Abstract: A differential pressure-responsive apparatus is disclosed which includes a primary base plate member, a cover member removably attached to the primary base plate and thereby forming a normally closed chamber, a bellows and range spring module removably attached to the primary base plate within the chamber and including a secondary base plate member, a bellows apparatus with one end thereof removably fixed to the secondary base plate and having an opposite normally closed movable end thereof movable in response to varying differential pressure thereacross, said bellows apparatus dividing the chamber into separate inner and outer chamber portions, and a first, normally closed, one-way opening valve in the normally closed movable end of the bellows apparatus, operable for interconnecting and disconnecting the inner and outer chamber portions of the apparatus with the first valve interconnecting the inner and outer chamber portions when the differential pressure therebetween exceeds a predetermined value in a fir