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Author: Subject: Modifying a diaphragm pump to achieve high vacuums - what is the limiting factor for lowest vacuum?
Sidmadra
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[*] posted on 17-4-2019 at 19:56
Modifying a diaphragm pump to achieve high vacuums - what is the limiting factor for lowest vacuum?


To the best of my knowledge, diaphragm vacuum pumps use check valves to direct the flow of the air, as is necessary. There are many diaphragm pumps on the market, and many cheap ones from China, but they often cannot achieve lower than 50-75mmhg of vacuum. There are more expensive diaphragm vacuum pumps on the US/EU market that can achieve down to around ~1mmhg. These more expensive pumps tend to have the same power ratings and airflow rates as the cheaper pumps, yet can achieve much lower vacuums?

Since power rating is roughly the same, I would speculate that the lowest achievable vacuum is limited by the speed and responsiveness of the check valves. Slower check valve = more vacuum suck back = worse vacuum threshold. Owing this functional difference to a mere check valve or 2 has implications. $3000 diaphragm vacuum pump that can reach 1mmhg vs $100 Chinese vacuum pump that can reach 75mmhg.

Can check valves alone account for this large difference? Surely a high-end check valve cannot cost that much, and replacing them on a pump or putting them inline should be no challenge.

[Edited on 18-4-2019 by Sidmadra]
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[*] posted on 17-4-2019 at 21:35


It probably has more to do with the relationship between swept volume and dead space in the "cylinder". This is true of any displacement pump.



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draculic acid69
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[*] posted on 17-4-2019 at 22:11


Are the more expensive pump that you are talking about also diaphragm pumps or oil or rotary vane pumps?
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wg48temp9
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[*] posted on 18-4-2019 at 00:39


The key parameters are the compression ratio (swept volume to deadspace) and the differential pressure required operate the inlet and outlet valves. Higher compression and valves that require low differential pressures to operate improves the ultimate vacuum achievable.

Putting the pumps in series increases the compression but does not change the differential pressure to operate each of the valves and the extra valves.

PS: putting a check valve in line to or from the pump will increases the dead space and therefore decreases compression. Diaphragm pumps tend to have large dead spaces. Many diaphragm pumps have two diaphragm pumps in series one at each end of the drive motor and arranged in opposite phase.



[Edited on 18-4-2019 by wg48temp9]




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[*] posted on 18-4-2019 at 03:46


I have a KNF 726.3 vacuum pump which can pull 10-15 torr. The check valves in it are pretty simple, just small rubber disks that flap open or closed for bypassing/sealing functionality. They aren't even spring loaded. I think the pump does so well because it's a two-stage pump, and the diaphragm pistons are high quality PTFE which seal well to the PTFE heads. I imagine the tolerances are much looser on the Chinese pumps.
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