Fugger, get a low stage expansion tank in , *now*. Don't let the static pressure rise above some 10 bar / 150psi. The compressor shell is probably the weakest point, and you don't want that to blow. Especially not at comdex.

Recover the R508B by pulling an old small refrigerant or propane tank vacuum, and pump the R508B in there. Reverse the process to get most of it back into the low stage again.

An old receiver connected with a piece of captube to the suction side of the low stage will do nicely as an expansion tank.

Also, get yourself a pressure switch for the low stage, and a simple tempcontroller (Eliwell EWPC-901, Dixell XR30C, Carel IR32, those things) to switch on/off the low stage.

Damn man, running an R508b cascade without any safety devices (I don't call a UPS a safety device; the high stage might break). You must be at least as nuts as I am.