FAQ: Pressure = Vacuum

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Richard Hull
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FAQ: Pressure = Vacuum

Post by Richard Hull » Mon Sep 29, 2008 4:06 pm

There is no such thing as a pure vacuum, only varying degrees of pressure.

We live on an air filled planet where the pressure at sea level is about 14.7 PSI or 760mm of mercury which is 760 torr. Even deep space has gas molecules in it, rare though they may be. We also live in a world where our bodies are comfortable, for the most part. Temperature, thermal conditions, can increase or reduce air pressure as it varies. Thus, all earth bound vacuums are specified against something called "standard temperature and pressure" or "STP". This is basically what is termed normal "sea level pressure" in dry air and at fixed temperature that is arbitrarily chosen as zero degrees centigrade.

For the sake of keeping this FAQ short, we will discuss only the units of pressure that we, and most books you might read, deal with. The main unit encountered is the "Torr". This is one millimeter of mercury column pressure rise. Atmospheric pressure is about 760 mm of mercury or 760 Torr under standard conditions of temperature and pressure. This has about 25 Quintillion gas molecules in each tiny cubic centimeter of air. Pressures less than STP are referred to as being at varying degrees of "vacuum".

There is no such reading as zero torr. It doesn't exist in this universe, to our knowledge. There are always molecules of gas; even in intergalactic space.

There are many pressures that various technologies would call "its" optimum vacuum level. Remember, all vacuums have a pressure associated with them. We are, therefore, trained to speak of "vacuum pressures". As such, the term vacuum literally seems a misnomer once this is comprehended. For most purposes a "vacuum" is a gas pressure significantly less than that of normal atmospheric pressure.

For a neon worker, a vacuum of 1-100mm or 1-100 torr is normal. For hard scientific vacuums, often termed "High vacuum", one needs to be at or below 10e-6 torr. This is where most vacuum tubes operate. Special, "ultra high" vacuums are found below pressures of 10e-8 torr. It is extremely difficult to achieve, hold and even measure vacuums below a pressure of 10e-12 torr. Even at this deep vacuum there are millions of gas molecules in every tiny cc of the chamber.

We often use a term in our fusion effort called the micron. This is an old term, but still used a lot by workers haunting the area called "technical vacuums". The micron is supposed to denote a millionth of an atmosphere but it falls short of that hope.

The reasoning, flawed though it may be, is that the atmospheric pressure is sorta' close to 1000 torr and one millionth of that is 10e-3 torr. This is the micron. It is merely a word for 10e-3 torr.

The traditional fusor we run and operate runs at anywhere from 5 to 20 microns or 5X10e-3 to 2X10e-2 torr of gas pressure. For us, in our work, the nice whole micron numbers with no exponentials, as in torr, are a lot easier to both visualize and verbalize. In short, the fusor works in the millionths of an atmosphere range. We have pumped out a lot of gas molecules, but still have over 10 trillion gas molecules in every cubic centimeter of our fusors.

No professional, scientific paper would, however, be accepted using microns, but scientists and workers speak in microns all the time. Microns are the region of "glow discharge" gas pressures.

You will notice there are no minus pressures for any units. All units go fractional. a deep vacuum of 10e-10 torr is actually 0.0000000001 torr or mm of mercury.

One is allowed to speak of submicrons i.e., 0.2 microns or 2X10e-4 torr. However, below this final decade, it is a no-no to speak in hundredths of microns. You must now speak in exponential torr units again as you have left the technical or glow discharge vacuum pressures far behind and are in the scientific vacuum pressure range. Down here when asked about your pressure you might just respond verbally with..."About ten to the minus seventh". One assumes the torr is the unit of choice.

Just remember that all vacuums are really just varying degrees of pressurized gas environments that are well below normal atmospheric pressure.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ: Pressure = Vacuum

Post by Starfire » Mon Sep 29, 2008 8:40 pm

Once again a concise FAQ and yet so very suitable for a novice.
I just love your FAQ's Richard - and the library builds.

Thanks Richard.

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Richard Hull
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Re: FAQ: Pressure = Vacuum

Post by Richard Hull » Tue Sep 30, 2008 3:13 pm

Thanks John. I take the time to do this work as I hope that such efforts will help avoid the same questions repeated over and over by newbies.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: FAQ: Pressure = Vacuum

Post by Dennis P Brown » Thu Jul 19, 2012 1:56 pm

Excellent overview and a lot of good information - especially on usage.

I would think, for completeness, a few lines on a common scientific unit of pressure - the Bar - would be helpful. I never use it (and all my gauges are ebay = cheap = old = use torr) but others do.

Just a thought.

Edit: Technically, a Pascal is the proper SI unit but 1 Bar = 10^-5 Pa; a Bar is a nicer sized unit but not strictly SI. (so that means 7.50*10^-3 torr about equals 1 Pa)

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Richard Hull
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Re: FAQ: Pressure = Vacuum

Post by Richard Hull » Fri Jul 20, 2012 3:21 pm

Dennis, Thanks for the additional input to this FAQ. I will allow your short dissertation here to suffice on the Bar/Pascal pressure update. Those interested enough to look at the replies will see it as an additional bit of info.

I have noted before that the micron and not the Bar or Pascal is a perfect unit for fusors. Our foreline is in the range 1-30 microns. The fusor's actual fusion run pressures are between 5 and 15 microns. As noted, most all instruments for reading "technical vacuums", that the amateur will acquire, will read in microns. Ultra low pressure gauges are almost 100% calibrated to read in Torr.

Whenever scientific notation is demanded the Torr, Bar or Pascal units can be referenced. However, 99.9% of the real workers here will use and understand only Torr, in that base pressures are in the simple 10e-5 to 10e-6 torr range and the micron is roughly 10e-3 torr. Torr and Microns tend to keep things in whole units such that the mental grasp of such ranges in pressure remains simple

I have advised, in the past, that anyone doing a scientific paper, especially one that seeks publication or is to be submitted to a teacher or professor, MUST use SI units. Unfortunately many of the scientific readers of such papers will have to mentally down convert the SI units to what they were taught.....Torr and Microns.

Until the last TC gauge, capacitive manometer, cold cathode gauge, ion gauge cal'd in microns and torr is in the junk yard under 10 feet of other waste, Pascal and Bar will forever remain foreign to the amateur vacuumist and even many pros.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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