Generally Pressure Gauges Differential pressure are provided with micrometer type pointer, by which zero can be adjusted after opening the Bezel & Glass. However for Gauges with Liquid (Glycerine / Silicone Oil) filled case, this arrangement is practically not suitable, since the filling liquid has to be drained before opening the Bezel & Glass. After doing the zero adjustment, the Bezel & Glass has to be re-assembled and again the case has to be filled with liquid.
In order to overcome this difficulty, General instruments consortium has developed a unique of external zero adjustment. By this arrangement, zero can be adjusted without draining the Glycerine, without opening the Bezel, without removing the Glass & without touching the pointer, just by rotating a knob provided out side the Gauges. This arrangement is highly recommended from the maintenance view, especially for liquid filled Gauges.