Thread: Vacuum as a percentage?

Hello,

Can anyone tell me if vacuum is able to be expressed accurately as a percentage?

I have a colleague who is using terms like 95% vacuum and 85% vacuum. What would he mean by this?

Your thoughts please.

Thanks.

A perfect vacuum at sea level is -29.92 inches of mercury, so 95% vacuum would be 28.42 in.-Hg and 85% vacuum is 25.43 in.-Hg.
http://www.hydraulicspneumatics.com/...460/Pneumatics

Originally Posted by missymig

Hello,

Can anyone tell me if vacuum is able to be expressed accurately as a percentage? I have a colleague who is using terms like 95% vacuum and 85% vacuum. What would he mean by this? Your thoughts please.
Thanks.

Maybe it simply means "95% of this volume contains no matter".

Thanks Harold.

What would you say to someone who is claiming that 20"Hg is equal to 95% vacuum?

OK. So percentage is probably an ambiguous term to use.

Originally Posted by missymig

OK. So percentage is probably an ambiguous term to use.

It's seldom used in my experience. In my work, I have maintained several types of equipment that uses venture type vacuum generators.

Anyway, look at the reverse. If you have 85% vacuum, the 1 - 0.85 = 0.15. You have 15% of the pressure. Now the question is, what is your reference source? I assume you are referring to atmospheric pressure, but that will still be different at sea level vs. a place like New Mexico.

The whole idea of expressing pressure as a pecentage is pretty much useless except to maybe compare two systems ie sys A can attain 95% of the vacuum of sys B. First off unless a base value is stated the number is meaningless, a pecent of what? STD conditions come to mind. 25% of 760 mm Hg means something, 25% of the ambient room pressure is meaningless unless a measured value is gven. Give me a Torr any time!

Why not express it in pounds per square inch? Does that not equate it to sea level atmospheric pressure?

Read the question!

Originally Posted by missymig

Thanks Harold.

What would you say to someone who is claiming that 20"Hg is equal to 95% vacuum?

If you want to express the pressure as gage pressure, then the maximum vacuum possible is whatever the atmospheric pressure is at your location.

If the atmospheric pressure is 21.05"HG, which is about 0.7 atmospheres, then 20"Hg would be 95%. That would be true at about 10,000 feet elevation.

Holy Mackrell!! just express it as a pressure torr, in Hg, tons per square mile or what ever.
% means nothing without a referrence point!
BYE!

Originally Posted by fizzlooney

Holy Mackrell!! just express it as a pressure torr, in Hg, tons per square mile or what ever.
% means nothing without a referrence point!
BYE!

LOL...

You're right. Getting a bit short are we?

At my previous work, we built a nice $3 million piece of semiconductor process equipment. See my avatar. The engineers spec'd out the vacuum to 650 mm if I recall. We would normally get 680 to 720. However, when we started installing the equipment in New Mexico, I think the installers were only able to get something like 580 mm. It took a long time before anyone realized the location we were installing this at was about 6,000 ft elevation. I immediately said that was the problem once someone brought up the elevation. I couldn't believe it. We had an engineer that just had to go to Timberline Lodge on Mt. Hood to test the venturies. Had to retest known sciences.

Oh well, maybe he just wanted a vacation day.

Now where I believe percentage comes in at, 700 mm of 760 mm is 92% vacuum. If the same 92% applies at the higher elevation, then pressure of 630 mm at 92% would be close to the 580 mm. At least within error limits of the SMC pressure sensors we used.

Now I don't recall the exact numbers except for normally seeing the 680 mm to 720 mm. Our manufacturing facility was less than 100 ft. above seal level. I didn't look at the pressure for a 6,000 ft elevation, but it wouldn't be too far from the 630 mm.

JEEEZ, how long can this idiotic thread go on? A vacuum chamber is a CLOSED system, atmospheric pressure has absolutely nothing to do with the pressure inside. Furthermore 650mm Hg vacuum is hardly a vacuum at all What that is saying is you have a PRESSURE equivalent to that of a column of mercury 650 mm tall . I assume you mean 760 mm minus the 650 mm or 110 mm (Torr) vacuum.
HERE:
Traditionally, the pressure in a system is stated in terms of the height of a column of mercury that may be supported by the pressure in the system. At one standard atmosphere the force is 1.03 kg/sq. cm (about 14.7 pounds per sq. inch). This pressure will support a mercury column 760 millimeter high (as in a barometer). One millimeter of mercury is the equivalent of 1 Torr. A thousandth of a millimeter is referred to as a micron of mercury or, in more current terminology, 1 milliTorr (mTorr). To be proper in the modern scientific world, the SI system of units is used. Here pressure is referred to in terms of newtons/sq. meter or Pascal (Pa). To convert Torr to Pascal, divide by 0.0075.

Just use Pa, it's easier than all those crazy and inconsistent units. I mean, mmHg even depends on the temperature (not much, but 2% difference between room temperature and 0°C), and inch is defined with mm, so why not directly use the easier units?

Anyway, for really, really high vacuums, I think molecules per is sometimes used.

As Wild Cobra explained, percentages make perfect sense to express the vacuum ability of a machine. It's the vacuum it can get relative to the ambient pressure. I'm a bit confused at his statement, though: "...in New Mexico, I think the installers were only able to get something like 580 mm..."
580 mm below 630 mm is better then 700 mm below 760 mm.

Originally Posted by fizzlooney

JEEEZ, how long can this idiotic thread go on? A vacuum chamber is a CLOSED system, atmospheric pressure has absolutely nothing to do with the pressure inside.

It does if you are using an ordinary vacuum gage to measure it.

Originally Posted by Bender

I'm a bit confused at his statement, though: "...in New Mexico, I think the installers were only able to get something like 580 mm..."

I think what he was saying is the installers didn't know to correct for the atmospheric pressure at that elevation.

Originally Posted by Bender

I'm a bit confused at his statement, though: "...in New Mexico, I think the installers were only able to get something like 580 mm..."
580 mm below 630 mm is better then 700 mm below 760 mm.

Remember, I not only gave a normal range for one, but a best for the other. On top of that, I was going by a cloudy memory.

Also, don't expect precision with venturi vacuum generation. Even minor changes in the scrubbed vacuum exhaust makes some difference.

Originally Posted by Harold14370

I think what he was saying is the installers didn't know to correct for the atmospheric pressure at that elevation.

That's exactly it. Then there was the worry that the small paddle size would lose the 200 mm wafers during the flips of the robotic movements.