SOME VACUUM BASICS
This article is condensed from material originally presented in Volume 1 (1992) of "the Bell Jar".
Material here is the work of Steve Hansen and is now also available in more complete form on the new Bell Jar Home Page.
This article is only meant to provide a very cursory overview of vacuum, how to produce a vacuum and and some of the more common applications of vacuum technology. Some noteworthy books will be referenced at the conclusion.
A vacuum system typically consists of one or more pumps which are connected to a chamber. The former produces the vacuum, the latter contains whatever apparatus requires the use of the vacuum. In between the two may be various combinations of tubing, fittings and valves. These are required for the system to operate but each introduces other complications such as leaks, additional surface area for outgassing and added resistance to the flow of gas from the chamber to the pumps. Additionally, one or more vacuum gauges are usually connected to the system to monitor pressure.
II. The More Common Units of Pressure Measurement
Traditionally, the pressure in a system is stated in terms of the height of a column of mercury that may be supported by the pressure in the system. At one standard atmosphere the force is 1.03 kg/sq cm (about 14.7 pounds per sq. inch). This pressure will support a mercury column 760 millimeter high (as in a barometer). One millimeter of mercury is the equivalent of 1 Torr. A thousandth of a millimeter is referred to as a micron of mercury or, in more current terminology, 1 milliTorr (mTorr). To be proper in the modern scientific world, the SI system of units is used. Here pressure is referred to in terms of newtons/sq. meter or Pascal (Pa). To convert Torr to Pascal, divide by 0.0075.
Measurement of pressure in a vacuum system is done with any of a variety of gauges which work through somewhat indirect means e.g. thermal conductivity of the gas or the electrical properties of the gas when ionized. The former are typically used at higher pressures (1 to 1000 mTorr), the latter in lower ranges.
III. Means of Producing Vacuum
Low grade vacuum may be reached using a variety of means. In the range to several 10s of Torr, sealed reciprocating piston compressors (as are commonly found in refrigerators) may be used. Piston compressors have the disadvantage of the dead space which exists above the piston. This, plus leakage past the piston, limits the degree of vacuum that can be achieved.
Better vacuum may be obtained with a rotary, oil sealed pump. This type of pump has a rotating off-center cylindrical rotor that "sweeps" air through the cylindrical housing in which the rotor is located. Air is kept from passing from between the vacuum and pressure sides by means of either a set of two vanes which are arranged across the diameter of the rotor or by means of a sliding single vane mounted in the housing. The entire mechanism of this type of pump is immersed in oil. The oil lubricates the moving parts and also acts as the sealing agent.
Single stage rotary compressors, as are used in some air conditioners, are usually good to 1 Torr. (These are typically manufactured by Matsushita and are rather tall and narrow with the wiring at the top of the unit. The inlet is at the bottom/side and the exhaust is at the top. Piston compressors are more squat and, as the internal mechanism is spring mounted, they can be identified by a characteristic 'clunking' sound when shaken.) Air conditioners from GE, Whirlpool, Westinghouse and Sharp commonly use rotary compressors.
To get below 1 Torr, a two stage (i.e. one stage in series with another) rotary pump should be used. Some success may be achieved by connecting two rotary air conditioner compressors in series. However, operation may be erratic. There is a type of rotary compressor which is used for the recharging of refrigeration systems. For less demanding applications, these can offer an economical alternative to industrial grade vacuum pumps. These refrigeration service pumps can be had for under $400, even in larger capacities (i.e. 3 to 4 cfm), and will readily reach 20 milliTorr.
- New, the industrial grade pumps (Welch, Alcatel, etc.) can cost well over $1000. However, a number of suppliers stock rebuilt pumps. In the smaller sizes, fully rebuilt and warranted pumps may be obtained for $500 or so. While the specifications on these industrial pumps will usually state an ultimate vacuum of 0.1 milliTorr, this level of vacuum is usually only attainable under ideal circumstances. A more practical value is 5-10 milliTorr.
At lower pressures, what is termed high vacuum, air doesn't respond very well to being squeezed and pushed around by pistons and rotors. At these pressures gas molecules don't really flow. They more or less wander into the pump. The most common type of pump for use in the high vacuum realm is the diffusion pump. This pump, invented by Irving Langmuir in 1916, utilizes a jet of vapor (generated by the boiling of hydrocarbon or synthetic oil) which forces, by momentum transfer, these stray molecules into the high pressure side of the pump. Since these pumps only work at low pressures, the outlet of a diffusion pump must be coupled to a mechanical 'backing' pump. Diffusion pumps are simple, quiet and only require simple (but sometimes tedious) maintenance. The major disadvantages are the backstreaming of oil toward the vacuum chamber (which may be minimized with baffles and or cold traps) and the catastrophic results from accidently opening the system to atmospheric pressure: the oil breaks down and goes everywhere. Mercury was the original pumping fluid. Mercury does not break down and higher forepressures may be tolerated. However, mercury also has a higher vapor pressure and liquid nitrogen cold traps are mandatory to prevent contamination. Oil pumps generally operate at a forepressure in the range of 100 mTorr or less. Ultimate pressures of 0.01 to 0.001 mTorr are readily achievable with small apparatus and simple baffles. Most of today's pumps have 3 stages with inlet sizes ranging from 2 inches on up. Pumping speed is related to the inlet area of the pump. A typical 2 inch pump will have a speed of about 100 liters/sec. For most amateur and small scale laboratory applications, pumps with inlets of 2 to 4 inches are the most convenient and economical to use.
