I am receiving odd signals at my cryogenic temperature transmitter (model DT500-CU silicon diode and DRC80 transmitter). Accuracy and sensitivity seem to have been lost.
The transmitter calibration has been checked and it is within error tolerance.
My main concern is that this is a failing diode. It is a significant undertaking to replace the diode as it resides inside a large coldbox in a contaminated area. Just prior to a maintenance outage in 2016, the signal began to drift. Since that outage , the signal has drifted further and has lost sensitivity at operating temperatures (22-30 Kelvin). Normal process operations that would result in a 1-2 Kelvin temperature response are now also hardly indicated. Additionally, with the system currently warm for a maintenance outage, the signal should be pegged off-scale high (40 Kelvin), but is currently indicating approximately 22 Kelvin. I am waiting for Maintenance to check the signal at the cold box junction box to determine if the problem lies within the cold box, but do not have these results as of yet.
The diode and transmitter are about 30 years old. What is a typical failure mode for silicon diodes? Does what is described above indicate a failing diode?
Post by Lake Shore Jeff M on Mar 13, 2019 7:28:37 GMT -5
A normal diode failure is that it either reads as an open circuit or a short circuit. What you are seeing could indicate a diode failure or an instrument failure.
The DRC-80 sources a 10uA current on the I+ & I- leads and then reads the voltage developed across the diode using the V+ & V- leads. It then performs a straight line interpolation between the two adjacent curve points to determine the temperature.
For the obsolete DT-500 diode, the difference in reading of 40K versus 22K is approximately 0.2V. 40K is 1.0746V and 22K is approximately 1.3V.
What you can do is to connect a precision 100K ohm resistor in place of the diode and verify what the DRC-80 reads in voltage which should be very close to 1V. If the DRC-80 reads correctly, the issue is in the diode or its wiring.