CTITestReport

Closed Cycle Refrigerators Temperature Gradient Test

On closed cycle refrigerators, one has to take care when interpreting the temperature reading. Depending on the configuration, one may have a considerable difference between the actual sample temperature and the temperature reading. And may be even more important is the time delay.

In our test, we mounted an empty vanadium can in the closed cycle refrigerator CTI2, with a test sensor at the bottom of the container. The 80 K radiation screen was always in place.

temperature difference
delta T at 12 K		time delay
delta t at 30 K		configuration
1 K33 minwith helium exchange gas
0.2 K30 minwith helium exchange gas, 12 K screen
0.2 K32 minwith helium exchange gas, 12 K screen and vacuum grease
2 K70 minwithout helium exchange gas
0.5 K62 minwithout helium exchange gas, but with 12 K screen

CTITestReport/cti2test.gif

Conclusion

  1. consider that the sample temperature may be delayed up to about 1 hour
  2. the difference between sample and sensor temperature may be lower when using a radiation shield on the cold finger (in addition to the 80 K screen).
  3. vacuum grease does not help to improve the thermal contact at low temperature

Remarks

At 4 K the situation is much worse