Post by cryogeorge on Mar 10, 2019 7:26:57 GMT -5
Hi,
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?
Thank you for any insight anyone has!
George
