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Vacuum Technology
Introduction to Vacuum
Definition of Vacuum
Vacuum spectrum
Pressure
Vacuum Fundamentals
Vacuum Pumping Terminology/Concepts
Production of Vacuum

Introduction to Vacuum

This section is intended to give a simple and straightforward explanation of fundamental vacuum concepts, basic behaviour of gases and common vacuum pumping terminology.

Definition of vacuum

A vacuum is strictly defined as a space that has had all the matter removed from it.
It is impossible however for the perfect vacuum to exist since even with the best vacuum ever created it is not possible to remove all the gas, as there would still be some molecules present in the vacuum area.

A vacuum is not a negative pressure.
A negative pressure cannot exist. All that can exist is a pressure of varying magnitude.

A vacuum is defined simply as a pressure below that of atmospheric pressure at sea level.
Atmospheric pressure is changeable however. Changes in weather usually mean changes in atmospheric pressure. Low atmospheric pressure is associated with poor weather and high pressure usually means warm weather. To cope with these fluctuations in local atmospheric pressure, standard atmospheric pressure has been defined as 1013 mbar.

When discussing vacuum it is important to check that you are dealing in terms of absolute pressure. Other industries use gauge pressure when discussing pressure (bar, psi, mm of water are common units). The table below shows equivalent gauge and absolute pressures and illustrates the confusion that can occur.

Absolute Pressure Gauge Pressure
mbar Torr Pa mm Water bar psi mm Water
2000 1520 2x105 2.04x104 1 14.7 1.02x104
1000 760 1x105 1.02x104 0 0 0
800 608 8x104 8.16x103 -0.8 -11.6 2.04x103
500 380 5x104 5.1x103 -0.5 -7.35 5.1x103
100 76 1x104 1.02x103 -0.9 -1.47 40.16
1 0.76 1x102 10.2 NA NA 0.4016
10-2 7.6x10-3 1 NA NA NA NA
10-4 7.6x10-5 1x10-2 NA NA NA NA

Note: If customer quote mm of water as the suction pressure this means they have a rough vacuum application above 800 mbar absolute.

HHV Pumps is in the business of manufacturing equipment to produce man-made vacuums but vacuums can exist naturally.

It is said that nature abhors a vacuum but in reality this is not the case.
For example the pressure at the top of Mount Everest is 300 mbar – less than a third of the pressure than that at sea level. In Bangalore the air pressure is 890 mbar we are in a vacuum. The vacuum of deep space is lower than any pressure that can be created artificially on this planet and as space is infinitely larger than the Earth natural vacuums are very much in existence.

Vacuum spectrum

The practical vacuum spectrum here on earth extends from 1013 mbar down to the lowest pressures so far artificially produced which are claimed to be of the order 10-14 mbar (it is incredibly difficult to measure such very low pressures). For convenience the wide spectrum of vacuum has been divided into the ranges detailed in the table below.

Low or rough vacuum 1013 mbar - a few mbar
Medium vacuum A few mbar - 10-3 mbar
High vacuum 10-3 mbar - 10-7 mbar
Ultra-high vacuum (UHV) Below 10-7  mbar


Comparison of range of vacuums

Atmospheric pressure at sea level 1013 mbar
Atmospheric pressure in Bangalore 890 mbar
Vacuum produced by vacuum cleaner at sea level 800 mbar
Summit of Mount Everest 300 mbar
Surface of the Moon 10-12 mbar
Highest ever man-made vacuum 10-14 mbar
Outer space 10-21 mbar

Remember - The lower the pressure the higher the vacuum and vice versa.

Pressure

Pressure is defined as a perpendicular force per unit area exerted by a liquid or gas on a body or surface. The SI unit for force is the Newton. The weight of a force is expressed by multiplying it by acceleration due to gravity, which is approximately 10 ms-2. Therefore the mass of an apple for example is about 1/10th kg multiplied by 10 gives 1 Newton of force. Other units of pressure more commonly used in vacuum are detailed below.

