Pressure monitoring device

An actuator device responsive to pressure variations in a fluid medium surrounding and external to the device, comprising:

• a vessel having wall means forming an enclosed chamber, said vessel being surrounded by the fluid medium;
• one-way inlet check valve means in the wall means for allowing fluid of the external fluid medium to flow into the chamber and including internal pressure responsive means for biasing the check valve means toward and holding it in a closed position when internal chamber pressure exceeds external medium pressure, so that the chamber assumes substantially the external medium pressure as the medium pressure rises, and retains the maximum medium pressure attained if the medium pressure drops;
• movable means connected to the wall means, biased toward a normally retracted position, for extending from the retracted position outwardly away from the wall means a predetermined limited distance in response to a decrease in pressure of the external fluid medium;
• and
• a relief valve in the wall means for allowing fluid to escape from the vessel whenever the pressure in the chamber exceeds the external medium pressure by a predetermined amount, said relief valve being operable in response to said predetermined amount of pressure differential independently of said movable means.