Vacuum pump operating at high altitude

[Ben_the_Druid]

Hi I live in a high Altitude around 2500 meter over sealevel. I have a vacuum pump, at max my vacuum meter shows 21 inHG inside the vacuum chamber . my vacuum pump(welch 2014B) has a max of 40 torr it say the technical datasheet.

What is now my endpressure in torr in my vacuumchamber?It's really confusing cause of the altitude...

Do vacuum pumps work better at sealevel or does the altitude has no effect on the efficency of my vacuum pump? Please help

 

[gleem]

I would think at higher altitudes you would be able to hold a lower chamber pressure. At 2500 m the standard atmospheric air pressure should be about 22.4 in Hg (.75 Atm) . I would expect the 40 Torr spec to decrease to 30 Torr (40 x .75). the ultimate chamber pressure should be determined by the difference in chamber pressure and atmospheric pressure. If you are only pulling 21 in Hg your pump should be struggling to suggest a major leak or defective pump.

How is the oil level?.

 

[Ben_the_Druid]

Thank you very much for your reply, my vacuum pump is a diaphragm pump, so its oilfree.
Why you think the vacuum pump would get a lower pressure(the lower the better) at higher altitude?
If 22.4 is max inHG at my altitude then 21 inHG is 93.75% of max vacuum at this altitude, is this the way to calculate the rest pressure in the chamber?

 

[Gordianus]

You should get 40 torr regardless the atmospheric pressure. I'd check for hidden leakages.

 

[gleem]

22.4 is not the max. the pressure is just what is known as the standard pressure at your altitude. It could be a bit higher or lower depending on the weather system over you.

 

[Ben_the_Druid]

Thank you for the reply, I am pretty sure my system is leakfree.
how should i measure the rest pressure in the vacuum chamber in torr if my meter shows only inHG?
I can clearly see a difference of the reading at sealevel, my vacuumgauge is analog. at sealevel it get to almost maximum which is 29.5 inHG in high altitude this meter shows only 75% of that value. So how do i know how to read the gauge in higher altitude?

 

[russ_watters]

Guys, I think the vacuum gauges measure using local pressure as a reference and assuming it's sea level, 30". So if atmospheric pressure is 22", then 22" of vacuum is almost there, as OP said.

Most pressure gouges are relative/differential pressure gauges.

 

[russ_watters]

Thank you for the reply, I am pretty sure my system is leakfree.
how should i measure the rest pressure in the vacuum chamber in torr if my meter shows only inHG?

Can you buy a barometer or get a local weather report? Then convert?

I can clearly see a difference of the reading at sealevel, my vacuumgauge is analog. at sealevel it get to almost maximum which is 29.5 inHG in high altitude this meter shows only 75% of that value. So how do i know how to read the gauge in higher altitude?

Add the difference between your pressure and sea level pressure.

 

[gleem]

You should read about 5% of your atmospheric pressure.

 

[gleem]

Thank you for the reply, I am pretty sure my system is leakfree.

And what is the reason you think it is leak-free?

 

[Ben_the_Druid]

Here(-0.175088, -78.360550) the meteo say its 1014 hpa, i know this is converted to sealevel.
We are at 2437.0 m over sealevel. so what is now the hpa at my altitude?
I haven't found a page where it display the atmospheric pressure for the altitude.
And thank you guys for all your support and all this answers!

 

[DaveE]

I haven't found a page where it display the atmospheric pressure for the altitude.

Really? This was the second link in m y google search, one of many:
https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

 

[Ben_the_Druid]

Thank you for the link, sure I have found some similar tables. I wasn't clear in my expressions sry. I wanted to say that I haven't found a page where it display the actual atmospheric pressure calculated for the elevation that this location is. it shows you only the sealevel pressure at this moment, but not for the meters over sealevel that this location is at the moment. so maybe a formula would help, as i know the elevation and the atmospheric pressure at sealevel at this location.

