Comparison Calibration; Calibration Of Multiple Probes - Fluke 7008 User Manual

7008
Users Guide

6.2 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: 15 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 slid over the probe before inserting it
in the bath or as complicated as a specially designed reflective metal apparatus.
When calibrating over a wide temperature range, better results can generally be
achieved by starting at the highest temperature and progressing down to the
lowest temperature.
Probes can be held in place in the bath by using probe clamps or drilling holes in
the access cover. Other fixtures to hold the probes can be designed. The object
is to keep the reference probe and the probe(s) to be calibrated as closely
grouped as possible in the working area of the bath. Bath stability is maximized
when the bath working area is kept covered.
In preparing to use the bath for calibration start by:
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6.3 Calibration of Multiple Probes

Fully loading the bath with probes increases the time required for the
temperature to stabilize after inserting the probes. Using the reference probe as
the guide is sure that the temperature has stabilized before starting the
calibration.
6-2
Placing the reference probe in the bath working area.
Placing the probe to be calibrated, the UUT, in the bath working area as close
as feasibly possible to the reference probe.