Abstract: A pressure switch for use within a pressure chamber containing a gas having a pressure of P.sub.2, said switch including a pressure vessel containing gas having a pressure of P.sub.1, said vessel circumscribed by an end wall at one end thereof, side walls and closed at the second end by a flexible diaphragm, said diaphragm biased in a first position and subjected to P.sub.2 on one surface thereof and to P.sub.1 on the other surface thereof, said diaphragm assuming said first position when P.sub.2 does not exceed P.sub.1 by a predetermined amount and assuming a second position under P.sub.2 when P.sub.2 does exceed P.sub.1 by said amount, and switch means operatively associated with movement of said diaphragm.