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10 Troubleshooting
In case you run into difficulty while operating the 1502A, this section provides
some suggestions that may help you solve the problem. Below are several sit-
uations that may arise followed by possible causes of the problem and sug-
gested actions you might take.
Incorrect Temperature Reading
While attempting to measure temperature the display shows an incorrect
value.
If the temperature readings seem to be incorrect you should first check to see
if the resistance is being measured correctly. Select ohms to display resis-
tance. If the resistance is incorrect refer to the next subsection for trouble-
shooting incorrect resistance readings. If the resistance is being measured
correctly but the displayed temperature value is incorrect consider the follow-
ing possibilities.
•
One or more coefficients are incorrect. This is a common mistake.
While entering coefficients it is easy to miss a digit or sign. Check all the
values carefully comparing them with the values on the calibration certifi-
cate for the probe.
•
The selected conversion type is incorrect. Check to make sure the cor-
rect conversion type (ITS-90, RTD, or IPTS-68) is selected.
• The measurement is out of range. The 1502A may not be able to calcu-
late temperature accurately if the resistance is outside the valid range. The
measured resistance may be too low or too high if the actual temperature
is too low or too high or if there is a problem with the sensor (see below).
Incorrect Resistance Reading
While attempting to measure resistance the display shows an incorrect value.
Consider the following possibilities.
•
Poor or incorrect connection of the probe. Acommon mistake is to con-
nect the wires of the probe to the wrong terminals. Check the wiring care-
fully (see Figure 1 on page 10).
•
Open, shorted, or damaged sensor or lead wires. Check the resistance
across the sensor using a hand-held DMM. Also check the resistance be-
tween common pairs of leads. Check to make sure there is no conductivity
between any of the leads and the probe sheath. Use a good-quality sensor
to avoid errors caused by drift, hysteresis, or insulation leakage.
•
Electrical interference. Intense radio-frequency radiation near the
1502A or the probe can induce noise into the measurement circuits result-
ing in erratic readings. The 1502Ais intended to operate in a laboratory en-
vironment with limited radio-frequency noise. If interference seems to be a
problem you might try eliminating the source of interference or moving the
1502A to a different location. A well-grounded, shielded cable should be
used for the probe leads.
•
Stem conduction error. The problem may be that the actual temperature
of the sensor is not what you expect. This is often the result of stem con
-
duction where heat flowing through the stem of the probe to ambient af
-
fects the temperature of the probe. It is very important that immersion
1502A Manual Rev. 952801 37
10 Troubleshooting
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