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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
Comparison Calibration of Other Instruments
The uniformity and stability of the heat source and the degree of accuracy re-
quired determine the number of temperature measurements necessary. How-
ever, to follow "good" practice procedures, always measure the triple point of
water (R
vides the most accurate measurement of the ratio:
=
W
t
All PRTs experience errors caused by self-heating of the element. Self-heating
is a combination of two factors, heat dissipation and heat sink. Self-heating er-
ror can be reduced to have a negligible effect if the PRT is used with the same
excitation current and medium in which it was calibrated.
6.3
Immersion Requirements
Stem effect can cause measurement errors for any thermometer not immersed
in the fluid at least 152 mm (6 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 152 mm
(6 inches) minimum immersion depth at 700°C. However, remember the handle
limitations. The handle is not designed to be immersed. The temperature limits
of the handle are –20°C to 200°C. Temperatures outside these limits can dam-
age the handle. 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.
) after each temperature measurement. The following equation pro-
tp
R
t
R
tp
6 Operation
General
17
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