Troubleshooting Guide

Temperature Controllers

Input channel voltage measurement is out of range.

Ensure that the sensor is connected and properly wired.

Ensure that the polarity of the sensor connections is correct.

Refer to the Sensor Connections section. Many sensors can be checked with a standard Ohmmeter. For resistor sensors, ensure that the resistance is correct by measuring across both the Sense and Excitation contacts. For a diode sensor, measure the forward and reverse resistance to ensure a diode-type function.

Input channel is within range, but measurement is outside the limits of the selected sensor’s calibration curve.

Check sensor connections as described above.

Ensure that the proper sensor has been selected. Refer to the Input Channel Setup Menus section.

Change the sensor units to Volts or Ohms and ensure that the resulting measurement is within the selected calibration curve. Refer to the User's Manual section on Sensor Setup to display the calibration curve.

The controller’s firmware has been corrupted (Invalid Checksum).

Locate new instrument firmware and utility software on the Cryo-con web site or the CD supplied with the controller.

Re-load the unit’s firmware. Refer to the User's Manual section on Downloading Instrument Firmware.

The control loops were disengaged by detection of an excessive internal temperature. Possible causes:

Shorted heater. Check heater resistance.

Selection of a heater resistance that is much greater than the actual heater resistance. Refer to the Control Loop Setup menu section of the User's Manual.

Selection of an AC Power line voltage that is much less than the actual voltage. Refer to the Fuse Replacement and Voltage Selection section.

Check that the instrument’s fan is running and that the sides and rear panel allow easy air flow.

If the system is oscillating, try de-tuning the PID values by decreasing P, increasing I and setting D to zero. If the oscillations cannot be stopped by this procedure, the cause is likely that your system has an excessive time delay. Linear control algorithms, including PID, cannot control systems with excessive time delay. These problems often occur in systems that use heat pipes, or depend on gas flow between the heater and temperature sensor elements. The only solution to such systems is to re-design the equipment to reduce the time delay, or to externally implement a time delay compensation algorithm, such as a Smith Predictor.

Do not try to control on Ohms or Volts. The controller will work correctly with either of these sensor units, but the PID values required are significantly different and most sensors are non-linear. Further, there is no advantage to controlling in sensor units.

Optimize the control loop parameters by using the Autotune feature described in the Autotuning section.

Most cryogenic systems require significantly different PID parameters at different temperatures. To ensure stable control over a wide temperature range, use the PID Table feature described in the PID Table Entry section.

If the heater is controlling with an output power level less than 10%, switch to the next lower heater range.

Possible causes:

Excessive noise pickup, especially AC power line noise. Check your wiring and shielding. Sensors must be floating, so check that there is no continuity between the sensor connection and ground. Review the System Shielding and Grounding Issues section of the User's Manual.

Note: Cryo-con controllers use a shielding scheme that is slightly different than some other controllers. If you are using cable sets made for use with other controllers, some shield connections may need to change. If pin 3 of the input connector is connected to the cable shield, disconnect it and either re-connect the shield to the backshell contact or leave the shield floating. No connection should ever be made to pin 3 of the input connector.

Check for shielding problems by temporarily removing the input connector’s backshell. If the noise changes significantly, current is being carried by the shields and is being coupled into the controller.

Use a longer display filter time constant to reduce displayed noise.

Possible causes:

The wrong sensor type or sensor calibration curve is being used. Refer to the Input Channel Setup Menu section.

DC offset in cryostat wiring. Review the Thermal EMF and AC Bias Issues section. Use AC bias, if necessary, to cancel the offset error.

A four-wire measurement is not being used. Some cryostats use a to a two-wire measurement internally. This can cause offset errors due to lead resistance.

Thermocouples: These sensors will often have DC offset errors. Use the CalGen® feature to generate a new sensor calibration curve that corrects for these errors.