User`s manual

Lake Shore MTD Series Cryotest System User’s Manual
Troubleshooting 5-1
CHAPTER 5
TROUBLESHOOTING
5.0 GENERAL
This chapter covers possible difficulties that may occur. Please read the entire chapter before
attempting to troubleshoot the MTD Cryotest System.
5.1 NO CRYOGEN FLOW AND NO COOLING
During a warm start cool down, latent heat must be removed from the transfer line. Therefore, the flow
rate may be low during this “flat” period and difficult to observe. Allow sufficient time (5 minutes) to make
sure that there is no flow before following the procedure below. If there is no cryogen flow and no
cooling there may be frozen air or moisture inside the system.
1. Depressurize the storage dewar.
2. Remove the transfer line withdrawal tube from the storage dewar and allow the tube to warm.
3. Attach a helium gas line to the transfer line and purge the transfer line with helium gas for
3
– 5 minutes.
4. Close the transfer line flow valve using (full counter-clockwise rotation when looking down at
the knob).
5. Remove helium gas line from the transfer line.
5.2 CRYOGEN FLOW BUT NO COOLING
If sufficient time has passed to have completed the normal “flat” period and there is cryogen flow but no
cooling, check the vacuum integrity of the transfer line. It may be necessary to re-evacuate the transfer
line. If the transfer line vacuum is “good,” check the vacuum in the MTD-150 vacuum chamber. The
vacuum chamber should not feel cold.
5.3 WATER CONDENSES ON OUTER SURFACE OF VACUUM CHAMBER
If the vacuum chamber “sweats,” check the vacuum integrity of the chamber.
5.4 INSTABILITY IN MAINTAINING SELECTED TEMPERATURE
If an automatic temperature controller is being used, check that the gain setting is not too high. Make
sure there are no electric shorts to the sensors and heaters.
5.5 UNABLE TO REACH COLD TERMINAL TEMPERATURE
This may be caused by any of the following:
• Experimental heat load too high.
• Improper anchoring of instrumentation and DUT lines or poor wiring practices.
• “Soft” vacuum in the vacuum chamber or transfer line.
• Thermal short between DUT and radiation shield or radiation shield and vacuum chamber.
• Poor interfacing between DUT and mount and/or between temperature sensor and cold finger.
• Insufficient cryogen flow rate.
5.6 NO RESPONSE TO CONTROL HEATER
First, make sure the problem is not in the cables to the 19-pin instrumentation feedthrough located on
the cryostat main body or in the power supply or controller. Then, check the heater lines with an ohm
meter. If approximately 25 ohms are not measured, visually observe the wiring for a solution to the
problem; otherwise, the heater will need replacement. Consult the factory for instructions.