Vacuum Science: A User's Guide to Vacuum Technology

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.

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. This pressure will support a mercury column 760 millimetres high (as in a barometer). One millimetre of mercury is the equivalent of 1 Torr. A thousandth of a millimetre 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 metre 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, for the most part, work through somewhat indirect means eg thermal conductivity of the gas or the electrical properties of the gas when ionised. The former are typically used at higher pressures (1 to 1000 mTorr), the latter in lower ranges. Such gauges are sensitive to the type of gas in the system, requiring that corrections be made. Accidents have occured when this was not taken into account. 

For example, the presence of argon in a system will result in a pressure reading on a thermal conductivity gauge (thermocouple or Pirani, for example) that is much lower than the true pressure. It is possible to significantly overpressure a system while the gauge is still indicating vacuum. 

The only gauges that are gas-type independent are those which directly measure pressure as force per unit area. Barometers, McLeod gauges and other liquid manometers are examples of direct gauges. Direct gauges also include instruments where the pressure causes a tube to distort (the principle of the common dial, or Bourdon gauge) or where the displacement of a metal diaphragm is measured by electrical means (the principle of the capacitance manometer).

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-centre 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, can serve as a good starting point for someone who desires to get into basic vacuum experimentation. These are usually good to about 1 Torr. Typically manufactured by Matsushita (although they may bear other manufacturers' labels), they 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 (ie 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. The most frequently seen "real" vacuum pumps are the industrial grade rotary pumps made by Welch, Alcatel, etc. New, these can cost well over $US 1000. However, a number of suppliers stock rebuilt pumps. 

In the smaller sizes, fully rebuilt and warranted pumps may be obtained for $US 500 and up. 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.

Described elsewhere, there is a type of low cost rotary pump that is used for the recharging of refrigeration systems. For the less demanding requirements these can offer an economical alternative to industrial grade vacuum pumps. These refrigeration service pumps can be had for under $US 400,  even in larger capacities (ie 3 to 4 cfm), and will readily reach 20  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. Instead 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, utilises a jet of vapour (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 minimised 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. Advantages include the fact that it does not break down and higher forepressures may be tolerated. However, mercury also has a much higher vapour pressure than diffusion pump oils and liquid nitrogen cold traps are mandatory to prevent contamination by backstreaming. It is also toxic. Oil pumps generally operate at a forepressure in the range of 100 mTorr or less and can achieve ultimate pressures at the inlet of 0.01 to 0.001 mTorr without much difficulty.

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 litres/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 hereand 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, cryo, and sublimation) which either entrap gas, freeze the gas, 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 molecular drag and hybrid turbo pumps which have very modest roughing requirements (in the Torr range) are becoming more common in industrial and research applications.

Some Vacuum Terminology

Mean Free Path. Reduction in pressure results in a lower density of gas molecules. Given a certain average velocity for each constituent molecule of air at a given temperature (at room temperature this is about 1673 km/hr) an average molecule will travel a certain distance before it interacts (collides) with another at any given pressure.  This average distance between collisions is the mean free path. At 1 Torr in air this distance is about 0.005 cm, a value that scales directly with pressure. 

Thus the mean free path would be 5 cm at 1 mTorr and 50 metres at 0.001 mTorr. The lengthening of mean free path at low pressures is a key enabler for devices such as vacuum tubes and particle accelerators as well as for processes such as vacuum coating where microscopic particles such as electrons, ions or molecules must traverse considerable distances with minimal interference.

Flow. Gases at very low pressures behave very differently from gases at normal pressures. As a reduction in pressure occurs in a vacuum system, the gas in the system will pass through several flow regimes. At higher pressures the gas is in viscous flow where the gas behaves much like a liquid. Viscous flow includes turbulent flow, where the flow is irregular, and laminar, where the flow is regular with no eddies. Moving deeper into the vacuum environment, Knudsen or transition flow occurs when the mean free path is greater than about one-hundredth of the diameter of the tubing. 

Full molecular flow, where molecules behave independently, begins when the mean free path exceeds the tubing diameter. Which flow regime the gas is in is dependent upon several factors including tube diameter and pumping speed. 

