Introduction to Vacuum Technology
by Rajka Krstic, Jennifer Trelewicz and Veena Mahesh
Introduction
- Why do we want to learn about vacuum?
- What is pressure?
- How do we measure the pressure?
- What is vacuum?
Introduction
Why do we want to learn about a vacuum?
Vacuum technology is widely used in a variety of industries. Here are some applications you would have certainly heard about:i) An early application of vacuum technology came around 1900 when the first major industrial use was for light bulbs and TV tube production (later on). It has been shown that filaments emit electrons under vacuum which is the major property used in television technology.
ii) The second major application is in the electronic industry. Many processes that occur in a semiconductor fabrication facility require vacuums of different levels, including the deposition of thin films of material on computer chips.
iii) Another major application is in space technology. The main issue in space technology is how to design the space station or shuttle in order to maintain a pressurized cabin. Also, it is important to design safe space-suits to protect astronauts during their missions in open space.
What is pressure?
Figure 1: Closed container with air inside.
Imagine a closed container
with air inside (see Figure 1). Air, as a gas, is composed of molecules
that you can imagine as round elastic balls. Molecules move in straight
lines until they collide with neighboring molecules or the container wall.
Molecules of gas hitting the wall impose a force on the wall. The amount
of this impact force per area of the container inner walls is called pressure.
The mathematical definition of pressure can be written as:
where F is the force of impact of molecules on the walls and A is the area of the walls. The unit for pressure in the SI-system (International System of units) is the Pa (Pascal) and it is named after the famous French scientist who clarified principles related to pressure measurements. Strictly speaking, the Pa is the only valid unit. However, there are some other pressure units which are still in use: bar, atm, and Torr. For example, the pressure at sea level is 101300 Pa at standard temperature, and the most common name for this specific pressure is an atmosphere or atm. The closest match to the atmosphere is 1 bar = 100000 Pa. The ratios between different units are given in Table 1.
Table 1: Ratios between
different pressure units.
| Pressure unit | Pa | Bar | Atm | Torr |
| Pa | 1 | 0.00001 | 9.869×10-6 | 7.501×10-3 |
| Bar | 100000 | 1 | 9.869×10-1 | 7.501×102 |
| Atm | 101325 | 1.01325 | 1 | 760 |
| Torr | 113.22 | 0.001133 | 1.316×10-3 | 1 |
It is important to understand that pressure defined above is a property
of gases. In the case of liquids the pressure at the certain position
in liquid is created by the weight of the fluid column above (remember
that as you dive deeper into a ocean, pressure is higher). The pressure
imposed by the height of water is called the hydrostatic pressure and
is directly proportional to the height of the fluid above and its density.
The mathematical definition of hydrostatic pressure is:
where h is the height of the
fluid, 
is fluid density and g is gravitational acceleration.
How do we measure pressure?
The general name for the device
which measures the pressure is the manometer. The simplest manometer is
the U - tube. The U - tube can be made out of glass or plastic tube shaped
in the form of letter U. The U - tube is partially filled with some fluid
of known density, like water, oil or mercury (see Figure 2). If both the
ends of the U - tube are open, the heights of the fluid columns in both
ends will be the same. However, if we connect one end of the U – tube
to the tank (or container) which has gas under the pressure, the gas will
push the fluid column down in the side of the U – tube which is connected
to the pressurized tank. The other side of the U – tube is usually connected
to the constant pressure (for example, if open, it is under the standard
atmospheric pressure). The difference of the fluid levels corresponds
to the difference of measured and atmospheric pressure. Therefore, the
pressure in the tank can be calculated as:
Figure 2: The U – tube manometer.
What is vacuum?
Imagine a closed container with nothing inside (see Figure 3). The space of the container with nothing in it is called a vacuum. The word comes from the Latin word vacuus which means empty. Vacuum is the most prevalent state in the Universe, and on the average, most space qualifies as a very good vacuum. However, around the objects with sufficient gravity one can find a trapped gas mixture, and thus, the enormous variation of pressure exists. Therefore, vacuum is usually divided into ranges with somewhat arbitrary cutoffs.
The change of pressure from the surface of the Earth to high altitudes is illustrated in Figure 3:
Figure 3: The change of pressure with height.
Page authored by Rajka Krstic, Jennifer Trelewicz and Veena Mahesh
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