Vacuum system can't hold vacuum

[Harrison Gieselman]

I am starting work on my demo vacuum system. My pump is a 3 CFM 2 stage with an ultimate vacuum of 3 microns. When I shut the pump off, the system immediately goes to atmospheric pressure and there is an audible hissing noise. Is this normal with a mechanical pump, or do I have a leak somewhere? If it is normal, I can't seem to get below 29 inHg with the pump. Thanks.

[Sarvesh Sadana]

If you hear a hiss, it means there is a leak. Try spraying some dust-off around suspect areas your to see if the pressure spikes with the pump running.

Also, some pictures would be good.

[Rich Feldman]

Wrong forum, Harrison! See where it says:

Now go fix your leak. A good starting point would be to learn the sound of your pump with an open inlet, and with your thumb plugging the inlet, and compare those with the sound pumping on leaky chamber. Or put some food in the leaky chamber, leave it outside, and wait for ants to find their way in through the leak.

Sarvesh, thanks for trying to help. You, too, have a few things to learn. When people talk about a vacuum level of 29 inches Hg, that's measured down from the ambient atmosphere. Very practical when the vacuum is used for suction, or other mechanical force generation. The "gauge" pressure is -736 torr, which is 97% of the way to a perfect vacuum.

So Harrison's absolute pressure is around +24 torr, 3% of ambient. That result is very sensitive to gauge errors, local weather, and geographic elevation. With a perfect gauge in Atlanta, GA, you'd need a perfect vacuum to get a reading of 29 inches. In Denver, CO, that gauge will never show a vacuum stronger than 25 inches.

[Sarvesh Sadana]

Ah, my mistake. Thanks for the tip. I have edited my post.

[Harrison Gieselman]

Sorry about that. Thanks for the help!