Differential pressure switch with an adjusting piston driven by a restoring spring for an oil-separating air filter

A differential pressure indicator for use in a system including an oil separating air filter element which separates an unfiltered air zone from a filtered air zone and which filters a mixture of air and oil, comprising a housing, a piston arranged in said housing and exposed to pressures of two different pressure zones, one of said pressure zones being in communication with said unfiltered air zone, and the other of said pressure zones being in communication with said filtered air zone, and a rolling member arranged in said housing adjacent one end of said piston, wherein said piston is movable in response to a relative change in pressure between said two pressure zones and is biased in one direction by a restoring spring, and wherein said different pressure zones are separated by said rolling membrane.