A variety of other styles of high vacuum pump have been developed but these are usually difficult to use in the type of environment we are discussing here (i.e. the home and small lab) and are more expensive to maintain and service. Such pumps include the turbomolecular (or turbo) pump, which is built roughly like a turbine, and the gas capture pumps (ion, cryoabsorption, and sublimation) which either entrap gas within a material or bury the gas under a constantly deposited film of metal. Most of these pumps are used in applications where extreme cleanliness is required or where very high vacuums need to be attained. However, the turbo is seeing increased use in more common applications. Wide range turbo pumps which have very modest roughing requirements are now coming on the market.
IV. Ranges and Applications of Vacuum
The minimum configuration of a system is dependent upon the most aggressive planned application. Here are some guidelines for the tailoring of an amateur's vacuum system based upon intended use. Elements of this section are based upon material originally published by Franklin B. Lee in his booklet of projects "Experiments in High Vacuum" dating from ca. 1960. My thanks to Mr. Lee for permitting the use of his material.
Low grade vacuum where a vacuum serves only as a source of pressure, as for example the application of a 'suction' at one end of a pipe to cause the same flow which could be produced by a pressure at the other end.
Air avoidance applications where it is merely desired to avoid some undesirable physical or chemical property of one or more of the constituents of air such as friction, convection currents, heat conduction, radiation absorption, or oxidation.
Thermodynamic applications where the temperature at which a chemical or physical process proceeds depends upon the absolute pressure of the system.
High purity environments where any foreign material at all is an impurity as gases dissolve in liquids and solids in amounts proportional to their pressure.
Atomic and molecular beam applications. As the distance that a molecular or atomic particle can travel is directly dependent upon the space between the stray molecules in its surroundings, beams of these particles will move in an increasingly unimpeded fashion as the ambient pressure is lowered (i.e. the mean free path increases at higher degrees of vacuum).
Some of the more common applications of vacuum technology, arranged by the required degree of vacuum, are as follows:
10 to 100 Torr
Hardly qualifying as a vacuum in the realm of experimental physics, this is about the correct level of vacuum for the pulsed ultraviolet nitrogen laser. Such lasers are simple to build and produce prodigious amounts of pulsed radiation. Some will even work at atmospheric pressure.
1 to 10 Torr
Continously pumped carbon dioxide lasers work in this range. Sealed He-Ne lasers are backfilled to the lower end of this pressure range.
0.1 to 1.0 Torr
The ambient pressure in gas filled discharge tubes e.g. neon and fluorescent lamps and gas filled electronic tubes. This range represents the upper decade for plasma pinch devices. The popular "plasma sphere" globes are backfilled to about 1 Torr. Sputter coating is commonly done in this range (or slightly higher); non electronic applications include vacuum melting.
0.01 to 0.1 Torr
Familiar applications include radiometers, incandescent light bulbs, and thermos bottles (dewars). Pulse plasma z-pinch apparatus such as the 'pseudospark' or 'hollow cathode' and coaxial plasma focus devices are receiving a great deal of attention for intense soft x-ray generation and the production of high current electron and ion beams.
10e-3 to 10e-2 Torr
Pulsed z-pinch devices for x-ray generation. High quality (L.L. Bean?) thermos bottles.
10e-4 to 10e-3 Torr
Cold cathode x-ray and 'Crookes' tubes, vacuum spectrographs, mass spectrometers, and evaporated films. Vacuum spark pulsed x-ray devices perform well in this range. While not particularly familiar with the device, I believe that this is about the right range for the Tesla 'button' lamp.
10e-6 to 10e-4 Torr
The beginning of serious vacuum, at least for the amateur. "Traditional" applications include low current dc particle accelerators (e.g. Van de Graaff), hot cathode x-ray tubes, electron microscopes, electronic tubes and other small particle accelerators (betatron, cyclotron, linac). Lower decade pressure range for vacuum spark devices (electron beam/x-ray) including MeV range pulsed accelerators.
Below 10e-6 Torr
Larger accelerators, surface science, photo electric research, high purity films.
V. Further Reading
Saul Dushman's "Scientific Foundations of Vacuum Technique" (John Wiley, 1962) and John Yarwood's "High Vacuum Technique" (John Wiley, 1945) are both excellent books. Yarwood, while being more dated, has a stronger emphasis on laboratory practice. Much of this is applicable to today's amateur. Both of these books are out of print but should be available from a university library or through your local library's interlibrary loan service.
The American Vacuum Society has begun a series of reprints of classic vacuum books. Recommended are "Handbook of Electron Tube and Vacuum Techniques" (Rosebury), "Vacuum Sealing Techniques" (Roth), "The Physical Basis of Ultrahigh Vacuum" (Redhead, Hobson and Kornelsen). These are all available from the American Institute of Physics, c/o AIDC, P.O. Box 20, Williston, VT 05495 for $35 each ($28 to members of AIP member societies) plus $2.75 shipping for the first book, $.75 each additional book. The toll free number is 1-800-488-book.
Copy from Basic Vacuum Technology Resources