Abstract: A spring-loaded differential pressure gage, particularly for testing the components of means for admitting fuel to the tanks of aircraft, has an upright glass cylinder for a reciprocable glass piston with a ring-shaped index which is movable relative to a fixed scale. The heat expansion coefficient of the material of the cylinder is identical with that of the material of the piston. The higher- and lower-pressure cylinder chambers are respectively located above and below the piston, and the piston is suspended on the stronger of two helical tension springs. The upper end of the weaker tension spring is connected to a fixedly mounted plug and the lower end of the weaker spring, as well as the upper end of the stronger spring, is connected to a second plug which is slidable along an upright guide rod having a stop at the lower end thereof. When the pressure of fluid in the upper chamber rises, the weaker spring expands and the stronger spring merely pushes the piston toward the lower end of the cylinder.