 

[Ben_the_Druid]

It is leakfree cause its very small and very simple and I think I would find a leak very quickly. its just the pump, the hose and the vacuum chamber attached to it. the vacuum chamber is a simple pan with a acrylic glass top with a silicon sealing on the border on it with a a vacuum gauge and a a simple air valve with the hose attached to it. pretty sure it don't leak. i guess that's not the problem.

 

[DaveE]

It is leakfree cause its very small and very simple and I think I would find a leak very quickly. its just the pump, the hose and the vacuum chamber attached to it. the vacuum chamber is a simple pan with a acrylic glass top with a silicon sealing on the border on it with a a vacuum gauge and a a simple air valve with the hose attached to it. pretty sure it don't leak. i guess that's not the problem.

But you're not describing a big leak in it's performance. How would you know if there's a small leak, maybe in the pump itself? Perhaps you could characterize this with a "leakdown" test. Make a vacuum, turn the pump off, and monitor the pressure over time. I have no idea what the absolute calibration is for how fast air can leak in but it should be a good repeatable test. This will also allow you to do some troubleshooting if you do need to find a leak.

BTW, most vacuum pumps have a separate valve at the exit to isolate the pump when it's off, since they do leak. I think the final vacuum is often achieved when the pumps ability to remove atoms is matched by the influx of atoms leaking in.

 

[gleem]

its just the pump, the hose and the vacuum chamber attached to it. the vacuum chamber is a simple pan with a acrylic glass top with a silicon sealing on the border on it with a a vacuum gauge and a a simple air valve with the hose attached to it. pretty sure it don't leak. i guess that's not the problem.

Still possibilities. The acrylic pan surfaces need to be smooth and scratch-free, the vacuum gauge itself and the manner in which it is fastened to the chamber, and the simple air valve, is it connected to assure that it is not leaking through the stem?

@DaveE suggestion is a good test. If leak-free except for possible outgassing it should hold the vacuum for hours if not more. Oh, is there anything in the chamber?

Considering the vacuum you are pulling is poor there has got to be air getting in from somewhere.

I don't know if your gauge is sensitive enough to notice alcohol being pulled in if you spray some on a leak or whether you could hear it.

 

[Baluncore]

Here are some conversions between atmospheric pressure and altitude in feet.

Double Po = 1000   ' in hPa, pressure at sea level in consistent units

Function altitude_to_pressure( Byval alt As Double ) As Double
    Return Po * ( 1 - 6.87535e-6 * alt )^5.2561
End Function

Function pressure_to_altitude( Byval Pa As Double ) As Double
    Return ( Exp( ( Log( Pa / Po )) / 5.2558797 ) - 1 ) / -6.8755856e-6
End Function

Edit: Source is from; https://www.brisbanehotairballooning.com.au/wp-content/uploads/PressuretoAltitudeConversion.pdf

 

[Ben_the_Druid]

Hey guys good morning,

yes the chamber can hold the vacuum for hours, even if its a poor vacuum, its stay at the same level over night. So I was thinking a lot about what you guys wrote and everything.
I come to this final answer:

Pressure inside the vacuum chamber = difference of chamber pressure and atmospheric pressure
chamber pressure is 21inHG = 533,4 torr
atmospheric pressure is 771 hpa(i don't have a barometer but i calculated this) = 578.2 torr
578.2 - 533.4 = 44.8 torr (4.8 torr away from the max of the pump)
also the gauges arent so exactly readable to make it more exactly.

So I guess my pump work just according to its specifications from the manufacturer.
But I can say to work with vacuumpumps at sealevel is much more easy to calculate things.
I wish meteosites would offer a altitude function, to see the actual pressure at the altitudelevel for any location.

Im still not sure if a vacuum pump work better at sealevel and get a deeper vacuum.
The reason why I think this is cause there are limitations for altitude from the manufacturers of vacuum pumps.

Lets say we have a 2 stage vacuumpump(with oil) that gets down to one micron at sealevel, would it be same or less or more at the mounteverest?

Or in other words the ability to remove atoms would be less or more when there are more atoms(higher pressure) around the pump?

 

[gleem]

@Ben_the_Druid: One more question what does your gauge read with no vacuum?

EDIT: Ignore this question, Sorry