Resistance thermometer systems are susceptible to three types of errors:
- The tolerances inherent in the resistance itself
- The temperature gradients between the thermometer and the medium to be measured
- The errors and defects in the connection between the sensor and the measuring instrument.
Some sources of error are electric; others result from the mechanical construction of the thermometer. Potential sources of error include interchangeability and conformity. Conformity indicates the extent to which deviation from the standard curve is permissible. Conformity has two components: a tolerance at the reference temperature, normally 0°C, and a tolerance on the slope.
These possible deviations are defined in the standards. For example, the standard DIN 43760 class B requires calibration at less than 0.12Ω (0.3°C) at 0°C, but allows the curve to deviate from the nominal 0.00385 by ±0.000012 Ω/Ω/°C. This can give deviation of 0.8°C at 100°C, 1.3°C at 200°C, and up to 3.8°C at 700°C.
It is, therefore, important to know precisely the tolerances applicable to the system used.
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