realization of the triple point of e-h 2, liquid helium was circulated to the two outer radiation shields to provide additional cooling power. On the cryostat, three monitoring thermometers (T1, T2, and T3) and two heaters (H1 and H2) were installed for temperature control. Specifically, silicon diode thermometers were installed on the outer most shield (T1), middle radiation shield (T2), and inner-most shield (T3). The two heater wires were made of Manganin alloy. Heater H1 controls the temperature of the inner-most radiation shield with monitoring thermometer T3. The resistance of heater H1 was about 70 at room temperature. The heater wire of H2 (room-temperature resistance of 21 ) was wound around the cell and Joule heat was directly applied to the cell to provide pulse heating during realization of the triple points. The resistance of heater H2 was measured at each triple-point temperature with the four-wire method to exclude the effect of lead wires. The resistance of H2 was 19.4 at the triple point of argon, and 18.2 at the triple point of e-h 2. The electrical wires, which were made of copper with a diameter of

6 2354 Int J Thermophys (2011) 32: Table 1 Set parameters of the resistance bridge F900 for this experiment Parameter Set value Sensitivity 10 4 Bandwidth 0.1 Hz Frequency Low Carrier current 1 ma (for argon and oxygen) 2mA(for neon and e-h 2 ) Standard resistor 25 (for argon, oxygen, and neon) 1 (for e-h 2 ) Source impedance 100 (for argon, oxygen, and neon) 1 (for e-h 2 ) 0.1 mm, were connected to measuring instruments at room temperature via vacuum feedthroughs. 2.3 Temperature Measurement and Control We used an F900 resistance ratio bridge made by ASL Ltd. (UK) for measurement of the resistance of the SPRT, with a nominal resistance of 25 at the triple point of water. The setting parameters of the bridge for this experiment are listed in Table 1. For temperature control of the inner-most radiation shield, heater H1 was connected to an LSCI 340 temperature controller (Lakeshore, USA) together with monitoring thermometer T3, which acts also as a control thermometer. The control resolution was 1 mk. In liquid-free operation, temperature control of about ±2 mk around the set temperature was possible. When liquid helium was used to achieve lower temperatures, temperature control of about ±10 mk was obtained. An LSCI temperature monitor Model 218 from Lakeshore was used to monitor the two silicon diode thermometers T1 and T2. The temperature was read in kelvin and recorded by a home-made data acquisition program. The resolution of this monitor is 0.01 K. To provide pulse heating to the cell to realize triple points, the power across heater H2 was controlled by an E3631A DC power supply (Agilent, USA) and a home-made control program. 2.4 Vacuum System To maintain a vacuum inside the cryostat, we used a turbo pumping system. When the seal was properly ensured, the pressure inside the cryostat was near Pa when pumped continuously for 24 h, and near 10 5 Pa when pumped for several days. 3 Results Figure 2 shows typical results of the temperature measurement monitored by T1 and T2 while the triple point of e-h 2 was realized. For realization of the e-h 2 point, the

7 Int J Thermophys (2011) 32: T2 Temperature, K T1 t 1 t Time, h Fig. 2 Temperature measured by T1 and T2 during cool-down for the realization of the triple point of e-h 2 lowest temperature without the use of liquid helium as a coolant can be reached after 15 h to 20 h of cooling from room temperature. In Fig. 2, liquid helium was slowly transferred to the cryostat at t 1, and then the temperature of the cryostat quickly fell below the triple point of e-h 2.Att 2, heater H1 was turned on for the adiabatic realization of the triple point of e-h 2. At each fixed-point realization, the point of the temperature controller must be as close as possible to the triple-point temperature for adiabatic realization. Using thermometer T3 un-calibrated would cause a temperature offset in the realization of the triple points. Therefore, through preliminary realizations, we first calculated the temperature offset of the T3 sensor at each fixed point, and used the offset value to realize the triple points thereafter. Figure 3 shows the results of the adiabatic melting curve through pulse heating at each cryogenic triple point. In the case of e-h 2, the results of the triple-point temperature measurement of the two cells coincided within 0.1 mk. The arrow in the figure indicates a temperature range of 10 mk. The realizations of the four triple points were performed through the same heater wires in the same cryostat system. According to the amount of triple-point material and its latent heat, the appropriate amount of current and heating time should be chosen. Table 2 summarizes the current I h, the voltage V h, the total heating time t h, and the total amount of Joule heat Q T for realization of the four triple points. As the resistance of heater H2 was measured with the four-wire method, Q T is the heat supplied to the cell, excluding the heat dissipated on the lead wires. The table gives the voltage applied by the DC power supply, not the voltage across the heater wire. Therefore, when calculating the total amount of Joule heat, both the heater wire resistance and the current have to be taken into account. For comparison, the latent heat of fusion Q T,cal calculated from the number of moles of gas sealed in each cell provided by the manufacturer was also indicated.

