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6 Operation
6.1 General
For best results, be familiar with the operation of the heat source and the
read-out instrument. Be sure to follow the manufacturer’s instructions for the
read-out instrument and the heat source.
6.2 Immersion Requirements
Stem effect can cause measurement errors for any thermometer not immersed
in the fluid at least 114 mm (4.5 inches). 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 inches) minimum immersion depth. However, remember the handle limita-
tions. The handle is designed to be immersed. The temperature limits of the
handle are –200°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 gradi-
ent test taking measurements approximately every 1.27 cm (.5 inches) 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.
6.3 Thermal EMF
Two factors contribute to thermal EMF, chemical consistency and physical con
-
sistency. 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 tempera
-
ture 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 can
-
celed by using an AC bridge or a DC bridge with reversible current.
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6 Operation