Vacuum Pump Performance and Altitude

I’ve read that “a good rule of thumb is to reduce the end vacuum level by 1” Hg per 1,000 feet gained”

How does that work in the inverse when operating below sea level?

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Below sea level you’d have increased pressure so it would raise the boiling point

Boiling happens when vapor pressure of whatever youre trying to boil is = to the atmospheric pressure at whatever Altitude.

Theres also a big difference between evaporating and boiling

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Nvm doesn’t apply here.

It really has more to do with the gauge calibration than what the actual absolute pressure is. In fact, most pumps can actually pull deeper abs. pressure at altitude because the discharge pressure is lower.

What your really seeing with a compound gauge like on a vac oven is that at “true” vacuum, the pressure is only 27" or whatever less than ambient because, well, ambient atmospheric pressure is less.

FYI, the local FBO is usually the best source of local atmospheric pressure on any given day, which is important if you’re say calibrating a pirani or thermocouple gauge

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I just skimmed through this, it’s a helpful lil guide.

Basically the maximum achievable vacuum is based on the atmospheric pressure. If atmospheric pressure is higher than a deeper vacuum is achievable.

In theory, as one travels below sea level, the max achievable vacuum increases.

From a theoretical perspective it’s mainly about the number of molecules of air per unit volume. If you’re in Denver, the max vacuum is not as deep because there’s less molecules of air per unit volume. But the RELATIVE vacuum between atmosphere and vacuum pump inlet should be the same in whatever altitude one is in.

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Did you find an old mine in Death Valley, or a submarine? Both would be kinda pimp Lab locations.

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You achieve about the same absolute vacuum (molecules per volume) regardless of where you are. It is only the fact that vacuum gauges read relative to atmosphere that makes this appear to not be the case. I did another post about it here:

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So it just takes longer to get to absolute? The link posted by @TheGratefulPhil seems to claim that the max vacuum depth is dictated by altitude

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In short, yes.

There are more molecules of air to remove to reach absolute vacuum at lower altitudes.

I also found an article I wrote for @sciphysystems a while back regarding vacuum, it’s a pretty good resource (in my opinion).

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Vacuum is a sealed system. It’s only relative to the guage. No matter the altitude the pump to chamber will nearly always be the same. It’s just how the guage opperates.

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No, the article he linked is saying the same thing i am. The max vacuum depth registered on the gauge is determined by altitude – but that only has to do with how the gauge defines vacuum, not anything actually different inside the system. You just need to correct your readings accordingly.

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This is just due to stupid vacuum oven gauges. This doesn’t even apply to most vacuum gauges. Everyone would be better off if they just ignored that whole thing. There might be some finer points but lets conclude that vacuum has nothing to do with altitude.

Seems this is very specifically saying you may need to buy a pump that goes to deeper absolute pressure ASL in order to compensate for altitude gain. Am I misreading this?

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Where is this from? That looks like something talking about what kind of work you can get the vacuum perform (eg a vacuum fixture for machining or the like), in which case yes that’s correct. In terms of how many molecules are loose banging around in the box, there’s an issue with the first sentence

Yes I’d say you’re misreading it (or rather they’re just doing a bad job of explaining it). What they’re saying is that if you get 20% of atmosphere at sea level, when you’re in denver you’ll get 20% of the reduced Denver atmosphere.

20% of 29.92 = 5.984 absolute
20% of 24 = 4.8 absolute

So you can see the vacuum is actually deeper in Denver. However what the gauge will read is:

Sea level: 0 - (.8 * 29.92) = - 23.936 gauge
Denver: 0 - (.8 * 24) = -19.2 gauge

So the vacuum will look worse on a guage in Denver, but actually, it’s better.

The guide is simply pointing out that you need to do this math when selecting a pump to determine appropriate sizing. Notice in the last sentence they actually switch to a totally different problem, and ask you to hit an arbitrary vacuum capacity in Denver. The question was not whether the new pump was bigger than in problem 1 or not.

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Think of it this way:

For the most part we’re talking about rotary vane or liquid ring pumps; they’re not technically positive displacement but they’re essentially positive displacement. For each rotation, some volume of gas is moved (displaced). As the pressure on the inlet side decreases, the amount of gas in that displacement also decreases. Ultimately, the pressure stalls when the amount of gas being moved out of the system is equal to the amount of gas being evolved/leaked into the system in the period of one rotation.

There is a factor of the discharge pressure on the amount actually moved on each rotation but in the grand scheme of things, it is probably of less impact than the rate of leakage into the system

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Vacuum chamber and vacuum pump is a closed chamber. Altitude has zero effect on the pumps ability to pull complete vacuum. Old articles are based on systems that would overcome leaks and leaks affect vacuum at altitude.

If you have any modern hardware it pulls identical rating at seas level, below sea level and above sea level.

The user has to calibrate the guage to sea level. Then go use it. If you calibrate at higher altitude the guage will not read properly. It will always given the quasi answer that the old articles are talking about.

There is zero effect on inside vacuum at altitude. Typically.

@Kingofthekush420 comin in clutch

This was a google search too

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I was trying to find that earlier actually

That really came up on Google?

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