Measurement Accuracy at Triple Point of Water and Gallium Melt Point Supports a Total Measurement Uncertainty of 0.0006 degrees C

Publication Date: 
Wednesday, June 18, 2014
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Triple-Point-of-Water Cell

The Triple-Point-of-Water (TPW) Cell consists of a cylinder of borosilicate glass with a reentrant tube serving as a thermometer well, filled with high-purity, gas-free water, and sealed. When an ice mantle is frozen around the well, and a thin layer of this ice mantle is melted next to the well, the triple point of water temperature can be measured in the well. The three states of water in equilibrium can only occur at the assigned value on the International Temperature Scales of 0.01 degrees C (273.16 K). (Figures 1 and 2)

 

Gallium Melt Cell

The gallium melt cell is a closed-end Teflon tube with a Teflon-tube reentrant well, aluminum shell, and Teflon jacket, filled with high-purity gallium metal. The frozen cell is heated above the gallium melt point (GaMP) temperature, establishing the gallium melt plateau, and allowed to melt over a period of 8 to 12 hours, achieving the assigned gallium melt temperature of 29.7646 degrees C. (Figure 3)


Figure 1: Triple-Point-of-Water Cell

Figure 2: Cross Section of Triple-Point-of-Water Cell

Figure 3: Gallium Cell

 

Measurement Uncertainties

Uncertainties in the achievement of high-accuracy temperature measurements for a Laboratory Standard Platinum Resistance Thermometer (SPRT) in the normal oceanographic temperature range include:

  • Accuracy of the measurement system at the fixed points
    The NIST uncertainty budget was used to evaluate Sea-Bird measurements at the fixed points of GaMP and WTP. Included in the evaluation is over three years of data measurements at Sea-Bird in the fixed point cells. State-of-the-art SPRT, automatic balancing bridge, and external standard resistor reference were used to make the measurements. The uncertainty budget tables provide the summary results. (Figures 4, 5, 6, and 7; Tables 1 and 2)
  • Accuracy of the measurement system between the fixed points
    Evaluation of the performance of the measurement system between the GaMP and WTP fixed points is not possible, but inferring from other subrange inconsistency evaluations and the narrowness of this range, the uncertainty would be very small.


Figure 4: Triple Point of Water Measurements — YSI SPRT S/N 4747
in Jarrett TPW Cell S/N 1866, 02 January 1998

 


Figure 5: Gallium Melts — YSI SPRT S/N 4747
in Isotech GaMP Cell S/N 114, 01 May 1996

 


Figure 6: Triple Point of Water Temperature — YSI SPRT S/N 4747
in Jarrett TPW Cell S/N 1866, 20 October 1998

 


Figure 7: Gallium MeltsTemperature — YSI SPRT S/N 4747
in Isotech GaMP Cell S/N 114, 03 January 1996

 

Table 1: GaMP Uncertainty Budget (Isotech GaMP Cell S/N 114)

Type A Bridge measurement (0.2 ppm) 0.0000005
Repeatability of bridge readings 0.000026
Non-linearity 0.000000
Quadrature effects in ac measurement ~0.000000
Total A 0.000027 (assumes ~0 non-linearity)
Type B Chemical impurities (6N purity) 0.000137
Hydrostatic-head (~ -270 microK) ~0.000010 (at end point)
Propagated TPW 0.000031
SPRT self-heating (-420 microK) 0.000010
Immersion ~0.000000
Moisture (dry ice test)  
Gas pressure 0.000000 (at GaMP assumed)
Insulation degradation (mostly high temperature problem) 0.000000
Total B 0.000188
Total Standard Uncertainty 0.000190
Total Expanded Uncertainty (k=2) 0.000380

 

Table 2: TPW Uncertainty Budget (Jarrett TPW Cells S/N 1682, 1866, etc.)

Type A Bridge measurement (0.2 ppm) 0.0000005
Repeatability of bridge readings 0.000026
Non-linearity 0.000000
Quadrature effects in ac measurement ~0.000000
Total A 0.000027 (assumes ~0 non-linearity)
Type B Chemical impurities (Jarrett aged glass) 0.000001 (bubble < 4mm diameter)
Hydrostatic-head (-198 microK) 0.000010
SPRT self-heating (-360 microK) 0.000005
Immersion 0.000000
Moisture (dry ice test) 0.000000
Gas pressure 0.000000
Insulation degradation (mostly high temperature problem) 0.000000
Total B 0.000016
Total Standard Uncertainty 0.000031
Total Expanded Uncertainty (k=2) 0.000062

 

SBE total calibration uncertainties also include:

  • Uncertainties of applying SPRT defined temperatures to in-house standard
    Analysis of the drift in Sea-Bird primary reference sensors against the SPRT indicates a variability of less than ±100 micro degrees C around the defined drift. (Figure 8)
  • Uncertainties of applying in-house standard defined temperatures to production sensors
    Sea-Bird secondary reference sensors indicate a variability of ±100 micro degrees C. (Figure 9)


Figure 8: Drift Trajectory - SBE 3S/N 1492 YSI SPRT S/N 4747, 01 April 2000

 


Figure 9: SBE 3 Variability

Adding the known uncertainties in the fixed points, the SPRT measurement system, and the transfer standards and technology yields a total known uncertainty of ±580 micro degrees C.

Repeatability of a typical Sea-Bird production sensor is shown. The sensors have typical drift rates of better than 0.001 degrees C in 3 months. (Figure 10)


Figure 10: SBE 3 Repeatability