There are usually two ways to mark vacuum degree, one is to use absolute pressure (absolute vacuum degree), and the other is to use relative pressure (that is, relative vacuum degree) to mark two.
The so-called absolute pressure refers to the vacuum pump is connected with the detecting container, after sufficient time for continuous pumping, the pressure within the container will no longer continue to decline while maintaining a certain value, then the pressure in the container is absolute pressure pump. If there is absolutely no gas in the container, then the absolute pressure is zero, which is the theoretical vacuum. In actual situation, the absolute pressure value of vacuum pump is between 0 and 101.325kPa. Absolute pressure values need to be measured with absolute pressure gauges, where the initial value of the instrument is 101.325kPa at 20 and 0 altitude.
The relative vacuum refers to the difference between the pressure of the measured object and the atmospheric pressure at the measuring point. Measuring with an ordinary vacuum gauge. In the absence of vacuum, the initial value of the table is 0. When measuring the vacuum, its values ranging from 0 to 101.325kPa (generally express with the negative number). For example, the measured value is 30kPa, said pump can be pumped into the vacuum measurement locations than the low atmospheric pressure 30kPa. The same pump is measured at different locations, and the relative pressure values may be different, because the atmospheric pressure at different measuring sites is different, which is caused by different objective conditions, such as altitude and temperature in different places.
The vacuum industry general, is also the most scientific is the identity with the absolute pressure; the method for measuring relative vacuum degree measuring instrument is simple, non universal, easily available and cheap, so it has been widely used. Of course, in theory, the two can be converted into one another. The conversion method is as follows: absolute pressure = pressure relative pressure at the measuring site.