Temperature Measurement Accuracy

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

Sea-Bird Electronics, Inc.
September 2001

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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)

Figure 1. Triple-Point-of-Water Cell

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

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 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 Melt Temperature -- YSI SPRT S/N 4747 in Isotech GaMP Cell S/N 114, 03 January 1998

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)	0.000000
	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 3 S/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

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