Abstract: This invention makes possible the use of a very small pressure relief valve for protecting pressure gauges, thermometers, humidity indicators and other detectors from excessive pressure in an element in which the fluid is contained and of which the indicators must measure a characteristic. The pressure relief valve is located in an exhaust branch outlet from passages that connect the detector with the element in which the fluid is contained; and the relief valve has a housing with an inlet port of a cross-section much less than the cross-section of the passage from the element containing the fluid. This inlet port throttles the flow of fluid into the relief valve. By thus reducing the cross section of the inlet port, the relief valve can be as small and inexpensive as desired, commensurate with the flow required by the detector where the detector is of a nature requiring a flow of the fluid.