Abstract: An apparatus for continuously monitoring clearances between a moving and a stationary machine part, utilizing compressible fluid flow. A measuring nozzle is disposed in the clearance space, and a conduit connects said nozzle with a space having a compressible fluid pressure different from the pressure in the clearance space. A measuring orifice is disposed along the conduit path. First and second comparison nozzles are situated in a pressure chamber and coupled through conduits to the same pressure space as that to which the conduit of the measuring nozzle is coupled. Measuring orifices are also situated in the conduits coupled to the comparison nozzles. Differential pressure meters are coupled between the conduit of the measuring nozzle and a conduit of one of the comparison nozzles; and between the conduits of the two comparison nozzles.