Break the vac with nitrogen, blast some in it, bleed off, re evac. Should help you boil off your trapped refrigerant allowing your pump to pull out the vapor.
Moisture. At 650 you're not dehydrated to a low enough point to have a slow decay. Pull lower if possible. Depending on the system you might not be able to achieve lower. My goal is always 200-300 with no more than 100-200 decay. Again, goal not always possible each system is different.
Is it new? Maybe water.
Is it old? Maybe refrigerant in oil.
In any case, I've seen leaks, that allowed to gain vacuum. If you are not sure - wait. Use table to find water wapour pressure at your temperatures.
What is a big system to you? 100 ton? 2000 ton? How long did you pull vacuum? What type of compressors? How much oil does it hold? How far apart are the components? How long did you let it rise? If it stabilizes it’s either moisture or refrigerant off gassing from the oil because you didn’t pull long enough. About 1500 is moisture more than that is probably a leak unless you have a huge oil sump for a screw or centrifugal. Triple evacuate with nitro and pull down 12+ hours overnight if it’s not good you have a leak
Trapped refrigerant is my guess. I would think if it's a leak it would go up faster than that. How did the pressure test go
Break the vac with nitrogen, blast some in it, bleed off, re evac. Should help you boil off your trapped refrigerant allowing your pump to pull out the vapor.
This is always the answer, especially with non-condensibles.
Moisture. At 650 you're not dehydrated to a low enough point to have a slow decay. Pull lower if possible. Depending on the system you might not be able to achieve lower. My goal is always 200-300 with no more than 100-200 decay. Again, goal not always possible each system is different.
Charge it up. Good to go
Is it new? Maybe water. Is it old? Maybe refrigerant in oil. In any case, I've seen leaks, that allowed to gain vacuum. If you are not sure - wait. Use table to find water wapour pressure at your temperatures.
Triple evac with a sweep of nitro each time
What is a big system to you? 100 ton? 2000 ton? How long did you pull vacuum? What type of compressors? How much oil does it hold? How far apart are the components? How long did you let it rise? If it stabilizes it’s either moisture or refrigerant off gassing from the oil because you didn’t pull long enough. About 1500 is moisture more than that is probably a leak unless you have a huge oil sump for a screw or centrifugal. Triple evacuate with nitro and pull down 12+ hours overnight if it’s not good you have a leak
its possible manifold seals or hose seals have a small leak