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miniature fixed-point cells and thermometer

 

Fixed-point cells are usually twin-wall casings. The hollow space is filled with a high-purity substance. During the melting or freezing of this substance, the inside temperature seems to remain constant for a while at the so called fixed-point temperature. A thermometer can be inserted in the centre of the cell. Standard laboratories have a few of those cells at their disposal to calibrate precision thermometers with an accuracy of about 1 millikelvin (or fractions of it). Industrial thermometers, in general, are calibrated on the basis of a comparison with precision thermometers performed in a bath or calibration furnace. The resulting uncertainties are much higher - they range from 0.1 to 2K typically. In contrast to usual fixed-point crucibles with a pure substance ingot of about 750ml, a miniature cell takes only a ten thousandth of this (50 to 100 mm³). The outer geometry of Ø5x20 mm permits an integration into thermometers (see Figs. 1-3). So, it is very easy to perform a calibration while a normal measuring process is running. The device-engineering and computational expenditure for external sensor calibration or preventive substitutions can be reduced. Compared with any other method, it also offers the greatest advantages for an in-situ calibration due to the inclusion of the whole measuring circuit.

 

 
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Fig. 1
cross-sectional view of a fixed-point cell with thermocoupl

 
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Fig. 2 Basic construction of a complete miniature fixed-point thermocouple

 
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Fig. 3 Set of miniature fixed-point crucibles. Due to diameters of 4 to 6 mm, their integration into a thermometer is possible (photo: electrotherm company). Special precision thermocouples of the manufacturer permit to exchange these cells. Thus, it is easy to perform an in-situ calibration at different fixed points ranging from 200 to 1600°C.