7.2.2 How to Use a Vacuum System

The easiest way to explain a vacuum system is to explain each part of the vacuum system individually. However, the only way to use a vacuum system is to use all parts. Therefore, there is a conflict of interest in coordinating a section that will explain the parts of vacuum systems in general and how to use your vacuum system in specific.

If you are starting to use a vacuum system for the first time, Simply skim this entire chapter. Afterward, study your vacuum system and see what components you recognize from what you've read. Finally, re-read the sections of this chapter that are pertinent to your system. This method of study may seem like a lot of work before you turn on a switch or twist a stopcock, but the vacuum system you save may be your own.

Due to variations in equipment, controllers, and designs, what you see on your system will probably not be what you see in this book. You will have to accommodate and respond accordingly. In addition, re-read Sec.7.1 and be sure you understand how accidents (disasters) can happen and how they can be avoided. That knowledge in itself will be the first major step to successful vacuum practice.

7.2.3 The History of Vacuum Equipment5"8

It is interesting, when looking back on history, to see how much has happened, and yet how little things have really changed. The following information is by no means comprehensive. At best, it is meant to provide the reader with an appreciation of how long ago some vacuum equipment was invented. For example, the McLeod gauge and Toepler pump were invented over a hundred years ago. Over the years, there have been many improvements made in such items as the ion gauge and diffusion pump; however, theprinciples on which they work are sofundamental that their importance and use have not been lost. Currently, advances in solid state electronics are providing new technology and methods for a variety of equipment, but they all are variations and/or adoptions of what we already have. Simply put, history shows us that there have been a variety of attempts to remove something from someplace and correspondingly, try and figure out how much has been removed.

Aristotle, some 1650 years ago, was one of the first to make any serious comments about vacuum that have survived time. Aristotle could not accept theconcept of a void, which led to his statement, "nature abhors a vacuum." This comment may not tell us very much about what is taking place in a vacuum, but it is as true now as it was then.

The first vacuum science work is credited to Evangelista Torricelli. In 1641, Torricelli was invited to Florence to serve as Galileo's secretary (during his last 3 months of life) as Galileo was still in his forced imprisonment and was blind. After Galileo's death, he was appointed court philosopher and mathematician to Grand Duke Ferdinando II of Tuscany. This title let him do research without having any teaching responsibilities at the university. It is not fully clear as to whether

326 Vacuum Systems

he was following up on a previous suggestion from Galileo or continuing some work of another Italian, Gasparo Berti,* but in 1644 he filled a glass tube (four feet long) with mercury and inverted it with the opened end pouring into a small basing As expected, some of the mercury did pour out, but most stayed in the tube, leaving an empty portion in the closed end of the tube. By doing this, Torricelli is credited for creating the first sustained vacuum. It was believed, at the time, that what had been created was a perfect vacuum. Fortunately, Torricelli was a good observer and noticed that the height of the mercury actually changed day to day, thus "discovering" the barometer. Oddly enough, Torricelli never formally published his observations.

Torricelli's work was taken up by Pascal, a French mathematician and philosopher, He repeated Torricelli's work, but he used glass tubes 46 feet long and wine. His work also demonstrated that the atmosphere could only support 33 feet of wine (whose specific gravity is similar to water). Pascal's brother-in-law, Perier, joined the experimentation by taking a mercury "barometer" up the mountain Puy de Dome (=1000m) and observed that the height of the column was 7.5 mm shorter than it was at the base of the mountain. This ingenious experiment helped to demonstrated that the force at work was the atmosphere pushing rather than a vacuum pulling, and that there was a vacuum at the top of the column.*

The next major technological breakthrough came from Otto von Guericke, a German burgomaster who invented a piston vacuum pump (which he called an air pump) that was capable of producing the best vacuum of the time. At first, Guericke pumped the water out of a barrel of water and observers were able to hear air leaking into the barrel refilling the void. Later he pumped out a copper sphere which immediately was loudly crushed (by air pressure). He then (wisely) had a better sphere created out of bronze, but did it in two halves. With these two hemispheres, Guericke became sort of a vacuum showman and is most known for these Magdeburg hemispheres. The two hollow bronze hemispheres were sealed by evacuating the space between the halves. Two teams of horses, each attached to a respective half, could not pull them apart. However, by opening the evacuated space to the atmosphere and allowing air into the sphere, the hemispheres fell

*In 1640, Berti had created a water barometer with a series of valves to eliminate the need of turning the apparatus upside down. He placed a bell in an enlargement of the glass tube near the top (evacuated) section. Berti correctly deduced that a vacuum would prevent observers from hearing the bell. Unfortunately, the rods connecting the bell to the glass chamber at the top of the device conducted the sound, making any evidence of vacuum impossible to detect. Because of poor design setup, an excellent scientific concept was lost and miscredited.

f The actual work may have been done by his colleague, Vincenzo Vivian.

*The difference between the two—a force pushing rather than a void pulling—is fundamental to understanding the movement of materials in a vacuum. Pressure is a force, and a vacuum is not an opposite (nor equal) force. A vacuum is simply a region with less pressure when compared to a region with greater pressure, and it is the molecules creating that pressure by pushing that creates a force rather than a lack of molecules creating a negative pressure by pulling.

apart on their own.* It was Guericke who correctly theorized that it was differences in air pressure that cause the winds.

