Diaphragm Vacuum Pump Ratings

[Orionstar]

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.

 

[Compositepro]

There is always some dead space in the cylinder between the diaphragm or piston and the exhaust valves. Pressure on the exhaust side of the valve will result in more gas remaining in the cylinder which re-expands on the intake stoke. Thus the intake capacity is reduced. High-vacuum pumps are generally oil-sealed so that no air can leak in and so that a small amount of oil always remains in the cylinder to displace air from the dead space at the end of the exhaust port. However, the main reason to use a diaphragm vacuum pump is to have a dry pump without concerns over oil contamination.

 

[Orionstar]

Compositepro - so you agree with me that it is highly unlikely that the max vacuum rating of a diaphragm pump (this one is published as 70 mmHg) won't change (become a higher number mmHg) if the pump output is into a 15 psig tank vs atmosphere?

 

[Compositepro]

No, quite the opposite. Say that the dead space in the cylinder is 5% of the volume swept by piston/diaphragm. That means that the maximum compression ratio is 20. When you reach a ratio of 20 all that will happen is that air in the cylinder will just be compressed and re-expanded in the cylinder because the peak pressure in the cylinder is less than it is on the other side of the exhaust valve. The compression ratio is the exhaust pressure divided by the inlet pressure in absolute pressure units. Thus, if the outlet pressure is doubled, then the the achievable inlet pressure also doubles from say 75 mmHg to 150 mmHg I assume you understand the difference between absolute pressure and gauge pressure. mmHg is usually means absolute pressure, which is referenced to a perfect vacuum (though sometimes it may mean differential pressure). psig is a gauge pressure, meaning that it is referenced to ambient air pressure (0 psig = about 14.7 psia).

 

[Orionstar]

I do understand the physics and I'm saying what your saying. I guess I didn't write it well, but if you re-read what I said, the mmHg will become higher (like you said in your example of 150 mmHg). Yes, that is a "lower vacuum" (higher abs pressure). I agree totally with your explanation of the pump and compression ratio.

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!

 

[Compositepro]

Wow, that is a pretty confusing paragraph and took me a while to figure out. The problem is that it discusses the affect of barometric pressure on a vacuum pump rating given in gauge pressure. If you convert the gauge pressures to absolute pressures it is far easier to understand. That the 83.4% figure is a constant is basically saying that the compression ratio stays constant.

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.