Fluke 7008 User Manual page 76

7008
Users Guide
The controller shows that the out- put power
is steady but the process temperature is
unstable
The controller alternately heats for a while
then cools
The controller erratically heats then cools,
control is unstable
13-4
temperature'.
•
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 Section 1.3) 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 intermittent 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 Authorized
Service Center (see Section 1.3) for
assistance.