Precision RTD Freezer Probe
Operation
5
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 not immersed in the fluid at
least 114 mm (4.5 in). 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. Therefore, we recommend a 114 mm (4.5 in)
minimum immersion depth. The handle is designed to be immersed. The temperature limits of the
handle are –100 °C to 156 °C. Temperatures outside these limits can damage the handle and the
probe. Convection of heat from the heat source must be kept within the handle limits. 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.