Robert Boyle (a founder of the Royal Society of England) heard about Guericke's pump and experiments and decided to scientifically analyze them. Aside from making improvements upon the design of the air pump, he also developed the relationship between gas pressure, volume, and temperature. This relationship was later called Boyle's law [see Eq. (7.2)].

The 17th century saw the beginning studies of electric discharges within a vacuum by Nollet in Paris in the 1740s. These studies were later refined by Faraday in England during the late 1830s and by Crookes in the late 1870s.

Manometers were eventually used as measurement devices. By the 1770s, mercury was boiled to increase measurement accuracy. Thus, albeit crudely, both outgassing and baking out of a vacuum system were instigated.

In 1851, Newman developed a mechanical pump that achieved a vacuum of 30.06 in. of mercury on a day that the barometer was reading 30.08 in. This pump was very impressive for the time. Vacuum technology was further enhanced by the invention of the Toepler pump in 1862, the Sprengel pump in 1865, and the McLeod gauge in 1874.

The "Dewar", invented by Dewar in the early 1890s, utilized the combined effect's of evacuating and silvering the empty space of a double wall container. Later, having the ability to keep things cold, Dewar developed the basic concepts of cryopumps by superchilling charcoal for absorbing gases.

In the 1910s, Gaede invented the mercury diffusion pump that was later improved by Langmuir (from the General Electric Co.), and improved yet again by Crawford, and yet again by Buckley. In the 1920s, industry began substituting refined oil for mercury in diffusion pumps.

The new century saw an abundant development of vacuum gauges. Both types of thermal conductivity gauges were invented in 1906 (Pirani inventing the Pirani gauge and Voege inventing the thermocouple gauge). The hot cathode gauge was invented by Von Baeyer in 1909 and the cold cathode gauge was invented by Penning in 1937. The early 1950s saw improvements in both the hot cathode gauge (by Bayard and Alpert) and cold cathode gauge (by Beck and Brisbane). Redhead made improvements on both types of gauges a few years later.

Since 1945, the fields of vacuum physics, technology, and technique have become the backbone of modern industrial production. From energy development and refinement, to silicon chips in watches, games, and computers, all are dependent on "striving for nothing."

* An interesting (and useful) way to duplicate this experiment is with an evacuated screw-top jar that you are unable to open. Take a knife, screwdriver, can opener (church key), or any prying device with which you can pry the lid away from the glass enough to momentarily distort the shape and allow air into the container. You will hear a momentary "pssst" as air enters, as well as a click of the metal as its shape goes from concave to normal. It will now be very simple to open the jar.

7.2.4 Pressure, Vacuum, and Force

Gas pressure can be loosely imagined as the pounding of atoms and molecules (a force) against a wall (a defined area). As molecular activity increases (for instance by heating), the atoms and molecules pound away with greater activity, resulting in an increase in pressure. In addition, if some gas molecules are removed, there is more room for the remaining ones to move around, and fewer are available to hit the wall. This activity results in a drop in pressure. We use atmospheric pressure (which varies itself) as a dividing line for describing what is a vacuum or pressure environment: That environment which is greater than atmospheric pressure is a pressure, whereas that environment which is lower than atmospheric pressure is a vacuum. However, a positive pressure can only be defined by comparing it with something else that has less pressure, even if both are vacuums when compared to atmospheric pressure.

Units of pressure and vacuum should be identified as force per unit of area, and pressure units typically are mbar, psi, and kg/cm2. Pressures below 1 torr (=10"3 atm) for many years were described in relationship to standard atmosphere* in various ways such as:

1.Millimeters of mercury (1/760 of a standard atmosphere)

2.Torr (= 1/760 of a standard atmosphere)^

3.Micron (|i) of mercury (1/1000 of a millimeter of mercury and/or 1/1000 of a torr)

4.Millitorr (also 1/1000 of a millimeter of mercury and/or 1/1000 of a torr)

Whether one uses millimeters, torr, microns, or millitorr, a unit of force is being determined by a unit of length. To maintain the relationship of force per unit of area used in pressure and vacuum, the SI*decided on the term Pascal (one Newton per square meter) as a unit of vacuum.** Regrettably, the acceptance of the Pascal has been as successful as metrics in the United States. The numeric relationship between all of these designation can be seen in Table 7.2.

Vacuum and pressure measurements were all originally made compared to atmospheric pressure, or "gauge" pressure. The term psig (pounds per square inch-gauge) refers to this comparison. Absolute pressure includes atmospheric pressure (14.7 psi) and is called psia (pounds per square inch-absolute). For example, your tire pressure is 35 psig or 49.7 psia. Generally, unless otherwise identified, the lone identification psi refers to gauge pressure.

*Standard atmosphere is the average pressure that the atmosphere exerts at 0°C, sea level.

+ One torr is equal to 1 mm of mercury.

*International System of Units, see Chapter 2.

**It is ironic that the first term torr, from Torricelli, and the later term Pascal, from the mathematician by the same name, were both selected as terms for vacuum units since neither man was really fundamental in vacuum history. Important yes, but not fundamental.