To summarise, when the ratio of the average mean free path in a tube to the radius of the tube is less than 0.01, the flow is viscous. When the ratio is greater than 1.00 the flow is molecular. Transition (or Knudsen) flow exists between the viscous and molecular flow regimes and we have a behaviour that is bit of both. One of the factors which determines pump applicability is the flow regime it needs to operate in. Mechanical pumps are not effective in the molecular region whereas diffusion pumps are.

Backstreaming. It is always hoped that the flow of gas and vapour in a vacuum system is away from the chamber, through the pump, and out to the atmosphere. However, this is not the case in molecular flow where molecules behave as individuals with some of them going against the main flow direction. This is not a good situation to have when there are undesirable things downstream of the chamber (like pump oil) that we would prefer not to have get into the experimental area. This is one reason why diffusion pumps always have some sort of baffle or trap - otherwise fairly large quantities of oil vapour will migrate out of the pump and into the chamber.

Pumping Speed and Throughput. The speed of a pump is the volume of gas flow across the cross section of the tubing per unit time. The standard units are litres/second. Since the density of a gas changes with pressure (ie the mass or number of molecules of gas in a given volume) an important measure is mass flow or throughput which is the product of pressure and speed with the units of Torr-litres/second. If you think of the vacuum system as an electrical circuit, throughput is like current flow and it is constant everywhere in the circuit. 

The various elements of the system (lines and pumps) are analogous to resistances except instead of voltage drops there are pressure differentials. In putting together a vacuum system you want minimal pressure differentials in the connecting lines and maximum throughput everywhere. A simple example will pull this together. 

Consider a small diffusion pump that has a rated inlet speed of 100 litres/second at 0.0001 Torr (0.1 mTorr). The throughput would be 100 x 0.0001 or 0.01 Torr-litres/sec. Now, connected to the outlet of the diffusion pump we have a mechanical forepump which is capable of maintaining a pressure of 0.1 Torr. Given the fact that throughput at the diffusion pump inlet must equal throughput at the outlet and that there is a pressure of 0.1 Torr at that outlet, the minimum speed of the forepump must be 0.1 litres/sec, a speed easily met by even very small mechanical  pumps. 

On the other hand, if the diffusion pump inlet pressure is 0.01 Torr (10 mTorr) - say just after the pump is started or if it is working against a very gassy load - the forepump would have to have a speed of 10 litres/sec to allow the diffusion pump to work at full speed. This would be a large pump.

To summarise all of this, at high diffusion pump inlet pressures, the speed most likely will be constrained by the speed of the forepump. At low inlet pressures there is so little mass flow that a very small forepump can keep pace with even a large high vacuum pump. In fact, in a tight system you can shut off the forepump once a low enough pressure has been reached simply because so little mass remains in the system. 

Conductance of Tubing. As mentioned above, the tubing in a vacuum system can represent a significant resistance. When one end of a tube is connected to a pump, that end of the tube will have a higher pumping speed than will the other end. For viscous flow, as would be the nominal case for roughing lines (ie mechanically pumped), the conductance, C, is dependent upon gas pressure and viscosity and, at room temperature and air, is (for a tube diameter of D cm, length of L cm and at an average pressure of P Torr): 

                                                                C = 180 x D4/L x P (litres/sec)

An example would be a foreline of 2cm diameter and 60 cm long. At one end is a venerable Cenco Megavac pump; the other end is connected to the outlet of a diffusion pump. Referring to the manufacturer's literature for the pump we find that the pumping speed of the roughing pump is 0.5 litre/sec at 100 mTorr, the maximum recommended foreline pressure of the diffusion pump. Plugging in the numbers, we find that the line conductance is 4.8 litres/sec. Thus, the line is not limiting the capabilities of the forepump.

Interestingly, pressure is not a factor in the molecular flow regime where, for example, a diffusion pump would operate. Here we have: 

                                                                C = 12 x D3/L (litres/sec)

An example here would be a 5 cm diffusion pump which has a specified inlet pumping speed of 100 litres/sec. The pump is connected to a small experiment chamber through 60cm of 2.5cm diameter tubing. Inserting the numbers, we find a line conductance of only 3.1 litres/sec. This may be adequate for the small chamber but it certainly throttles the pump significantly. If a 5cm line were substituted (same length) the conductance would rise to 25 litres/sec. 

