Suction Pressure

Suction pressure is a negative difference in pressure generated between two points which draws a gas or a liquid from a higher to a lower pressure state.
For example if the inlet pressure of a vacuum pump is at 900 mbar absolute and it is connected to a closed chamber at an atmospheric pressure of 1000 mbar absolute, the resulting suction pressure is 100 millibar negative gauge, which will draw down the pressure in the chamber from 1000 to 900 mbar absolute.

For suction pressures in the range of 0-10 mbar (~0.15 psi) up to 0-1000 mbar (~15 psi) it is possible to use strain gauge diaphragm technology. For ranges 0-0.1 mbar up to 0-10 mbar the sensitivity is too small and errors are too great to use a strain gauge diaphragm, so for very low pressures a more sensitive technology such as variable capacitance, inductive or LVDT is used.
Suction pressure is always measured by the amount of pressure below atmospheric pressure e.g. a suction pressure of 100 mbar means 100 mbar below the atmospheric air pressire. Vacuum pressure is similar but should be referred to perfect vacuum e.g. a vacuum of 100 mbar means 100 mbar above a perfect vacuum.

However vacuum is often confused with suction pressure so a pressure described as a vacuum should always be verified to ascertain whether it is referred to atmospheric pressure or a perfect vacuum.