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6.4 Thermal EMF
Two factors contribute to thermal EMF, chemical consistency and physical
consistency. Variations in chemical structure due to impurities can contrib-
ute 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
beasourceofthermalEMF.ThethermalEMFiscausedbyadifferencein
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 re-
versible current.
6.5 Transition Junction
Exceeding the temperature range of the transition junction will cause a
breach in the seal of the instrument. Maintaining the seal is critical to pre-
venting moisture from entering the device. If moisture penetrates the seal,
the PRT's short term repeatability, hysteresis, and insulation resistance may
be adversely affected. Insulation resistance also decreases rapidly as the
transition junction temperature increases, even if the seal is not broken.
When the insulation resistance becomes sufficiently low, performance suf-
fers. A good rule of thumb is that the transition junction is too hot when it is
hot enough to burn your thumb.
5608/5609
User’s Guide
14
1.888.610.7664 sales@GlobalTestSupply.com
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