The D50 two-position level switch operates on Archimedes' principle. When non-floating solid 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.

The weight change is detected and used to actuate a magnetically isolated switch. Surface agitation, froth, floating or submerged solids do not affect the action of the D50 switch.

The displacement principle depends on the specific gravity of the liquid where the displacers are suspended. The D50 is set to operate in water with a specific gravity of 0.95 or greater.