LD5050
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Vacuum pressure question
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...
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Praxichys
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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.
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wg48
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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...
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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.
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LD5050
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Awesome thanks for the reply.
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macckone
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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.
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Sulaiman
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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]
CAUTION : Hobby Chemist, not Professional or even Amateur
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PirateDocBrown
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Could be helpful to get a good lab barometer.
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Sulaiman
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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
CAUTION : Hobby Chemist, not Professional or even Amateur
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Bezaleel
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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.
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wg48
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| 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.
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