Understanding Pressure and Measurement - Ali Cihan Şengül

Pressure, Q=SP fundamental vacuum relationship's important component, but in practice and application means much more. Pressure is one of three main quantities that forms the fundamental relationship governing the behavior of all vacuum systems. This relationship is expressed with the formula Q (Gas Load) = S (Pumping Speed) x P (Pressure). In many cases, reaching a certain pressure is taken as the main goal in vacuum technology. Under these conditions, P in the formula can become just a number in the mind of the vacuum practitioner. However, there are many ways to examine the pressure concept and how you examine it will depend on what you are trying to do. Pressure is defined as force per unit area. In this case, force is applied by the impact of gas molecules with the surface where impact energies are transferred. More impacts mean more force and pressure. Since all molecules have the same energy at a particular temperature, it does not matter what the gas is. This is true for all pure gases or gas mixtures like air. Lighter molecules like helium (He) will move faster than a heavier gas like argon (Ar) and the energy transfer in the impact will be the same.

When in 1644 Toricelli discovered that air pressure supports a column of mercury (Hg) inside a closed-ended tube, the concept of force applied by air pressure opened the door to vacuum technology. Today a standard atmosphere is defined as a 760 mm (760 torr) Hg column as measured with a Hg manometer.

Yes, torr comes from Toricelli. A wide and important portion of vacuum technology depends on the use of the force concept. A vacuum vessel designer must take this into account. As gas molecules are pumped out of a vessel, there will be fewer molecular collisions inside the vessel, while the air outside the vessel will continue to provide a higher and constant number of collisions. This difference results in a pressure differential, which is also a force differential on the walls of the vessel. This explains the groans and moans sometimes heard when a vacuum vessel is evacuated, whistling and noise or the sound of an oil can from a rotating large and flat area. This effect was first demonstrated by von Guericke in 1650 when he connected two hemispheres together and pumped some of the air out of the resulting sphere. Two teams of horses could not pull them apart, but they separated into two when the sphere's air was evacuated. As increasingly more molecules are removed from the vessel, pressure and force differentials increase. This continues until the pressure inside the vessel drops to a few torr. At lower pressures, the differential forces are too small to have any practical effect or significance.

The physical force applied by pressure differentials can have numerous practical applications. Pneumatic tubes used to transport materials, vacuum cups or pressure plates and even old suction cups are perfect examples.

In fact, this technique was used in New York in 1870 on the first American subway to move cars a single block. These forces can also be used to operate vacuum/pressure gauges. When a vessel is evacuated, the decreasing pressure will apply a smaller force differential on a thin-walled diaphragm or closed-ended Bourdon tube and this results in a movement proportional to the pressure differential between the vessel and the atmosphere. This movement can be connected through mechanical linkages to a dial to produce an indication of the pressure inside the vessel. Typically, these gauges only respond to low pressures of a few torr, because the pressure differential will produce small force differential below these pressures, but there are some of these types of gauges that reach lower pressures. Capacitance manometers are a noteworthy example where clever sensing techniques are used to measure very small differences.

The use of mechanical measuring instruments has created a potentially confusing situation that often makes the transmission of specifications and references problematic. We have both absolute pressure and gauge pressure to deal with.

Absolute pressure uses perfect vacuum as zero, while gauge pressure uses atmospheric pressure as zero. This means that a mechanical gauge typically calibrated in English units can have a needle reading from zero to a value called 30 inches of vacuum. What is meant here is generally quite confusing for most vacuum practitioners unless conversions are made from the dial to absolute pressure equivalents. For example, a gauge reading of 29.5 inches of vacuum is actually 11 torr in absolute pressure. The use of gauge pressure is generally limited to some special application areas but there is still some confusion. For example, a vacuum practitioner may be concerned with a diaphragm pump that can only produce vacuum levels of a few torr and then will see that the manufacturer's specifications are listed in inches of vacuum. In fact, most of these manufacturers tend to take everything below a few torr as "deep vacuum" and this is clearly not a term very much appreciated by practitioners working at lower pressures. Vacuum practitioners working at pressures below a few torr tend to think in terms of absolute pressure from a logarithmic perspective, using torr, millibar (mbar) or Pascal (Pa) units.

