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7/27/2019 The Method of Instantaneous Comparisons
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© Q u e e n ’ s P r i n t e r a n d C o n t r o l l e r o f
H M S O
, 2 0 1 3
. 1 0 6 4 3 / 1 0 1 3
www.npl.co.uk
The Method of Instantaneous Comparisonsapplied to the Calibration of SPRTs
in Liquid Nitrogen and Liquid ArgonR L Rusby, G J M Sutton, L R Stanger and R I Veltcheva
National Physical Laboratory, Hampton Road, Teddington, TW11 0LW, UK
Email: [email protected]
References1. Euramet Technical Guide No 1, Extrapolation of SPRT Calibrations below the Argon Triple Point, 83.8058 K, and Traceability in baths of Liquid Nitrogen
at ~77.3 K , www.euramet.org (2011).
2. Ward SD and Compton JP, Intercomparison of Platinum Resistance Thermometers and T68 Calibrations, Metrologia 15, 31-46, (1979)
3. Inseok Yang, Chang Ho Song, Kee Hoon Kang and Yong-Gyoo Kim, First-order Compensation or Time Lag in the Dynamic Calibration o Industrial Thermometers, Measurement Science and Technology20 (2009) 015109 (6pp)
Guide tubesupport plate
L-SPRT
Loose foamcover
Copper block
C-SPRT
2 of 4connections
to C-SPRT
1 of 3 steelsupport rods
1 of 4 SPRTguide tubes
Liquid nitrogen
Dewar vessel(stainless steel)
Foam plugs
Foam liner
Polystyreneplatform
Vacuum
The method
Instead o measuring the SPRT resistances separately with respectto a standard resistor, a calibrated SPRT is selected as the reerence,R
re . All the other resistors (standard R
s, and SPRTs R
1, R
2, etc.) are
measured relative to it.
Step 1: measure Rs
/ Rre
to obtain the value o Rre
,
Step 2; measure R1
/ Rre
rom which R1
is determined,
Step 3: similarly or R2, R3
, etc.
The R1
/ Rre
etc., are essentially instantaneous comparisons withrespect to R
re . For like thermometers they are substantially
independent of temperature: between LN2 and LAr the SPRT ratiostypically change by < ~200 parts in 106, equivalent to about 1 mK per kelvin temperature change.
Thereore
• they are substantially independent of (uniform) temperaturechanges during the comparison sequence, and
• the comparisons apply as if they had all been made at the initial temperature of R
re .
The ratios can be measured at an additional current to establish thesel-heating corrections, with no concerns over drit except in thecase o R
s/ R
re.
Three experiments are described in which the method is appliedto comparisons o SPRTs in liquid nitrogen and liquid argon in asimple apparatus without active control. Repeatabilities o < ~0.1 mK are achieved, in spite o signicant temperature drits.
Is temperature drit a problem in your comparison calibrations?
Then the method o instantaneous comparisons is or you! Further considerations
Because the ratios are independent o temperature, the exact valueo R
re is not important: the (R
re , T
90) datum can be taken at any
convenient near-by point, or rom a value in a previous experimentor in a certicate.
What matters is that the ratios are accurately measured and that Rre
is accurately related to T 90
.
Conclusions
In the three experiments it has been ound that
• C-SPRTs can be compared with standard deviations <~0.1 mK
• ve NPL reerence C-SPRTs have been stable within ± 0.15 mK over 37 years
• diferences between measurements in two argon L-SPRT triple-point cells have been conrmed
• dynamic comparisons have been made with reasonableaccuracy in a warm-up rom 77 K to 293 K.
The method has potential for application in all comparisons of likethermometers, eg
• comparisons in liquid baths, metal blocks and urnaces,
• proling the baths, etc, to detect non-uniormity,
• investigations into (eg) thermometer cycling stabilityand hysteresis
• comparisons o thermometers in conditions o rapid drit.
In comparing industrial PRTs and L-SPRTs in water baths, the ratiosensitivity is still only ~ 14 mK per kelvin change in temperature.
Figure 1: the comparison apparatus, consisting o a copper block, resting ona polystyrene platorm, surrounded by porous oam insulation, located ina stainless steel 8-litre liquid nitrogen Dewar (vacuum) vessel 15 cm insidediameter by 48 cm deep. The hold-time is about 24 h.
L-SPRTs are inserted through guide-tubes at the top plate. C-SPRTs are locatedin close-tting holes, and our electrical connections lead out to the measuringsystem (selector switch and ASL F18 bridge).
Experiment 2: Agreement o fve NPL reerence C-SPRTs ater 37 years
Figure6:comparisonsbetweenC-SPRTsduringthe warm-upfrom77K to273 K.
