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Introduction and Specications
Specications
2.2.5.4.2 Example 3: Measuring Zero-Power Resistance
The purpose of this example is to demonstrate how the Relative Current specication applies when performing
a zero-power measurement. The zero-power uncertainty calculated in this example would be added to the rest
of the uncertainties involved in the measurement as explained in previous examples. The intention of the zero-
power measurement is to remove measurement errors due to self-heating of the SPRT.
In this example a 1595A is used to measure a 25 Ω SPRT at the triple-point of water using the Zero-Power
function. The SPRT is measured at nominal current, 1.0 mA and 1.4142 mA (double-power current). In this
example, the self-heating sensitivity of the SPRT in a triple-point of water cell is 0.0024 °C/mA. This was
found by using the Zero-Power function and reading the SELF-HEATING eld in the Zero-Power results
screen. This value will vary signicantly depending on temperature, measurement medium, and probe
construction.
The uncertainties used to calculate zero-power uncertainty are:
●
Ratio accuracy of the 1595A
●
Measurement noise
●
Relative current accuracy of the 1595A
●
Reference resistor stability
2.2.5.4.3 Resistance Ratio Accuracy at 0.01 °C
The ratio accuracy at 0.01 °C is based on using the internal 25 Ω resistor to measure a resistance of 25.5 Ω.
The 1595A standard uncertainty of resistance ratio is 0.03 ppm. To convert this value to temperature divide by
1.0 × 10
6
and multiply by 1.02 (Rx/Rs). The result is then divided by 0.004 (W
T90
sensitivity, dW/dT, at 0.01
°C, see above). The nal result is 0.000008 °C.
2.2.5.4.4 Measurement Noise
The Zero-Power function reports the standard error of the mean of the zero-power value in the STANDARD
ERROR eld. In this example, the standard error of the mean is 0.0000018 Ω. To convert this value into tem-
perature, divide by the resistance sensitivity (dR/dT) of the SPRT at 0.01 °C. dR/dT at 0.01 °C is 0.1 W/°C (see
tip above). The result is 0.000018 °C.
2.2.5.4.5 Relative Current Accuracy
The relative measurement current specications are listed in Table 7 of the Specications section. The stan-
dard uncertainty of the measurement current over the range 1.0 mA to 1.4142 mA is 0.0015 mA. This is
converted to temperature by multiplying by the SPRT self-heating sensitivity at 0.01 °C which is 0.0024 °C/
mA. The result is 0.0000036 °C.
2.2.5.4.6 Reference Resistor Stability
Since the individual measurements of the zero-power measurement are taken in close proximity in time, the
drift of the reference resistor is considered negligible.
2.2.5.4.7 Combining the Uncertainties
The uncertainties of the zero-power measurement are combined by RSS with the rest of the uncertainties
involved in the measurement. See the previous examples for the other uncertainties.
2.2.5.5 Example 4: Measuring a 100 Ω PRT
In this example the temperature of a typical 100 Ω PRT is measured at 420 °C with a 1595A. The uncertainties
in the measurement associated with the 1595A are as follows:
●
Resistance accuracy of the 1595A
●
Measurement noise at 420 °C
2.2.5.5.1 Resistance Accuracy at 420 °C
First, calculate the 1595A absolute resistance accuracy at 257 Ω (the resistance of the 100 Ω PRT at 420 °C).
The one-year absolute resistance standard uncertainty (k = 1) of the 1594A, using the internal 100 Ω resistor,
is 2.0 ppm. To convert this to an uncertainty in temperature, multiply 2.0 ppm by 1.0 × 10
6
then multiply by