6
Test Probe Calibration
For optimum accuracy and stability, allow the calibrator to warm up for 10
minutes after power-up and then allow adequate stabilization time after reach-
ing the set-point temperature. After completing calibration, allow the block to
cool before switching the power off.
6.1
Calibration Methods
Never introduce any foreign material into the probe hole of the insert. Flu-
ids, etc., can leak into the calibrator causing damage to the calibrator or
binding and damage to your probe.
6.1.1
Comparison Calibration
Comparison calibration involves testing a probe against a similar reference
probe. The advantage to this method is that better accuracy can be achieved
since errors due to dry-well inaccuracy, stem effect, and drift can be reduced.
After inserting the probes to be calibrated, allow sufficient time for the probes
to settle and the temperature of the dry-well to stabilize.
It is best if both the reference probe and the probe under test are the same size
and construction. Using probes with different lengths, diameters and materials
will have different stem effects causing an unknown temperature difference. All
dry-wells have horizontal and vertical gradients that change with temperature .
This is an unknown variable which can be factored out if probes are the same
type, length, diameter, and material. Probes should be inserted to the same
depth in the well.
The following procedure can be used to calibrate a probe against a reference
while eliminating error due to temperature gradients between wells.
•
Place the reference probe in one well.
•
Place the probe to be calibrated, the unit under test (UUT), in another
well.
•
With the reference inserted into one well and the probe under test inserted
into a second well, make measurements of each.
•
Swap the locations of the reference probe and probe under test. Allow
plenty of time for thermal settling.
•
Make another set of measurements of the reference probe and the probe
under test.
•
Average the two measurements of the reference probe. Average the two
measurements of the probe under test. Averaging the two measurements
in this way eliminates error due to temperature gradients between the two
wells.
13
6 Test Probe Calibration
Calibration Methods