An oil well fluid depth or pressure sensing system for sensing pressures adjacent the bottom of the oil well and providing signals on the surface indicative of the pressure range in the well. The sensor comprises a pressure sensitive switch utilizing a nestable welded bellows. Information is transmitted to the surface through a single wire, with grounded shield return. A phase sensitive detector circuit is located at the surface for providing signals indicative of normal pressures, over pressures and under pressures, as well as any malfunction in the system.