Pascal extended Toricelli's work to demonstrate that the altitude of a Hg column changes and he deserves his own unit for this. All three of these unit systems have decreasing numbers pointing to a decreasing number of molecules in a vessel as it is evacuated.

Although mechanical pressure measurements down to a few torr do not need to consider the actual types of gases, this is not true for a series of applications that require evacuating that vessel to a sufficiently low pressure to remove atmospheric gases before refilling the vessel to a higher pressure with particular gases. In such cases, the actual gas or gas mixture is of extreme importance. On the other hand, processes that only require pumping down ignore gas accumulation at pressures above the few torr level where most mechanical gauges bottom out. Satisfactory progress has been made here with the market introduction of convection-enhanced thermal conductivity measurement instruments that allow readings from atmospheric pressure down to a few millitorr or so. This has made it possible to use the same logarithmic units throughout pumping down and process cycles. Still, differences in actual gas accumulation remain effective even when we only consider total pressure.

For the vacuum practitioner, total pressure is the sum of the partial pressures of the various gases that make up the vacuum environment. Attempting to work within total pressure alone can cause some problems.

A good example of this is the use of thermal conductivity gauges where the various gases used or encountered can have very variable thermal conductivity. If you try to refill a vessel with atmospheric pressure using a gauge calibrated in Ar with nitrogen (N2), the gauge will show such a low value that you may end up overfilling the vessel with pressure before reaching an atmospheric pressure value. This can lead to a very dangerous and costly method of taking apart a vacuum system. Perhaps more importantly, the partial pressures inside the vessel can have critical effects on the process. Total pressure gauges calibrated in N2 equivalents can continue to show the same total pressure while being able to have much variable partial pressures of gases.

At pressures below 10-3 torr, the main gas type inside the vessel is usually water vapor, so many process specifications will require a certain total pressure value after the process starts, but this will also mostly assume that water vapor is always the main constituent.

A small air leak can change this environment, but the total pressure value on a hot or cold cathode ion gauge can show the same value. This can ruin a process. Since each gas will have a different sensitivity to ion gauges, total pressure measurements can always be misleading. Residual gas analyzers (RGA) can offer a solution to this problem. They measure and display the partial pressures of all residual gases within certain pressure ranges. Generally, an atomic mass unit (AMU) range between 1 and 50 is sufficient unless heavy hydrocarbon contamination is expected. Using an RGA will not eliminate vacuum or process problems, but it will help prevent them before process problems occur. The concept of pressure in terms of force applied by molecular impacts changes to really evaluate the effects of impacts. For example, a growing thin film of reactive metal will react with active gas molecules bombarding it at a rate proportional to the number of impacts per unit time, which is proportional to pressure.

The actual units you use do not matter as long as you are consistent. Torr is probably the most commonly used unit in the US, but mbar is generally used in Europe while Pa is common in Japan.

For rough work, it is possible to use torr and mbar interchangeably, but for precise work, multiply mbar by 0.75 to obtain torr and multiply torr by 1.33 to obtain mbar. Pa is somewhat more cumbersome and resistance to its use in many cases can be explained by the fact that Pa is used throughout the text of Vacuum Technology User's Guide written by John O'Hanlon, one of the most common vacuum books. Still, the conversion is not that difficult; multiply torr by 1.33 or mbar by 102 to obtain Pa. The concept of pressure in vacuum technology is certainly important, but it is always necessary to evaluate what this term really means for your process or application. The differences between total and partial pressure must always be kept in mind and this is especially important for cross-communication among vacuum practitioners. Maintaining consistent units is equally important. And also never forget that the really experienced vacuum community refers to "ten'a'minus" and special exponential numbers.

Ali Cihan Şengül / Industrial Engineer - Sales Representative - Gücüm Pompa Makine - Sanayi ve Ticaret A.Ş.