Reading a vacuum gauge

Science
#1

Continuing the discussion from Anyone Good at Evacuating Refrigeration Systems?:

Forgive my virtually complete ignorance…
I’ve done A/C work for years, and we just hook up a gauge, pull a vacuum, and look for ~ -29 gauge units (I think it’s PSI). Evacuate at this rate for 15 or so minutes, and then cap off. Check after 1 hour to make sure the gauge has not moved, and if not, charge with refrigerant and get that shyte out the door.

So, on the chart @Photomancer posted, very cool, BTW, the PSI does not go negative. I assume this is because these are “absolute psi” instead of “gauge psi”, wherein the “gauge psi” 14.7 (or so) reads zero. In other words, 1 atmosphere is equal to zero on the average a/c gauge setup. Right? OK.

So, the venturi vacs like the one available in the 'space and, therefore, I assume the one the Science Committee has used, claims to pull 28.3 inches of mercury. I assume that’s “gauge” on the chart, and therefore is roughly 1.57 inches of mercury absolute, or 5.3kPa, or 5300 Pa, or 40,000 Micron. Right?
This is 5.3X10^3, which falls under “medium vacuum” on the Vacuum Ranges chart, right?

So, this pump David’s talking about (lending or donating) is capable of <50 micron, (similar pumps to what I think this is specify 35 microns), which, as is pointed out, is .00065 kPa, which is .65 Pa. Right?
.65 Pa is 6.5X10^-1, which falls under the “High Vacuum” category on the Vacuum Ranges chart, right?

Thank you to anyone who can confirm or correct my assertions. It’s clear I don’t do this stuff enough any more, and that I should pay more attention to the things I do.

#2

If the number is 29, it is not like in pounds per square inch, but rather inches of mercury in a barometer. The precision of gauges that measure in negatives of this unit are rough. Basically they are estimating the draw down from standard atmospheric pressure (which can vary +/- 1-2 inches of mercury. And the -29 is further likely another +/- 1-2"

The difference is really one of the intended purpose of the pump. The ones you are describing are really intended for HVAC and similar applications which simply don’t need the precision that an application like Richards might call for.

You will note that Dave’s chart shows a range of six orders of magnitude that qualifies as ‘high vacuum’. That is a huge variation.

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#3

Usually we shoot for under 500 microns but settle if it’s under 2000. Basically you want to boil out all the moisture but at a rate that you don’t freeze it. If you put to big of a pump you can freeze the moisture in the system.

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#4

Also we used to use Mercury manometer for years to see what vacuum readings we have.

image.jpg640×960 173 KB

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#5

It’s right on the boundary between medium and high vacuum, according to this chart. I’d only point out that the range from one vacuum domain to another is only approximate, not some universal constant.

#6

So are "microns"on this chart “millitorr” as noted here?

#7

No. Torr and mm Hg are often confused, as they are close to the same, but they aren’t the same, at least philosophically. A micron is one millionth meter of Hg.

“Historically, one torr was intended to be the same as one “millimetre of mercury”. However, subsequent redefinitions of the two units made them slightly different (by less than 0.000015%). The torr is not part of the International System of Units (SI), but it is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr.”

“As vacuum pumps became more efficient it also became necessary to have a smaller vacuum measurement unit than the mm of Hg. This was only possible after the invention of electronic vacuum gauges because you really can’t see any divisions of a millimeter on a linear scale. The mm of Hg was divided into 1000 smaller parts which were called microns. The word micron means a one millionth part of a meter.”

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#8

For completeness, I’ll add the following links, intended from the perspective of HVAC systems, rather than vacuum technology:

“JB: Deep Vacuum: It’s Principle and Application”
http://www.jbind.com/pdf/Deep-Vacuum-Principles.pdf

“JB: Deep Vacuum Principles and Applications”
http://www.jbind.com/marketing/Principles%20of%20Vacuum%20Presention.pdf

As you might imagine, the two links include similar material, but they have enough differences to include both of them.

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#9

Lol. I used a 7 cfm vacuum pump to pull a 5-1000 ton chiller overnight. Usually under 300 microns lately if we were able to get it assembled well.

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#10

HVAC produces vacuum in the last millimeter of atmospheric pressure. The micron divides that millimeter into 1000 parts.

High vacuum technology takes one micron and divides it into another 1000 parts. Ultra-high vacuum technology then takes one of those and divides it, again, into another 1000 parts.

So, much of our high- and ultra-high vacuum technology takes place with pressures that are 1000 to 1,000,000 times less than the best you could get from an HVAC system. No mechanical pump can reach these regimes, no matter how many stages they have.

http://www.belljar.net/basics.htm

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#11

Are you excluding turbo molecular pumps from the set of mechanical pumps, or are you really trying to say that no piston or rotary vane pumps that you find routinely are capable of high vacuum?

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#12

I am excluding turbomolecular pumps from the set of mechanical pumps, as they operate in completely different domains. It’s been a few decades since I took Vacuum Technology, but, as I recall, turbomolecular pumps cannot be operated anywhere near atmospheric pressure, on either inlet or outlet. Mechanical pumps are designed to work in the fluid domain, whereas turbomolecular pumps operate in the particle domain.

#13

So basically a non common sense definition for a common word.

It is difficult to accept a common language definition of mechanical that excludes bearings and rotors operating typically over 22,000 RPM.

But you are correct that TM pumps do require decent backing vacuum.

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#14

I don’t believe that “turbomolecular” is a common word.

There are many ways of categorizing pumps. What I’ve termed mechanical pumps also could be categorized as roughing pumps or displacement pumps (either positive or non-positive displacement). Turbomolecular pumps, however, are neither roughing pumps nor displacement pumps, but are momentum transfer pumps. In all these cases, it isn’t the components of the pump that are the focus of interest, but, rather, the operating principle of the pump. The fact that a pump consists of bearings and gears is not as important to its operation as the mechanism these components are using to pump. If that is so, then “mechanical” doesn’t refer to the components of the pump, but to the method of pumping.

https://www.lesker.com/newweb/vacuum_pumps/pdf/kjlced09_sec04_pages2-7_technicalnotes.pdf

#15

It also wasn’t that long ago that there were no high vacuum pumps with moving parts, aside from the oil vapor in a diffusion pump. So there may be a historical usage of them being synonymous.

TM pumps are also different than other momentum transfer pumps (diffusion pumps) in that the collision is with a moving solid part of the pump, rather than a working fluid. (diffusion oil vapor)

The DOE and university labs I’ve worked in always used the nomenclature of backing pump or roughing pump, vs turbo molecular pumps. They rarely spoke of the other outcast high vacuum pumps stuffed in the corners that they never wanted to use again, but couldn’t quite convince themselves to surplus.

#16

The turbomolecular pump was invented in 1958, based on molecular drag pumps that were developed in 1913. So, between 50 and 100 years.

http://www.vacuumlab.com/Articles/Sorting%20Out%20Turbo%20Drag.pdf

I mentioned that roughing pump is another name used for mechanical pumps. I don’t recall ever seeing any other type of pump referred to as a mechanical pump, though. The reference I provided in my last post distinguished between mechanical pumps and high vacuum pumps, for example. The following reference also considers mechanical pumps as low-vacuum pumps:

http://www.cientificosaficionados.com/libros/CERN/vacio2-CERN.pdf