The D30 two position top entry level switch operates on the principle of displacement. When non floating solid polypropylene displacers, suspended from the switch, are immersed in liquid their weight is reduced by an amount equal to the weight of the liquid they displace, This weight change is detected and used to actuate a magnetically isolated switch. Surface agitation, froth, floating or submerged solids do not effect the action of the D30 switch. The displacement principle depends on the specific gravity of the liquid in which the displacers are suspended. The switch is set to operate in water, at a specific gravity of 1+/- 5%.