I have a cryostat controlled with a Lakeshore 340 coupled to a Lakeshore DT-470-CO-13 diode. What i am having trouble with is the temperature displayed is very unstable, it will flutter and jump around by up to 10 K every time the display refreshes. Under 10 K it is more stable but above 100 K it is a real problem. When I fix the heater output and let the temperature equilibrate it will still jump around like crazy so I am thinking that it is a problem related to the diode?
I have found that it will stabilize but this can take hours and changing the set point or heater output by only a small amount is enough to have it jumping around again.
I am wondering whether this behavior could be caused by interference on the cable run from the diode to the controller? What should the output from the diode look like on an oscilloscope?
Post by Lake Shore Ogi on Jun 29, 2018 11:39:46 GMT -5
If you're able to manually control the temperature (with fixing the heater output and waiting for an equilibrium), I don't believe then that you're having a control-type issue and we can look at the instrument, the diode and its wiring.
You can start testing the controller first, to rule out any issues with it. If you have a 100kOhm resistor, you can "mimic" the diode sensor by connecting the resistor the same way (you connected the sensor). With the fixed 10uA excitation current, the 100kOhm resistor will give a 1V which should read approximately 87K (depending on the resistor accuracy). If you're reading in that ballpark, and the measurement is stable, you can focus on checking the sensor and its wiring.
The Model 340 essentially outputs a 10uA DC current to excite the sensor. So if you're looking at it with a scope, you should be seeing a flat line (DC). You can further check the sensor and wiring by measuring the resistance as follows (unplug sensor from 340 during the measurement): - Measure the resistance across diode sensor in forward bias (V+ to V-) [expected = several Meg Ohm] - Measure the resistance across diode sensor in reverse bias (V- to V+) [expected = open/infinity] - Measure resistance from each sensor lead to your system ground [expected = open/infinity]