Helium System Maintenance


Customer called PTB Sales technical support with a problem regarding a CTI-Cryogenics Cryo-Torr 8 and a CTI-Cryogenics 8200 Compressor. The compressor was rebuilt by PTB approximately 20 months ago, the Cryo-Torr 8 was not sold or serviced by PTB therefore, the last service date was unknown. After installing the pump and compressor on the tool, an initial startup sequence was initiated and the pump placed into regeneration. The pump and compressor appeared to be operating correctly however, after an hour of operation the cryopump was not cooling down.

Suggested Actions

During the initial call, an action plan was developed to isolate the problem to either the pump or the compressor. To preclude the most likely suspects on the pump motor, the customer was asked to confirm cold head power from the compressor first by listening to the pump motor for both motor sound and movement. This was done by removing and reattaching the pump motor power cable. If the motor did not appear to have movement then he was instructed to check the power from the compressor cable. A “pin-out” diagram of the compressor cable connector was provided to aid in discovery.

In addition, the customer was asked to confirm the compressor static (when pump is off and compressor running) and dynamic pressures (when both pump and compressor are running). The customer was also asked to verify all Aeroquip fittings were tight.

The second call from the customer advised PTB that the pump motor was operating and there was movement. The compressor however was displaying curious pressures. OEM stated static pressure for a 8200 compressor is 245 psig with dynamic pressure at ~257 psig. When checking the 8200’s pressure the customer confirmed the static pressure was a little low ~240 psig, but the dynamic pressure did not change (increase) at all when in operation. It maintained 240 psig with no fluctuation of pressure. Typically, as the helium passes through the Gifford-McMahon refrigerator the helium dynamic pressure will cycle 5-10 psig rhythmically as the helium flows through the inlet and return valves.

This information does not in itself preclude the pump as the problem, but it made the compressor a likely suspect. The customer was asked to verify voltage settings, compressor amp draw and finally to confirm again that the Aeroquip fittings were tight.


The customer called and reported that the problem was found to be a loose Aeroquip fitting on the 8200 supply connection. Once this was tightened, cooling in the pump was seen immediately.

The customer was advised to perform the decontamination procedure on the system, as it is possible the helium was contaminated due to the loose fitting. After decontamination, the compressor helium should be charged to correct static pressure and verification of the dynamic pressure.

If technical assistance is required, please contact our Customer Service Representative.

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Document # PTB-T35-0004