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
required determine the number of temperature measurements necessary.
However, to follow “good” practice procedures, always measure the triple
point of water (R
tp
) after each temperature measurement. The following
equation provides the most accurate measurement of the ratio:
W
R
R
t
t
tp
=
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 error 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 im-
mersed 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 addi-
tion, heat losses occur due to radiation losses from the sensing element to
the housing.
The immersion depth for standards is dependent on several factors includ-
ing accuracy requirements and type of liquid. Therefore, we recommend a
152 mm (6 inches) minimum immersion depth. However, remember the
handle limitations. The handle is not designed to be immersed. The temper-
ature limits of the handle are 0°C to 80°C. Temperatures outside these limits
can damage 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
gradient 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.
13
6
Operation
