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Explanation to Unit of Vacuum Degree
    There are usually two ways to identify vacuum degree: one is by ultimate pressure (absolute vacuum) and the other is by gauge pressure (relative vacuum).

    The ultimate pressure is supposed to mean the pressure of air in the vessel where the air is pumped continuously long enough for the pressure of air in the vessel cease to drop and maintain a certain stable degree. If there be absolutely no air in the vessel, the ultimate pressure should be zero, which is only theoretically valid. In reality, the ultimate pressure of vacuum pump is between 0~101.325kPa. Ultimate pressure needs to be measured by ultimate pressure gauge. The initial value of the gauge is 101.325kPa at 20℃ with altitude of 0.

    Gauge pressure means the difference of the pressure of measured object with the atmospheric pressure on the location of the measurement, which is done by ordinary vacuum gauge. The initial value of the vacuum gauge is 0 when there is no vacuum. When vacuum is measured, the value is between 0 and -101.325kPa (generally indicated with negative number). For example, if the measuring value is -30kPa, it indicates that the pump is capable of producing a vacuum of pressure 30kPa lower than the atmospheric pressure on the measuring location. The gauge pressures of the same pump measured at different locations may read different due to different atmospheric pressure of the different measuring location. This is due to different elevations and temperatures of different locations.

    The industry standard and the most scientific way to identify vacuum pressure are by ultimate pressure. However, gauge pressure measurement is also enjoying wide application due to the simplicity of measurement, availability of measuring devices and their affordability. Of course, theoretically the two methods are convertible. The conversion equation is: Ultimate pressure = Atmospheric pressure at the measuring location – Gauge pressure.