In pressure measurement, except for a few direct measurements, most of them are indirect measurements. It is to cause a certain physical phenomenon in the measured gas, and then measure the physical quantity related to the pressure in the process, and then try to determine the pressure value. This is the characteristic of vacuum measurement, and it will cause some problems.
1. A vacuum gauge is an instrument for measuring the pressure of a gas or vapor below one atmospheric pressure.
2. Absolute vacuum gauge is an instrument that directly calculates (or obtains) the pressure from the physical quantity measured by the vacuum gauge itself.
3. The relative vacuum gauge must be calibrated by absolute vacuum gauge before vacuum measurement.
The relationship between any specific physical phenomenon and pressure is the most significant within a certain pressure range. Beyond this range, the relationship becomes weak. Therefore, any method has a certain measurement range, which is the “range” of the vacuum gauge. Expanding the range of each method as much as possible is one of the important contents of vacuum science research.
The pressure range of modern vacuum technology is as wide as 19 orders (105-10-14 PA). No vacuum gauge can measure such a wide pressure range. Therefore, several kinds of vacuum gauges are always used to control a certain area. However, due to the differences in the principle of various vacuum gauges, large errors are often caused in the areas where they are connected. From the above point of view, in order to improve the performance of vacuum gauge and improve the accuracy of vacuum measurement, it is necessary to highlight the main phenomenon and restrain the parasitic phenomenon.
When the pressure is higher (105 ~ 1pA), U-tube manometer, McCullough vacuum gauge, film gauge and spring tube manometer are used. In the middle pressure range of 103-10-2 PA, the piezometer and viscous vacuum gauge are made by using the change of gas properties (heat conduction, viscosity). At low pressure, an ionization vacuum gauge is used to measure the ionization of gas molecules.