We have a LS 218 controller which we are using to control a spectrometer at ~80 K. Two channels are actively controlling elements within the spectrometer and work very well at the ~1 mK level for many days. The other two channels (UNCONTROLLED) show the correct temperature ~79 K, but with a ~5mK oscillation with 8-10 minute period. There are no obvious heat sources which could be producing this. Also we swapped the inputs to these channels to other stable ones and these then started showing the same oscillation pattern.
Channela 5 and 6 always show an oscillating temperature no matter what cable or sensor is connected
All the thermometers are silicon diodes, DT-600 series (model DT-760A1-CU). I believe all the LS218 channels were setup identically, same sensor curves.
Post by Lake Shore Jeff M on Jul 16, 2019 10:01:50 GMT -5
Hello Chas, Sorry for the delay in responding. Can you tell me how you have the sensors wired and if the leads are in a cable with a shield, where you have the shield grounded. Also, since the Model 218 is only a monitor, how are you doing the control you describe?
Thanks for the reply! Sorry for the confusion about the plot above. We are actually reading out 4 DT-600s: Channel 1 and 2 on cable 1, and Channels 4 and 5 on cable 2. Channels 4 and 5 had the oscillations while channels 1 and 2 did not. We then switched the cables and had the same oscillations on the same channels. We then switched the cables at the cryostat only, channels 4 and 5 still showed oscillations but with the DT-600s previously readout out by channels 1 and 2. This suggests that we aren't having problems with the cable or the DT-600s. It suggests that the oscillations are coming from within the 218. To answer your question, the cable does not have a shield.