Operation; General; Comparison Calibration Of Other Instruments; Thermal Emf - Fluke 5606 User Manual

Full immersion prt

Table of Contents

6 Operation

For best results, be familiar with the operation of the heat source and the readout in-

strument. Be sure to follow the manufacturer's instructions for the readout instrument

and the heat source.

The uniformity and stability of the heat source and the degree of accuracy required

determine the number of temperature measurements necessary. However, to follow

"good" practice procedures, always measure the triple point of water (R

temperature measurement. The following equation provides the most accurate mea-

surement of the ratio:

All PRTs experience errors caused by self-heating of the element. Self-heating is a

combination of two factors, heat dissipation and heat sink. Self-heating error can be

reduced to have a negligible effect if the PRT is used with the same excitation current

and medium in which it was calibrated.

Two factors contribute to thermal EMF, chemical consistency and physical consisten-

cy. Variations in chemical structure due to impurities can contribute to thermal EMF.

Also discrepancies in crystal structure can contribute to thermal EMF. These factors

are minimized by annealing the full length of wire before construction of the PRT.

Likewise, connection to extension lead wires and readout instruments can be a source

of thermal EMF. The thermal EMF is caused by a difference in temperature between

two connections. If the two connections are the same temperature, there will be little

or no thermal EMF effects. However, if there is a substantial temperature difference

between connections, the thermal EMF effects will be significant. Therefore, cover or

insulate any exposed bridge or galvanometer terminals to lessen the source of error.

The effects of thermal EMF can be canceled by using an AC bridge or a DC bridge

with reversible current.

Operation

ThermalEMF

) after each

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