Blow-off valve and pressure indicator for fire extinguishers

Claim 1

A GAUGE FOR USE WITH PRESSURIZED FIRE EXTINGUISHERS OR LIKE DEVICES HAVING A PRESSURIZED TANK, CMPRISING A CYLINDER HAVING A BORE IN COMMUNICATION WITH THE INTERIOR OF THE TANK, A PISTON IN THE CYLINDER; A SPRING ENGAGING THE PISTON YIELDINGLY RESISTING MOVEMENT OF THE PISTON IN RESPONSE TO GAS PRESSURE; MEANS CONNECTED WITH THE PITTON TO INDICATE THE AMOUNT OF GAS PRESSURE PRESENT IN THE TANK, THE BORE OF THE CYLINDER HAVING AT ONE END A DIAMETER LARGER THAN THAT OF THE PISTON SO THAT AT A PREDETERMINED PLACE IN THE TRAVEL OF THE PISTON, BEYOND ITS NORMAL OPERATING RANGE OF MOVEMENT, TO WHICH IT IS MOVED WHEN PRESSURE IN THE CYLINDER EXCEEDS A PREDETERMINED SAFE VALUE OTHE GAS MAY ESCAPE PAST THE PISTON TO THE ATMOSPHERE, THEREBY AVOIDING THE CREATION OF EXCESSIVE PRESSURE WHICH WOULD DAMAGE THE DEVICE SAID SPRING RETURNING THE PISTON TO NORMAL OPERATING RANGE WHEN THE GAS PRESSURE IN THE TANK IS REDUCED TO A SAFE VALUE, AND AND AN OBTURATING RING FOR THE PISTON, SAID PISTON BEING FORMED OF TWO RELATIVELY MOVABLE PARTS, THE ADJACENT FACES OF WHICH ARE SHAPED TO AND RECEIVE SAID OBTURATING RING, ONE OF SAID PARTS BEING SECURED TO THE PISTON ROD AND FORMING THE HEAD OF THE PISTON AND THE OTHER OF SAID PARTS BEING SLIDABLE ON SAID PISTON ROD AND URGED AGAINST SAID RING BY OPERATIVE ENGAGEMENT WITH THE END OF THE SAID SPRING FACING THE PISTON, WHEREBY AS THE GAS PRESSURE AGAINST THE HEAD OF THE PISTON INCREASES THE GRIP UPON THE RING BY THE TWO PISTON PARTS INCREASES.