A self contained breathing apparatus (SCBA) served by a source of breathing gas with a mask and a demand regulator connected to the mask. The regulator has a flexible diaphragmatic member having a side exposed to pressure differentials to cause a valve mechanism to introduce breathing gas to the mask. An expanding and contracting member and a chamber fluidically connected to the diaphragm compensates for movement of the diaphragm while shielding the diaphragm from ambient contaminants.