It have the function of regulating the negative vacuum pressure, keeping it stable at set value regardless of flow and variations in the degree of vacuum in the primary network.
The units are of a double diaphragm construction and have been designed to exploit to their advantage any existing pressure differential between the secondary depression and atmospheric pressure. The degree of vacuum is obtained by rotating the adjusting knob, rotating it clockwise to increase the amount of vacuum and anti clockwise to decrease it.
They are used in all centralized systems where regardless of the vacuum in the main network a lower vacuum level is required for other applications.