6 Operation
6.1 General
Forb est 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 instrumentand the heat source.
6.2 Immersion Requirements
Stem effectcan 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 elementthrough the thermometer stem. In addition, heat losses
occur due toradiation losses from the sensing element to the housing.
The immersion depth for standardsis dependent on several factors including
accuracyrequirements 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
handleare –200°C to 156°C. Temperatures outside these limits can damage the
handleand the probe. Convectionof heat from the heat source must be kept
within the handle limits.
The exactimmersion depth required can be determined by performing a gradi-
ent test taking measurements approximately every1.27 cm (.5 inches) until
there is a significant differencein read ings. Allow the thermometer to stabilize
at each newdepth. Plot the results to see the stem effect.
6.3 Thermal EMF
Twofactors contribute to thermal EMF, chemical consistencyand 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
constructionof 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 twoconnections. If the two connections are the same temperature,
there will be little or no thermal EMF effects. However,if there is a substantial
temperature differencebetween connections, the thermal EMF effectswill 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 anAC bridge or a DC bridge with reversible current.
5623A 13
6Operation