In either case, the most important thing to bear in mind is that conductance is strongly influenced by the tube diameter 1cm to the third or fourth power is a whole lot less than 3cm to the same powers. The bottom line is: go for fat tubes, and keep them short, particularly in high vacuum lines. 

Outgassing and Vapour Pressure. Assuming that a system is tight, as the pressure gets lower most of the load is from gases evolving from the surfaces of the materials in the system. This becomes significant below pressures of around 100 mTorr. Outgassing will be the main limiting factor with regard to the ultimate pressure which any  particular system may reach, assuming that leaks are absent. Leaks may be either real leaks, like holes in the chamber, or virtual leaks that are caused by gas escaping from, for example, screw threads within the system or porous surfaces that contain volatile materials. 

The level of outgassing is reduced by keeping the system clean and dry and with a proper selection of materials. If the construction of a system is appropriate to the practice, adsorbed layers of water vapour and other gases may be evolved by heating the system in an oven or with a hot air gun to a temperature of at least 150°C and usually more. For most of the applications that we will be discussing this level of cleaning is not required. However, the system components should be kept clean (no fingerprints or other grime), dry and, as much as possible, sealed off from room air (a major source of moisture). 

Related to outgassing are the vapour pressures of the materials used in the system. All materials evolve vapours of their constituent parts and these vapours will add to the gas load in a system. Water is the worst commonly encountered material and is a good example of what vapour pressure means. At 100 °C, the vapour pressure of water is 1 atmosphere (760 Torr). Under those circumstances, when the vapour pressure is equal to the surrounding pressure, we know what happens - the water boils. At room temperature, the vapour pressure of water drops to 17.5 Torr and it will boil at that pressure. Water is not a good material to have in high vacuum systems. 

Other materials having high vapour pressures include some plastics, particularly those with volatile plasticisers, and metals such as mercury, lead, zinc and cadmium. Low vapour pressure materials include glass, copper, aluminium, stainless steel, silver, some other plastics and some synthetic rubbers. As vapour pressure is a function of temperature, some higher vapour pressure materials, eg zinc bearing brass, are quite acceptable in many applications as long as excessive temperatures are not encountered.

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.

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. Gases dissolve in liquids and solids in amounts proportional to their pressure and contaminants settle on surfaces are a rate that is dependent upon the molecular density of the gas above the surface.

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 anincreasingly unimpeded fashion as the ambient pressure is lowered and the mean free path increases.

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 designs will even work at atmospheric pressure. The so-called Plasma Sphere uses a mixture of gases that permits a discharge to easily form at pressures approaching 1 atmosphere.

1 to 10 Torr: Continuously pumped carbon dioxide lasers work in this range. Sealed He-Ne lasers are backfilled to the lower end of this pressure range. This also represents the ambient pressure in gas filled discharge tubes eg neon and fluorescent lamps and gas filled electronic tubes. An outgrowth of space propulsion research, the coaxial dense plasma focus devices work in this range. They are used to produce high fluxes of soft x-rays and neutrons.

0.1 to 1.0 Torr: This range represents the upper decade for plasma pinch devices. Sputter coating and other plasma processes are commonly done in this range. Non-electronic applications include vacuum melting and the freeze drying of pharmaceutical products and biological specimens.

0.01 to 0.1 Torr: Familiar applications include the Crookes radiometer (the thing with the vanes that spin when exposed to bright light), and incandescent light bulbs. Pulse plasma z-pinch apparatus such as the "pseudospark" or "hollow cathode" devices are receiving a great deal of attention for the production of high current pulsed electron and ion beams. 

10-3 to 10-2 Torr: Thermos bottles and pulsed z-pinch devices for x-ray generation.

10-4 to 10-3 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. 

10-6 to 10-4 Torr: The beginning of serious vacuum, at least for the amateur. "Traditional" applications include low current dc particle accelerators (eg Van de Graaff), hot cathode x-ray tubes, electron microscopes, electronic tubes and other small particle accelerators (betatron, cyclotron, linac). This is also the lower decade pressure range for vacuum spark devices (electron beam/x-ray) including MeV range pulsed accelerators.

Below 10-6 Torr: Larger accelerators, fusion reactors (such as Tokamaks), surface science, photo electric research, high purity films. Many of these require a vacuum in the ultra-high vacuum range, that is, below 10-8 Torr.

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