A pneumatic pressure detector comprises a first electrical terminal, a second electrical terminal and a deformable diaphragm configured to deform between first, second and third positions. When the diaphragm is in its first position, the first and second terminals are open. When the diaphragm is in its second position, the first terminal is open and the second terminal is closed. When the diaphragm is in its third position, the first and second terminals are both closed. The pneumatic pressure detector may be connected to a sensor tube.