8 2356 Int J Thermophys (2011) 32: (a) Ar 10 mk (b) O 2 10 mk Resistance, Ω (c) Ne mk (d) e-h mk Time, h Fig. 3 Realization of the cryogenic triple points by the pulse-heating method for (a) argon, (b) oxygen, (c) neon, and (d) e-h 2 Table 2 Parameters related to the pulse heating during the realization of the triple points Fixed point I h (ma) V h (V) t h (min) Q T (J) Q T,cal (J) Argon Oxygen Neon Hydrogen From the results of the melting curves induced by pulse heating, we calculated the resistance of the SPRT at the triple points. For this, we first expressed the equilibrium temperature as a function of the melting fraction F. We then applied two methods suggestedin[2]. In the first method, the initial and final 15 % of the data are discarded,

9 Int J Thermophys (2011) 32: Fig. 4 Analysis to deduce the resistance at the triple point of e-h 2. Solid line indicates R 15 85, the resistance measured from the middle 70 % of the melting plateau, while the dashed line indicates the linear fit of the resistance as a function of 1/F Table 3 Resistance deduced from two methods and the difference of two results in temperature R ( ) R 1/F ( ) T (mk) Ar O Ne e-h and the average of the middle 70 % of the resistance measurement is taken to calculate R In the second, the resistance is plotted as a function of 1/F, a linear fit is obtained using the data in a range of 2 < 1/F < 5, and the fitted line is extrapolated to 1/F = 1 to calculate R 1/F. Figure 4 shows the analysis of the results of the pulse-heating melting of the e-h 2. The solid line represents R 15 85, and the dashed line represents R 1/F. Table 3 summarizes the results of this analysis for each fixed point. The differences in temperature between the two methods of determining triple points were less than or equal to 0.1 mk for Ar, O 2, and Ne and 0.3 mk for e-h 2. Figure 5 shows the deviation function of the SPRT measured at each fixed point. For calculation of the resistance at the fixed points, the zero-current corrected value was used. In the case of the triple points of water and mercury, a hydrostatic correction was applied. Furthermore, in the case of mercury, a correction due to the results of the international comparison was also applied to the calculation of W. For the oxygen cell, the α β and β γ solid solid transitions were also observed. The temperatures of these transitions were measured and compared to values reported in the literature [3]. Table 4 summarizes the results of the temperature measurement of the solid transitions with their uncertainties (k = 2). The two results were consistent with the combined uncertainties.

10 2358 Int J Thermophys (2011) 32: ΔW W Fig. 5 Deviation W at the cryogenic fixed points for the SPRT calibrated in this work Table 4 Temperatures of the solid solid transitions of oxygen measured in this work and cited in the literature Solid solid transition Measured in this work (K) Literature value (K) α β ± ± β γ ± ± Uncertainties Table 5 shows the calibration uncertainty of the capsule-type SPRT at the cryogenic triple points. The sealed-cell uncertainty was taken from the calibration certificate provided by INRiM. The uncertainty due to the determination of the plateau value was taken from the observed variation of the resistance ratio at the state of the triple points. The plateau repeatability was estimated from the repeated measurements. The self-heating effect was taken as 100 % of the measured self-heating effect, but the amount was negligible relative to the combined uncertainty in the case of the capsule-type thermometers. The heat-flux effect was estimated from the measured triple point while the temperature controlled by heater H1 was varied. The uncertainty from the standard resistors was taken from their calibration certificate, and converted to temperature. The hydrostatic head correction was calculated assuming that the center of the thermometer was located at a depth of 2 cm from the liquid gas boundary, and 100 % of this value was used as the uncertainty of the depth. Finally, the propagation from the TPW uncertainty was calculated from the realization uncertainty of the triple point of water at our lab, 0.1 mk. Including all the effects above, the combined and expanded uncertainties were calculated. As shown in Table 5, the uncertainty of the calibration at the fixed points was about 0.5 mk (k = 2). Even after including the uncertainty due to the nonuniqueness of the subranges of ITS-90 [4], SPRT

11 Int J Thermophys (2011) 32: Table 5 Uncertainty budget of the calibration of the SPRT at cryogenic triple points Uncertainty components Ar (mk) O 2 (mk) Ne (mk) e-h 2 (mk) Sealed-cell uncertainty Determination of the plateau value Plateau repeatability Self-heating Heat flux Standard resistors Hydrostatic head correction Propagation from TPW uncertainty Combined uncertainty (k = 1) Expanded uncertainty (k = 2) calibration in the low-temperature ranges of ITS-90 with an uncertainty 1mK(k = 2) is possible using this system. 5 Conclusions From this work, it is now possible to calibrate capsule-type SPRTs from the triple points of neon to water. An open-type system for realization of the triple points was constructed and used for CCT-K2. However, KRISS did not submit the results of equilibrium hydrogen to the international comparison. The next step will be realization of all the cryogenic triple points using the open-type triple-point system, and comparison of the results of the two realization systems. References 1. H. Preston-Thomas, Metrologia 27, 3 (1990) 2. Bureau International des Poids et Mesures (BIPM), Supplementary Information for the International Temperature Scale of 1990 (BIPM, Sèvres Cedex, France, 1997) 3. R.E. Bedford, G. Bonnier, H. Mass, F. Pavese, Metrologia 33, 133 (1996) 4. B.W. Mangum, P. Bloembergen, M.V. Chattle, B. Fellmuth, P. Marcarino, A.I. Pokhodun, Metrologia 34, 427 (1997)

European Association of National Metrology Institutes

European Association of National Metrology Institutes EURAMET GUIDELINES ON TEMPERATURE: Extrapolation of SPRT calibrations below the triple point of argon, 83.8058 K, and traceability in baths of liquid