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Problem
The controller controls or at-
tempts to control at an inaccurate
temperature
The controller shows that the out-
put power is steady but the pro-
cess temperature is unstable
The controller alternately heats
for a while then cools
The controller erratically heats
then cools, control is unstable
Causes and Solutions
The controller operates normally except when controlling at a speci-
fied set-point. At this set-point, the temperature displayed does not
agree with the temperature measured by the user's reference ther-
mometer to within the specified accuracy. This problem may be
caused by an actual difference in temperature between the points
where the control probe and thermometer probe measure tempera-
ture, by erroneous bath calibration parameters, or by a damaged con-
trol probe.
•
Check that the bath has an adequate amount of fluid in
the tank and that the stirrer is operating properly.
•
Check that the thermometer probe and control probe are
both fully inserted into the bath to minimize temperature
gradient errors.
•
Check that the calibration parameters are all correct ac-
cording to the Report of Test. If not, re-program the con-
stants. The memory backup battery may be weak
causing errors in data as described in the problem: 'The
display flashes "cutout" and an incorrect process tem-
perature'.
•
Check that the control probe has not been struck, bent,
or damaged. If the cause of the problem remains un-
known, contact an Authorized Service Center (see Sec-
tion 1.3, on page 5) for assistance.
Possible causes are an improper proportional band setting or the fluid
being used.
•
If the bath temperature does not achieve the expected
degree of stability when measured using a thermometer,
try adjusting the proportional band to a narrower width
as discussed in Section 9.7, Proportional Band.
•
Check to ensure the fluid has not deteriorated or is not
too thick.
The bath is not stable and the duty cycle is not constant.
•
The proportional band being too narrow typically causes
this oscillation. Increase the width of the proportional
band until the temperature stabilizes as discussed in
Section 9.7, Proportional Band.
If both the bath temperature and output power do not vary periodically
but in a very erratic manner, the problem may be excess noise in the
system. Noise due to the control sensor should be less than 0.001°C.
However, if the probe has been damaged or has developed an inter-
mittent short, erratic behavior may exist. The probe is located inside
the stirrer motor cover.
•
Check for a damaged probe or poor connection between
the probe and bath.
•
Intermittent shorts in the heater or controller electronic
circuitry may also be a possible cause. Contact an Au-
thorized Service Center (see Section 1.3, on page 5) for
assistance.
12 Troubleshooting
Troubleshooting
57
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