Experiment 3: Comparisons o C-SPRTs and L-SPRTs.
Figure3: Comparisonsof C-SPRTsversusRef1(1676928) inLN2
Experiment 1. Calibration o six C-SPRTs by comparison with NPL reerence C-SPRTs
Figure2: ComparisonsofC-SPRTsversusRef1 (1676928)in LAr
Six C-SPRTs (H. Tinsley Co) were calibrated against two NPL reerence C-SPRTs (Leeds and Northrup) inliquid nitrogen (77.3 K) and liquid argon (87.3 K).
Bridge readings were recorded about every 2 s. The rst 10 readings were discarded and the next 10readings were averaged, to give one measurement about every 40 s.
Measurements were made at currents o 1 mA, √2 mA and 1 mA.
A 6 Ω standard resistor was used so that Rs/R
re1~1.25 (the exact value o Rs is not important).
The diferences in LAr (Figure 2) were mostly within ±0.2 mK over 19 h, and the standard deviationswere < 0.10 mK.
These data were used to derive the calibrations o the thermometers, at ~87.3 K.
The standard deviations in LN2 (Figure 3) were similar but the mean diferences, −0.10 mK to +0.30 mK,were larger than in LAr because o the propagation o calibration error at the triple-point o mercuryand the extrapolation rom ~87.3 K to ~77.3 K. This is consistent with Euramet Technical Guide No 1 orextrapolations below the argon triple point [1].
Nine comparison sequences were made in LN2 over a period o 3.5 h. Then the LN2 was drained of andreplaced by LAr, and the temperature rose initially to ~82 K.
In comparisons between 82.4 K and 83.2 K (18 h) the average drit was <0.001 K/min.
The results are shown with respect to the Ward and Compton International Intercomparison o Realisationso the IPTS-68 [2].
The average diferences rom 1728839 in LN2 were -0.24 mK to -0.11 mK, with standard deviations o 0.06 mK or lower.
The average diferences between 82.4 K and 83.2 K were -0.04 mK to -0.13 mK, with standard deviations
up to ~0.13 mK as a result o the l arger drit rates.1728839 reads high with respect to the other our C-SPRTs, and 1832689 lies somewhat below the restin LN2. Both these efects had been detected in intervening comparisons.
These results are remarkable demonstrations of the stability of the C-SPRTs over 37 years, and also of the precision of the 1976 comparisons in a vacuum cryostat and the present comparisons in liquid or partially solid cryogens.
NPL has used LN2 as a comparison medium or the calibration o L-SPRTs since the mid 1980s (previously liquid oxygen had beenused, as specied in the IPTS-68). Agreement between the reerenceL-SPRTs was typically < ~ 0.5 mK.
Recently calibrations o L-SPRTs in the NPL Ar triple-point cellshowed diferences o ~-0.3 mK with respect to calibrations in acommercial cell.
To resolve this discrepancy more precise comparisons with C-SPRTsin LN2 and LAr were made, using the instantaneous comparisonmethod.
The mean diference between two reerence C-SPRTs and twoL-SPRTs in LN2 was -0.08 mK with a standard deviation o 0.06 mK.
In LAr at 87.1 K the mean diference was -0.26 mK ± 0.09 mK.
This broadly conrms the discrepancy, though the reason or it isnot yet clear.
Dynamic calibrations
Ater Experiment 2, above, the logging continued as the temperatureramped up to 293 K over ~ 15 h. Ater the nal evaporation o LN2 theramp rate rapidly increased rst to 10 K/h and then to a peak o 32 K/h(~9 mK/s) at ~ 170 K. During the ramp the C-SPRTs agreed within ~ 10 mK,rising to ~ 20 mK, and then reducing as the temperature approached 293 K.
At ramp rates o 9 mK/s the SPRT response times are critical. The TinsleySPRTs have poorer thermal contact (evidenced by larger sel-heatingefects) and so read lower. The maximum diference o ~25 mK isequivalent to a lag o ~3 s. The two Leeds and Northrup SPRTs showdiferences o ~10 mK, most likely because 1832689 was connectedwith ner wiring, so less heat was conducted to it than to 1676928. Tinsley B409 deviates above 220 K, probably because some moisturehad condensed on the capsule.
A check comparison in a steady low-temperature bath is needed to conrmdynamic calibration errors. Alternatively Inseok Yang et al[3] have discussedcompensation or time lag in dynamic comparisons where measurementsare taken rst during the cool-down and then in the warm-up.
Figures5:internalconsistencyat~ 83K aftera periodof37 years.Figure4:internalconsistencyat~77.3K afteraperiodof 37years.
