Image Number 3 for United States Patent #4579001.
A conventional rupturable curved disc in a pressure relief valve for high pressure cylinders serves as one plate of a capacitor. A curved segmented disc, alterable in curvature in response to deformation of the adjacently located curved disc, serves as the other plate of the capacitor. The segmented disc accommodates and conforms to the normal expected deformation of the curved disc, which deformation is due to repeated fill cycles of the cylinder, without altering the maximum and minimum capacitance during each cycle. The segmented disc physically bears against a translatable electrically conductive plunger element. The plunger element is electrically connected to an impedance variation sensing circuit, which circuit senses and provides an output signal responsive to a change in the capacitance between the curved and segmented discs during each cycle; such output signal reflects the actual pressure, and hence state of fill, of the cylinder.