Vacuum Plate

The most vacuum you can achieve is obviously 0 psia, or near this value.

Pumps have a curve showing the fluid flow X working pressure. Every pressure to be achieved correspond to a maximum flow. They can be obtained from the pump manufacturer, and depend on the pump size, power, technology, and current integrity.

For exemple, inexpensive chinese air compressors that you plug in your car 12 VDC outlet can easily reach 150 psig pressure. However, an industrial air compressed central delivering high flow will demand several KVAs of power in an installation running at 3 fase 480 VAC with a dedicated substation, just to keep a high flow of around 100 psig.

It's a matter of energy transformation, not pressure itself.

You can get verrrrry close to zero pressure or a perfect vacuum but not an absolute vacuum!

A bit like absolute zero you can get within millionths of a degree or torr of it but never achieve it totally...

Not sure what you mean about the 'size of your pump'!!

The size isn't important (so my woman friend says!) Take for example the getter in a vacuum valve or a 'tube' as you Americans call it, the getter is sealed inside the valve envelope after a vacuum has been established and the glass envelope is sealed, the getter is then fired by rf radiation to remove the last atoms of gas present, and it remains active removing molecules of gas until it expires due to the glass seal being breached this causes the colour of it to change to a whitish shade which is a technician's delight to find, as its an easy sign of a 'soft' valve that needs replacing....

John.

Achieving an absolute vacuum is theoretically impossible just like attaining a temperature of absolute zero; because zero = 1/ (infinity), and infinity is impossible to achieve (or so I'm told).

The Pump size only means how long it will take to get to where you want to go. In a past life, we pulled everything down to within 2.5 torr and then tried to hold that for 30 min. if we could we were assured a leak free unit. Anyway the size of the pump related to the time it took to pull it down. We started out with really small pumps and spent the day watching grass grow. We ended up with these monster pumps with 2" hoses and could achieve 2.5 torr within 5 or 7 min. depending on any moisture in the unit. The under vacuum, water will boil away and dry out.

The most vacuum is negative barometric or atmospheric pressure and varies constantly at a given location as influenced primarily by altitude, wind velocity and direction, ambient temp, moisture

At sea level when 14.696 psi gauge equals barometric ambient pressure

a perfect vacuum of zero(0) psia is equal -14.696psig

The barometric or total pressure (14.696 psig) is equal to the sum of all the partial pressures of the constituents of the gas (ambient air). The pressure exerted by nitrogen plus the pressure exerted by oxygen plus the pressure exerted by moisture (relative humidity) plus the pressure exerted by each of the other (including pollutant) gases present will add up to a total barometric pressure of +14.696 psia (0 psig) which would yield a theoratical perfect vacuum of -14.696 psig vacuum (0 psia). Vacuum may be measured in units torr.

The most vacuum is negative barometric or atmospheric pressure and varies constantly at a given location as influenced primarily by altitude, wind velocity and direction, ambient temp, moisture

Sorry I must disagree with that definition. A vacuum doesn't vary with climatic conditions, a perfect vacuum is the absence of any gas molecules and doesn't vary from that.

What you are saying is that ambient atmospheric conditions vary, a vacuum doesn't.

John.

"What is the most vacuum I can hope to get. Is there NO limits or is based on the size of my pump?"

Mechanical, diffusion, molecular, or other extreme type of pump and/or combination of same?

For just a mechanical pump a gage reading of -14.7 psig is all you can expect.

For the exotic laboratory pumps the more you spend for equipment the fewer stray molecules in the evacuated chamber you will have, hence a lower pressure.

Your question is much like asking; "How close to a temperature of absolute zero can I get with a cooler of unlimited capacity?"

Various approximate ranges are:

Low vacuum; 101325 to 3000 Pa

Medium vacuum; 3000 to 0. 133 Pa

High vacuum; 0.133 to 1.333x10-4 Pa

Very high vacuum; 1.333x10-4 to 1.333224x10-7 Pa

Ultrahigh vacuum; 1.333224x10-7 Pa and below.

Some Conversions

1 bar = 1x105 Pa 1 standard atmosphere (atm) = 1.01325x105 Pa

1 lbf in-2 (psi) = 6894.76 Pa 1 tonf in-2 = 15.4443x106 Pa

1 torr = 1 mmHg 1 mmHg = 133.322 Pa

1 inHg = 3386.39 Pa 1 inH2O = 249.089Pa

1 mmHg = 13.5951 mmH2O 1 mmH2O = 9.8066 Pa

1 kilo ps = 1 ksi 1 ksi = 1000 psi

This should be enough information to answer your question.

There certainly is a limit: the style of your pump limits the absolute vacuum.

A membrane pump can reach a decent vacuum but need to exhaust to the free air, making them work in series can solve some issues.

Centrifugal pumps are limited to the fluid you use: water will start to boil.

What sometimes is done to reach a high vacuum: heating up the walls to speed up the gas that is still inside. The hotter gas the quicker it goes to the pump. When your vacuum is established, switch off the heating and your vacuum will go up.

To reach perfect vacuum you can replace the air with a fluid, let it pull a vacuum like in a barometer and organise a chemical reaction that consumes the evaporated residue of your fluid. (and this reaction needs to result in a solid)

The gas atoms in the metal walls will come out (you can speed this up with the heating procedure)

Never forget: it is the movement of the gas atom that makes it ending up in your pump, your pump does not pull them. The pump only needs to prevent it from going back.

We regularly achieve 1.3x10-9 Pa using vacuum pumps for roughing then ion pumps to grab the remaining molecules.

In addition to the size/type of pump you use, you also have to know how many and how big your leaks are. There is no such thing as a chamber with no leaks, just chambers with leaks so small they are hard to measure.

You will need at least two pumps to get to high vacuum. One a roughing pump to back the high vacuum pump. This reduces the outlet pressure of the main pump to allow it to get dowm to its base pressure.

The size of your pump ( volume moved over a period of time) is less important if you have time to let it work. A bigger pump just gets to its base pressure faster.

All pumps have a compression ratio to help you calculate what its possible base pressure is. See manufacturing data sheets for this info.

RBeadle@pcc.edu