Unstable Cryo Temps

[twilliams10]

Hi All,

I'm running an Agilent 7890B/5977B with LN2 cryo cooling (dewar set to 20 psi) on the oven for permanent gas analysis. The method I'm following calls for a starting temp of -60 C but I'm having trouble stabilizing at the temperature. The oven doesn't seem to have a problem getting to -60 C but it fluctuates to the point that the instrument often never become ready. If it does stabilize and become ready for a run, it still fluctuates a enough to where the TCD noise is unbearably high. I've also been informed that there weren't any stability issues at the factory. So far I've tried insulated/insulated LN2 transfer lines and removed/repaired the oven flaps which weren't tightly closed.

The oven method is:
-60 C hold for 6.5 min
50 C/min to 240 C hold for 3.5

Any thoughts how to improve the stability?

[70 eV]

How much is the temp bouncing around? Is it holding a specific value and then jumping 5-15 degrees, or just slowly falling at a consistent rate and the cryo is doing a poor job actuating temp?

If it's jumping it could be a bad oven thermocouple, otherwise I'd look into the cryo valve. Do a dry run and stare at the oven temp, if it ever goes down while ramping once you're above ambient its the thermocouple.