Hi, I'm very new to this forum and I found this forum consist a lot of information with a lot of expertice kind helpful advice.

Hi...I'm a contractor for repairing this Ultra Low freezer (-86C), model ULT2586 which consist of 2 compressor. Basically, it run in 2 compressor.
This compressor low stage ( second stage ) is running using 43gram R404 and 300g R508B and it consist of oil separator. But, without expansion tank.
I'm wondering, most of the Thermo Revco -86C freezer usually built with expansion tank.

FYI, i just perform flushing on the system as the system get clog after not runnin gfor a year.

Basically, the freezer temperature able to reach the setpoint of -75C if i set it to -75C. During the ramping until reaching -75C, the compressor current is app 2.6A with low side P=26psi amd high side P=app90->100psi.

The freezer compressor will stop when temperature reach -76C and will run back when the temperature -74C. During this process, the high side pressure tend to gradually increase slowly until the high side pressure shoot up to app 300 psi and causing overpressure occur or overpressure switch trigger.

I'm wondering what is the problem. Hence, I need expert advice here.
As fas as my experience, normally overpressure will occur only during ramping stage at about -60C if overfill the refrigerant.