What is vacuum degree, and what is its unit?

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“\"Vacuum level\" literally refers to the degree of vacuum. It is a key parameter for vacuum pumping equipment such as vacuum pumps, micro vacuum pumps, micro air pumps, micro exhaust pumps, and micro exhaust and inflation pumps. The so-called “vacuum” refers to a gas state in a given space where the pressure is lower than 101325 pascals (which is equivalent to one standard atmospheric pressure of about 101 KPa). In a vacuum environment, the degree of thinness of a gas is usually indicated by its pressure value; obviously, the lower this pressure value, the thinner the gas. There are usually two methods for indicating vacuum level: one is to use “absolute pressure” or “absolute vacuum level” (that is, the pressure above the \"theoretical vacuum\”) ； In practice, the absolute pressure value of a vacuum pump ranges between 0 and 101.325 KPa. The absolute pressure value must be measured using an absolute pressure gauge; at 20°C and an altitude of 0, the initial reading of a gauge used to measure vacuum level (an absolute vacuum gauge) is 101.325 KPa. (That is, one standard atmosphere.) Second, it is indicated using “relative pressure” or “relative vacuum level” (that is, the amount of pressure lower than atmospheric pressure). "Relative vacuum degree refers to the difference between the pressure of the object under measurement and the atmospheric pressure at the point of measurement. Measure using a regular vacuum gauge. In the absence of a vacuum (i.e., at normal pressure), the initial value of the gauge is 0. When measuring vacuum, its value lies between 0 and –101.325 KPa (usually expressed as a negative number). For example, a micro vacuum pump model PH2506B (http://www.weichengkj.com/PH.htm) has a measurement value of –75 KPa, which indicates that the pump is capable of creating a vacuum level 75 KPa lower than the atmospheric pressure at the location where it is used. The term \"vacuum level\" commonly used in the international vacuum industry is, scientifically speaking, best expressed in terms of absolute pressure ； It refers to \"ultimate vacuum, absolute vacuum level, and absolute pressure\", but \"relative vacuum level\" (relative pressure, gauge pressure of a vacuum gauge, negative pressure) is also widely used because its measurement method is simple, the measuring instruments are very common, easy to obtain, and inexpensive. Theoretically, the two can be converted into one another, and the conversion method is as follows: Relative vacuum level = Absolute vacuum level (absolute pressure) – Air pressure at the measurement location. For example, if the absolute pressure of a micro vacuum pump model VM8001 (http://www.weichengkj.com/VM.htm) is 80 KPa, then its relative vacuum level is approximately 80 – 100 = -20 KPa (assuming the air pressure at the measurement location is 100 KPa); on a regular vacuum gauge, this would be displayed as -0.02 MPa.