One is to use the "absolute pressure", "absolute vacuum" (that is, how much higher than the "theoretical vacuum" pressure) mark;
In the actual situation, the absolute pressure of the vacuum pump is between 0 and 101.325KPa. The absolute pressure should be measured with the absolute pressure meter. At 20℃ and altitude =0, the initial value of the instrument used to measure the vacuum degree (the absolute vacuum meter) is 101.325KPa(that is, one standard atmosphere).
The second is to use "relative pressure", "relative vacuum degree" (that is, how much lower than "atmospheric pressure" pressure) to mark.
"Relative vacuum" refers to the difference between the pressure of the measured object and the atmospheric pressure of the measured place. Measure with an ordinary vacuum gauge. In the absence of a vacuum (that is, at normal pressure), the initial value of the table is 0. When measuring a vacuum, its value is between 0 and -101.325KPa (usually expressed as a negative number).
For example, a miniature vacuum pump with a PH2506B measurement of -75kpa means that the pump can pump a vacuum at a pressure of 75KPa lower than the atmospheric pressure at the measurement site.