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Operation
For best results, be familiar with the operation of the heat source and the readout instrument. Be
sure to follow the manufacturer's instructions for the readout instrument and the heat source.
Immersion Requirements
Stem effect can cause measurement errors for any thermometer. This error is due to heat lost or
gained by the sensing element through the thermometer stem. In addition, heat losses occur due to
radiation losses from the sensing element to the housing.
The immersion depth for standards is dependent on several factors including accuracy
requirements and type of liquid. However, remember the handle limitations.
The handle is not designed to be immersed. The exact immersion depth required can
be determined by performing a gradient test taking measurements approximately every
1.27 cm (0.5 in) until there is a significant difference in readings. Allow the thermometer to
stabilize at each new depth. Plot the results to see the stem effect.
Thermal EMF
Two factors contribute to thermal EMF, chemical consistency and physical consistency. 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.
Transition Junction
The transition junction has the appearance of a probe handle but functions as a transition point
from sensor wire to lead wire. Lead wires pass through an epoxy seal and are soldered or welded
to the sensor wires. Exceeding the temperature range of the transition junction will cause a breach
in the seal of the instrument (see Transition Junction Temperature Range in Table 2) Maintaining
the seal is critical to preventing 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
suffers.
Small Diameter Industrial RTD
Operation
5
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