Sciencemadness Discussion Board

Vacuum pressure question

LD5050 - 1-3-2017 at 06:35

I have a harbor freight vacuum pump and I purchased a vacuum gauge that reads in mmhg. When I pull a vacuum the pump maxes out at around 730-750 mmhg ( its not as wide of range but giving number off the top of my head) I think it pulls around 740 to be precise. Now when I read about people speaking of vacuum pressure they say a good vacuum is around 25mmhg. Why does my vacuum pull all the way to 740mmhg? The gauge maxes out at 760 so do I subtract 740 from 760 to give me a vacuum pull of 20mmhg? Sorry I know I feel stupid for asking probably such a silly question...

Praxichys - 1-3-2017 at 07:12

Yes, the reason most vacuum gauges measure "negative pressure" is that you are measuring "gauge pressure" which is pressure relative to ambient, or the outside of the gauge. The minimum pressure a vacuum pump can reach is also relative to ambient atmospheric pressure since the pump has to push against it, so it is easier to measure relative pressure (relative to the atmosphere) rather than absolute pressure.

So yes, assuming the barometric pressure that day is 760mmHg, a reading of -740mmHg on the vacuum gauge is equivalent to an absolute pressure of 20mmHg.

wg48 - 1-3-2017 at 07:27

Quote: Originally posted by LD5050  
I have a harbor freight vacuum pump and I purchased a vacuum gauge that reads in mmhg. When I pull a vacuum the pump maxes out at around 730-750 mmhg ( its not as wide of range but giving number off the top of my head) I think it pulls around 740 to be precise. Now when I read about people speaking of vacuum pressure they say a good vacuum is around 25mmhg. Why does my vacuum pull all the way to 740mmhg? The gauge maxes out at 760 so do I subtract 740 from 760 to give me a vacuum pull of 20mmhg? Sorry I know I feel stupid for asking probably such a silly question...


If the gauge shows 0mmHg when open to the air then yes subtract the reading from 760 (or more accurately the ambient air pressure in mmHg) to get the absolute pressure (perfect vacuum is 0mmHg) Many gauges have a minus sign on the scale to indicate the that reading is mmHg below air pressure. ie a pressure relative to air pressure.

LD5050 - 1-3-2017 at 14:42

Awesome thanks for the reply.

macckone - 1-3-2017 at 19:13

If you live above sea level, then 760 might not be the correct value.

http://www.engineeringtoolbox.com/air-altitude-pressure-d_46...

For high vacuum, you really need a gauge that shows absolute pressure
and that is designed for high vacuum work.

Sulaiman - 2-3-2017 at 05:23

just to add to the above;
last week I sucked mercury up a 9mm i.d. glass tube to test my vacuum pumps,
my dual-stage rotary only managed 735 mm Hg !
due to being 190m above sea-level and in a local low-pressure weather system, and the mercury was at 6oC

[Edited on 2-3-2017 by Sulaiman]

PirateDocBrown - 2-3-2017 at 08:06

Could be helpful to get a good lab barometer.

Sulaiman - 2-3-2017 at 09:07

Quote: Originally posted by PirateDocBrown  
Could be helpful to get a good lab barometer.


cheap wall-hanging barometers are good enough for lab use,
you can adjust for absolute pressure, or compensated for height above sea level

between atmospheric pressure and a few mm Hg absolute pressure, a mercury manometer works well
but I mostly use my cheap Bourdon gauge as it is good enough for my purposes and a lot easier and safer to use.

I can't measure below a few mm Hg ... yet

Bezaleel - 3-3-2017 at 06:22

Same here. I never took the effort to look for a gauge capable of measuring below a few mBar. If anyone has experience with such gauges and/or knows which type are practical in use and/or knows where to buy them at a good price, I'm interested to read your post.

wg48 - 3-3-2017 at 11:24

Quote: Originally posted by Bezaleel  
Same here. I never took the effort to look for a gauge capable of measuring below a few mBar. If anyone has experience with such gauges and/or knows which type are practical in use and/or knows where to buy them at a good price, I'm interested to read your post.


The obvious place is ebay for a used one. The most robust type are the absolute reading diaphragm types but only good to a about 0.1mtorr. I have bid on several but they all sold too high for me at a round 30£/$ and up.