Unit of vacuum Country
mbar (milli bar) Europe
Pa (Pascal) Japan
Torr/mTorr USA


1 Newton per m2 is equal to 1 Pa (Pascal)

1 Bar = 1 x 105 Pa = 1 x 103 mbar (millibar)

Unit Pa bar mbar atm Torr Psi
1 pa = 1 N/m2 1 10-5 10-2 9.8692 ×10-6 750.06×10-5 1.4503×10-4
1 bar = 0.1 MPa 105 1 103 0.98692 750.06 14.503
1 mbar = 10-2 Pa 102 10-3 1 0.9869 ×10-3 0.75006 14.503×10-3
1 atm = 760 Torr 101325 1.0133 1013.25 1 760 14.69
1 Torr = 1 mm Hg 133.22 0.00133 1.333 1.3158× 10-3 1 0.01934
1 psi 6895 0.06895 68.95 0.06805 51.717 1


What is atmospheric pressure?
The atmosphere is a mixture of gases surrounding the planet. The gases would escape into space if was not for the gravitational pull of the Earth. As the gravitational pull is greater the closer you get to the planet’s surface, the air is denser.

Gases Percent by Weight Percent by Volume Partial pressure  (Torr)
N2 75.51 78.1 594
O2 23.01 20.93 159
A 1.29 0.93 7.1
CO2 0.04 0.03 0.23
Ne 1.2 X 10-2 1.8 X 10-3 1.4 X 10-3
He 7 X 10-3 5.24 X 10-4 4 X 10-3
CH4 2 X 10-4 2 X 10-4 1.5 X 10-2
Kr 3 X 10-4 1.1 X 10-4 8.4 X 10-4
H2O 6 X 10-3 5 X 10-3 3.8 X 10-4
H2 5 X10-4 5 X 10-3 3.8 X10-4
Xe 4 X 10-3 8.7 X 10-4 6.6 X 10-3
O3 9 X 10-4 7 X 10-4 6.3 X 10-3
  S100% S100% S100%
50% RH AT 20°C 1.6 1.13 8.75


Standing on the ground we are surrounded by the atmosphere that rises many miles above us. The air molecules stack up on one another to create the pressure 1013 mbar at sea level. The higher you rise in the atmosphere the shorter the stack of molecules becomes so the lower the pressure.

Below is a comparison of air pressures at various altitudes.

Altitude (km) Air pressure (mbar)
Sea level 1013
80 10-3
160 10-6
320 10-7
480 10-8
640 10-9
800 10-10


Air itself weighs 1.18 kg per cubic metre at sea level. The weight of the Earth's atmosphere pushing down on each unit area of the Earth, at sea level is actually about 10000kg m-2.

If this sound surprising, remember we have said that atmospheric pressure at sea level is 1013mbar - let's say 1000mbar for simplicity and using SI units.

1000mbar = 1 bar         
1 bar = 1 x 105 Pa (N/m2)
Acceleration due to gravity (g) = 10ms-2
Force = Mass (M) x Acceleration due to gravity   (g) F = Mg
Force per unit area (FA-1) = Pressure (P)
Then P = Mg. A-1
Re-arrange M.A-1 = P.g-1
Then M. A-1 = 100000/10 = 10000 kg.m-2 = 1bar

Another unit of pressure that you may encounter is Torr, which is widely used, in vacuum practice. Torricelli was an Italian physicist and mathematician who amongst other things invented the barometer.  He realised that the pressure of the atmosphere at sea level would support a column of mercury 760mm high. 1 Torr is the amount of pressure required to support a column of mercury 1mm high.

It is useful to approximate that if one atmosphere is 1000 mbar and 760 Torr then:
1 mbar = ¾ Torr approximately.

Effect of pressure on structures
Using a Coke can as an example, consider the affects of pressure on its structure. The can is designed to contain pressure above that of atmospheric (to take into account whether the can has been allowed to get too warm for example). The structure would consist of elements in tension. If the can was empty (i.e. contained a vacuum) the entire structure would be subject to a compressive load due to atmospheric pressure.

Vacuum

Compression is a force that acts to compress or shorten the object that it is acting upon. A failure by compression is called buckling.

Tension is a force that acts to extend or lengthen a thing that it is acting upon. A failure by tension is snapping.

Vacuum Pressure

The coke can is designed to deal with the forces exerted upon it by its contents (internal pressure) and can deform elastically to cope with this. When its contents are gone (replaced by air) the can is not subject to any forces (internal or external). If the contents (air) were replaced with a vacuum the can would be crushed by the atmospheric pressure. The can is not designed to withstand external pressure and deforms plastically (permanently). When dealing with vacuum it is important to be aware of pressure differentials and their affects on wall thickness.

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