The Purposes and Limitations of Vacuum Gauges

There are two basic ways for a vacuum gauge to read a vacuum direct and indirect. For example, say that on one side of a wall you have a known pressure, and on the other side of the wall you have an unknown pressure. If you know that a certain amount of deflection implies a specific level of vacuum, and you can measure the current wall deflection, you can then determine the pressure directly. This process is used with mechanical or liquid types of vacuum gauges. On the other hand, if you know that a given gas will display certain physical characteristics due to external stimuli at various pressures, and you have the equipment to record and interpret those characteristics, you can infer the pressure from these indirect measurements. This indirect method is how thermocouple and ion gauges operate. 

Each method of vacuum reading has its advantages and disadvantages. No single method of vacuum reading is entirely easy and/or comprehensive, and no vac- 

In molecular flow, there is a time factor before pressure can equalize within a vacuum system. Small-diameter (and/or long-length) tubing can compound this problem. 

If the entry tube to the gauge is too narrow (and/or long), the statistical movement of molecules to enter the gauge may be inaccurately low. On the other hand if the angle of the tube to the gauge is in direct line with a molecular stream, the count of molecules could be inaccurately high. 

Ion gauges have their own pumping capability providing lower pressures in the gauge region than in areas located some distance away (see Sec. 7.5.19). 

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