General; Comparison Calibration - Fluke 7341 User Manual

Read this chapter entitled BATH USE before placing the bath in
service.
The information in this section is for general information only. It is not designed to be the
basis for calibration laboratory procedures. Each laboratory needs to write their specific
procedures.

General

Be sure to select the correct fluid for the temperature range of the calibration. Bath fluids
should be selected to operate safely with adequate thermal properties to meet the
application requirements. Also, be aware that fluids expand when heated and could
overflow the bath if not watched. Refer to Chapter 8, General Operation, for information
specific to fluid selection and to the MSDS sheet specific to the fluid selected. Generally,
baths are set to one temperature and used to calibrate probes only at that single
temperature. This means that the type of bath fluid does not have to change. Additionally,
the bath can be left energized reducing the stress on the system.
The bath generates extreme temperatures. Precautions must be taken to prevent personal
injury or damage to objects. Probes may be extremely hot or cold when removed from the
bath. Cautiously handle probes to prevent personal injury. Carefully place probes on a
heat/cold resistant surface or rack until they are at room temperature. It is advisable to
wipe the probe with a clean soft cloth or paper towel before inserting it into another bath.
This prevents the mixing of fluids from one bath to another. If the probe has been
calibrated in liquid salt, carefully wash the probe in warm water and dry completely
before transferring it to another fluid. Always be sure that the probe is completely dry
before inserting it into a hot fluid. Some high temperature fluids react violently to water
or other liquid mediums. Be aware that cleaning the probe can be dangerous if the probe
has not cooled to room temperature. Additionally, high temperature fluids may ignite the
paper towels if the probe has not been sufficiently cooled.
For optimum accuracy and stability, allow the bath adequate stabilization time after
reaching the set-point temperature.

Comparison Calibration

Comparison calibration involves testing a probe (unit under test, UUT) against a
reference probe. After inserting the probes to be calibrated into the bath, allow sufficient
time for the probes to settle and the temperature of the bath to stabilize.
One of the significant dividends of using a bath rather than a dry-well to calibrate
multiple probes is that the probes do not need to be identical in construction. The fluid in
the bath allows different types of probes to be calibrated at the same time. However, stem
effect from different types of probes is not totally eliminated. Even though all baths have
horizontal and vertical gradients, these gradients are minimized inside the bath work area.
Nevertheless, probes should be inserted to the same depth in the bath liquid. Be sure that
all probes are inserted deep enough to prevent stem effect. From research at Hart
Scientific, we suggest a general rule-of-thumb for immersion depth to reduce the stem
effect to a minimum: 20 x the diameter of the UUT + the sensor length. Do not submerge
the probe handles. If the probe handles get too warm during calibration at high
temperatures, a heat shield could be used just below the probe handle. This heat shield
could be as simple as aluminum foil placed around the handle or as complicated as a
specially designed reflective metal apparatus.
When calibrating over a wide temperature range, starting at the highest temperature and
progressing down to the lowest temperature can generally achieve better results.
Probes can be held in place in the bath by using probe clamps or drilling holes in the
Caution
6
Bath Use
General
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