scruffy_hair86 - 20-7-2011 at 01:37
Hey everyone, thanks for your time to read this and help out.
I recently bought a second hand vacuum pump that has a vacuum gauge connected to the unit, I got home plugged it in but can't seem to figure out how
to read the gauge.
It says it reads in m.m. Hg. but starts at 0 being the weakest vacuum and as the suction gets stronger the gauge increases to 760 which is the
strongest suction it has.
How do I read this scale to then find the expected boiling point of liquids?
Pic below.
woelen - 20-7-2011 at 02:29
The only reasonable explanation I can give is that this gauge shows the difference between air pressure and the vacuum achieved so far. So, per
example, if the value shown by the gauge is 500, then I expect the vacuum to be 260 mm Hg.
Endimion17 - 20-7-2011 at 04:57
That one just shows the difference between the air and the chamber you're evacuating. It's a relative measure. You need a barometer to read the
absolute current pressure, and then you can find the pressure in the chamber.
Mr. Wizard - 20-7-2011 at 05:26
From the markings on the label it was from a medical or hospital suction, it may have been exposed to bodily fluids internally. The most logical
explanation is the numbers corresponds to how many millimeters of Mercury the suction would lift. When it's at atmospheric pressure it will lift 0 mm
and when at full suction it would lift 760 mm of Mercury (Hg), which amazingly enough is Standard Atmospheric Pressure, at sea level, or it used to
be.
[Edited on 20-7-2011 by Mr. Wizard]
Magpie - 20-7-2011 at 06:41
That is simply a vacuum gauge calibrated in mmHg. When the pump is off the vacuum is zero. At full suction it could be as high as 760mmHg, depending
on the ambient pressure. At sea level it averages 760mmHg.
To determine when a liquid will boil at a given vacuum you can use this handy nomograph (thanks to peach
):
http://www.sigmaaldrich.com/sigma-aldrich/areas-of-interest/...
But as already stated, you will have to convert the "vacuum" reading of your gauge to "absolute pressure" by:
Absolute pressure = ambient pressure - vacuum (gauge reading)
Assuming you don't have a barometer call the airport to get the ambient pressure.
I guess the previous responders have said the same thing so this post may be redundant.
[Edited on 20-7-2011 by Magpie]