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6 Operation
6.1 General
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.
6.2 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 ele
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ment to the housing.
The immersion depth for standards is dependent on several factors including
accuracy requirements and type of liquid. However, remember the handle limi
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tations. The handle is not designed to be immersed.
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
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celed by using an AC bridge or a DC bridge with reversible current.
6.4 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 preventing mois
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ture from entering the device. If moisture penetrates the seal, the PRT's short
term repeatability, hysteresis, and insulation resistance may be adversely af
-
fected. Insulation resistance also decreases rapidly as the transition junction
13
6 Operation
General
1.888.610.7664 sales@GlobalTestSupply.com
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