provides a means of quantifying uncertainties in temperature measurement in
order to optimise sensor and/or system accuracies.
Uncertainties result from various factors including:
a) Sensor tolerances which are usually specified according to published standards and manufacturers specifications.
b) Instrumentation (measurement) inaccuracies, again specified in manufacturers specifications.
c) Drift in the characteristics of the sensor due to temperature cycling and ageing.
d) Possible thermal effects resulting from the installation, for example thermal voltages created at interconnection junctions.
A combination of such factors will constitute overall system uncertainty. Calibration procedures can be applied to sensors and instruments separately or in combination. Calibration can be performed to approved recognised standards (National and International) or may simply constitute checking procedures on an “in-house” basis. Temperature calibration has many facets, it can be carried out thermally in the case of probes or electrically (simulated) in the case of instruments and it can be performed directly with certified equipment or indirectly with traceable standards. Thermal (temperature) calibration is achieved by elevating (or depressing) the temperature sensor to a known, controlled temperature and measuring the corresponding change in its associated electrical parameter (voltage or resistance). The accurately measured parameter is compared with that of a certified reference probe; the absolute difference represents a calibration error. This is a comparison process. If the sensor is connected to a measuring instrument, the sensor and instrument combination can be effectively calibrated by this technique. Absolute temperatures are provided by fixed point apparatus and comparison measurements
are not used in that case. Electrical Calibration is used for measuring and control instruments which are scaled for temperature or other parameters. An electrical signal, precisely generated to match that produced by the appropriate sensor at various temperatures is applied to the instrument which is then calibrated accordingly. The sensor is effectively simulated by this means which offers a vary convenient method of checking or calibration. A wide range of calibration “simulators” is available for this purpose; in many cases, the operator simply sets the desired temperature and the equivalent electrical signal is generated automatically without the need for computation. However this approach is not applicable to sensor calibration for which various thermal techniques are used.
Temperature Scale of 1990
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