Diaphragm Vacuum Pump Ratings
Diaphragm Vacuum Pump Ratings
(OP)
I have questions relating to a diaphragm vacuum pump. I am looking for a diaphragm vacuum pump that can pull down a tank to between 50 and 100 Torr and at the same time push the outlet to another tank sitting at 15 psig. I see some pumps that have ratings for both max vacuum inlet and max pressure output but it’s not clear to me that the individual ratings apply to doing both at the same time. Intuitively it seems to me that for a given diaphragm vacuum pump, its ultimate vacuum capability would change (lessen) as the output has to go up above atmospheric. I have been told the output pressure doesn’t make a difference to the input capability –but I haven’t accepted this answer yet. Does anyone else know the answer to this, and if what I heard it true, can you explain it? Thanks.





RE: Diaphragm Vacuum Pump Ratings
RE: Diaphragm Vacuum Pump Ratings
RE: Diaphragm Vacuum Pump Ratings
RE: Diaphragm Vacuum Pump Ratings
Here is what one manufacturer says:
"Atmospheric pressure determines the maximum vacuum force that can be achieved. And standard atmospheric pressure at sea level is 29.92 in.-Hg. But what happens at locations a mile above sea level? The maximum vacuum that can be achieved in locations above sea level will be less than 29.92-in.-Hg. The force will be limited by the ambient atmospheric pressure. Vacuum pumps have maximum vacuum ratings based on sea level conditions and must be re-rated for operation at higher elevations. First, determine the local atmospheric pressure. A rule of thumb is that for every 1000 ft. of altitude above sea level, atmospheric pressure drops by 1 in.-Hg. Using rounded-off figures, for a city at an elevation of 5,000 ft, the atmospheric pressure is about 25 in.-Hg. To adjust a pump rating, think of that rating as a percentage of atmospheric pressure at sea level. If a pump is rated for 25 in.-Hg, it can achieve 83.4% (25 29.92) of a sea level perfect vacuum. At a 5000-ft elevation, that same pump can achieve 83.4% of 25 in.-Hg – or a vacuum of 20.85 in.-Hg."
They are saying that, at lower discharge pressures (below atm), the max achievable inlet vacuum is less (higher abs pressure). This goes to reason that at higher discharge pressures (above atm), the maax achievable vacuum would be higher (deeper, less abs pressure). Yet we are both saying (and I believe correctly by rational physics) the opposite.
That is what confuses me!
RE: Diaphragm Vacuum Pump Ratings
At a barometric pressure of 29.2" the pump can achieve a vacuum of 25" gauge or 4.2" absolute. At 25" barometric pressure the pump can achieve a vacuum of 20.85" gauge or 4.15" absolute (25"-20.85").
The only reason I can think of for such a confusing explanation is that the manufacturer assumed that their audience did not understand the difference between gauge and absolute pressure and did not want to get into it.