Do the AC vacuum pumps for automotive usage typically use a check valve like air compressors do? I would think so, as to me it is just a compressor running in reverse. I would also think that a check valve would allow the pump to achieve's maximum vacuum potential. Yes? No?
More like running a compressor in the forward just like air compressors but connecting the output to the inlet rather than the outlet. Ones I have seen use reed valves for unidirectional air flow. Seems like using a rotary vane type pump is more common than using pistons and even two rotary vane pumps in cascade for a dual stage.
But if you want to leave your vacuum pump in for checking vacuum leak down or preventing refrigerant back flow when charging so you don't have to break lines, better to add an real manual inline valve.
Rotary vane pumps have no check valve at the inlet, and reed valves at the outlet. The outlet valves depend on being submerged in oil to seal properly. If the pump is even a little low on oil, it will not develop full vacuum. You won't see this on a dial gauge but you would with a micron gauge.
The reason there is no check valve at the inlet is because it always takes some pressure to push open a check valve. Thus if a valve were fitted, the pressure in the system being evacuated would always be higher than the pump is truly capable of. At high vacuum there is practically no physical pressure available to move a mechanism.
There is a manual isolation valve on most pumps. To conduct a leak-up test, close this valve before turning the pump off. Or you can just close the valves on the manifold. Vacuum pumps are not intended to hold vacuum while off. Typically the oil will be sucked out of the pump if you try to do that.
What I was refering to, and I should have been more specific, was the use of an oil less piston vacuum pump. Or maybe an oil less piston vacuum pump cannot be used for air conditioning serviceing. No? Yes?
I was thinking that a vacuum pump of any kind could be used for A/C work. Maybe this is not the case, or, what are the pro's and cons of different type pumps?
What do you guys think?
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