obvious impact of this statement is the potential for confusion due to the various unit options people have when it comes to reporting pressure. Fortunately, it is not an issue that the atmospheric pressure at sea level is not one fixed value, nor is there any deleterious impact of defining 1 atmosphere of pressure?the pressure at sea level on a clear cold nonstormy Industrial vacuum measurements are expressed in pressure units that reflect a pressure range and historic preference of the industrial practice. Thus, it is common for vacuum applications that are near atmospheric pressure to be described in inches of water or mercury. However, it is also expected that the pressure associated with a semiconductor metalization industrial application that uses physical vapor deposition be described in Pascals, microns, or perhaps millitorr units. The reader should also be prepared to deal with pressure unit selection variations with respect to the pressure units that appear on various vacuum pressure gauges. The unfortunate realities when dealing with industrial applications that require a vacuum environment is that there is no universal vacuum gauge that will work for all applications and that different types of gauges will use different pressure units. The normal measurement ranges for various gauges are summarized in Figure 5.14a. A variety of pressure units will be used in this section.People new to vacuum gauges and vacuum technology should also be prepared to deal with potentially confusing vocabulary when describing various pressure situations. People with different technical backgrounds may use the terms low vacuum, low pressure, and/or high vacuum, and high pressure imprecisely at different times. The reader should refer to a pressure that is very much lower than atmospheric pressure as a low pressure or a high vacuum environment. For a process situation that is very close to but below atmospheric pressure, the terms to use would be low vacuum or high pressure. In all situations, when a vacuum is referenced or described, the maximum pressure value being considered is atmospheric pressure. Therefore, vacuum chambers located in Denver, CO, or Geneva, Switzerland, have maximum pressure values that are less than 1 atmThere are a variety of ways to categorize vacuum gauges.

Grouping gauges by operational principle is one common way. Thus, the Bourdon tube and the bellows gauge are classified as mechanical gauges, but the spinning element gauge is know as a momentum transfer gauge. Figure 5.14a provides a list of vacuum gauges and their operating range grouped by operational principle. Vacuum gauges that function because of a mechanical principle typically perform a differential pressure measurement. Gauges that operate within the same pressure range can also be grouped together. Pirani gauges work in the same general pressure range as thermocouple (TC) gauges but theion gauge would not be grouped with these two. A third way to categorize vacuum gauges is to identify if they actually make a direct or an indirect pressure measurement. Thus, a direct vacuum pressure measurement gauge will provide a new measurement signal value simply because the pressure being measured changes. The indirect pressure measurement gauge is actually monitoring some other parameter of the system that proportionally responds as the pressure of the system changes. A vacuum gauge that uses a TC as the pressure-sensing element is one example of an indirect pressure gauge. The increase or decrease in pressure near the thermocouple does not directly alter the electromagnetic force generated by that TC. However, that pressure change does reflect a change in the molecular population near the TC. This molecular density change affects the heat transfer near that TC, which, in turn, alters the value of voltage, measured across the TC. Finally, it is also important to identify if the vacuum gauge performs an absolute pressure measurement. Similar to pressure gauges designed for above atmospheric pressure measurement applications, vacuum pressure gauges may provide an absolute or gauge pressure reading. Naturally, it is important for instrumentation and controls personnel to know which type of pressure measurement is being taken. The adjustment from gauge to absolute pressure is straightforward and may be provided as an electronic correction within the gauges. Elastic element based vacuum gauges are simple and inexpensive examples.

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