The reproducibility of some thermometric fixed points and the accuracy of temperature measurements using platinum resistance thermometers

Abstract

The reproducibility of some thermometric fixed points and the accuracy of four platinum resistance thermometers (PRTs) were studied. It was found that the fixed points of aluminium (Al), zinc (Zn), tin (Sn), indium (In) and gallium (Ga) were realized reproducibly within ± 0.17 mK; ± 0.11 mK; ± 0.10 mK; ± 0.13 mK and ± 0.12 mK, respectively. Because the actual impurities and their concentration in our samples (of 99.9999 % or 99.999 99 % purity) are unknown, the systematic uncertainty due to impurities cannot be estimated. However, any of the samples of Ga, In, Sn, Zn and Al is consistent with the rest within ± 0.2 mK, using a cubic or quadratic deviation function, in the temperature range 0 C to 660 C. This indicates that the effect of impurities is negligible.Four PRTs were selected at random. They were calibrated repeatedly, first up to the Zn point and then up to the Al point. The resistance of each PRT drifted. From time to time, for each PRT, a seemingly well-established resistance drift suddenly and unpredictably changed to a different rate of drift. Occasionally, the resistance of the PRTs shifted. Such unpredictable changes obviously limit the accuracy of temperature measurements using PRTs no matter what the accuracy of their calibrations. In the case of our four PRTs, the uncertainty of temperature measurements near 660 C ranged from about ± 1 mK to about ± 2.5 mK even though they were all calibrated at all fixed points well within ± 0.25 mK uncertainty. Possible explanations are offered for the apparently permanent drifts and the erratic shifts in the resistance of the PRTs. Some comments are made concerning the ambiguity of "immersion tests" in general. The furnaces of the National Research Council of Canada used in this work are high-temperature adiabatic calorimeters.

System for Realizing the Triple Point of Mercury to Calibrate Industrial-Type Platinum Resistance Thermometers

“…On the other hand, simple experiments have been reported [4,5] for open cells and very simple experimental conditions (using slim cells) where the sample is frozen when placed in a mixture of "dry ice" (CO 2 ) and alcohol, and afterwards the cell is placed in a Styrofoam pouch to let the melt proceed naturally. More stringent experimental conditions are required to use adiabatic calorimetric techniques to reproduce the TP-Hg [5].…”

Comparing fixed points of the ITS-90 as realized in Canada, Argentina and Brazil

“…A small sample of Al (66 g) in its quartz housing units, similar to that described in [3], was also carried to be used in a furnace of the host laboratory. The INMETRO Ar point was compared with the portable Ar calibrator (also carried as luggage, and developed at the NRC [4]). In the first host laboratory (INTI), as mentioned above, the entire comparison was closely observed by the workshop participants.…”

“…During high-temperature use, the states of anneal and oxidation of the PRT can rapidly change. During thermal cycling, its dimensions can change unpredictably and cause irreproducibility [5].…”

“…Its quenched will usually be 7 mK to 30 mK above its well-annealed value (see Tables 8, 9 and 11), a state that is also ambiguous. A state may be considered to be well annealed if both the and of a deoxidized PRT remain reasonably constant during repeated calibrations at the Zn point (apart from the occasional, erratic, shifts of the resistance on thermal cycling [5]).…”

“…The above procedure is rapid and highly reproducible (see Figure 16 of [5]). For high-purity samples, it is equivalent to conventional methods of realizing fixed points.…”