Abstract: A biomedical pressure sensor having a pressure sensing diaphragm is provided with a distensible membrane overlying the diaphragm, and the diaphragm is provided with one or more holes. During normal operation, externally applied pressure forces the membrane against the diaphragm so that the membrane and diaphragm deform under pressure as a unit and transmit pressure-applied forces to a force transducer within the probe housing. When a reference pressure exceeding the externally applied pressure is supplied within the probe housing, such reference pressure passes through the hole in the diaphragm and forces the membrane away from the diaphragm, thereby isolating the diaphragm from the externally applied pressure and bringing the diaphragm to a zero-differential pressure condition. Use of a hole in the diaphragm permits an extremely compact construction.