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Micro-Bath
Comparison Calibration
Stir Bar
The stir bar (Figure 4) sits in the bottom of the well for mixing the fluid providing better accuracy,
uniformity, and stability.
Figure 4. Stir Bar
Well Extender (Optional)
An optional well extender is available to increase the depth of the well. The extender screws onto the
well and features an O-ring. Thread the extender down until the O-ring makes a good seal. Some fluids
may expand up to 6.35 cm (2.5 in). Therefore, when you use the well extender, keep in mind how much
the fluid in use expands. Do not overfill the well.
Comparison Calibration
Comparison calibration involves testing a probe (Device Under Test, DUT) against a reference probe.
After you insert the DUT into the bath, allow sufficient time for the probes to settle and the temperature
of the bath to stabilize.
One significant benefit of bath use rather than a dry-well to calibrate multiple probes is that the probes
do not need to be identically constructed. The fluid in the bath allows you to calibrate different types of
probes at the same time. However, stem effect from different types of probes is not completely
eliminated. Even though all baths have horizontal and vertical gradients, these gradients are minimized
inside the bath work area. Nevertheless, make sure to insert probes to the same depth in the bath
liquid. Be sure that all probes are inserted deep enough to prevent stem effect. Fluke Calibration
recommends a general rule for immersion depth to reduce the stem effect to a minimum: 20 x the
diameter of the DUT + the sensor length. Do not submerge the probe handles. If the probe handles get
too warm during calibration at high temperatures, use a heat shield just below the probe handle. This
heat shield could be as simple as aluminum foil placed between the probe handle and above the bath
before you insert it in the bath or as complicated as a specially designed reflective metal apparatus.
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