The spring-disk pressure sensor (Figure 400-6) is used in pneumatic and electronic pressure switches (see Section 417). The sensor is a convex metal spring disk (Belleville spring). Pressure applied against the convex side gradually deflects the disk until it snaps to a concave shape. When the pressure is released, the disk snaps back to its original shape. The snap action is connected to an electrical or pneumatic switch. The switch usually includes a helical spring behind the disk for adjustment of the trip point. The switch contacts can be ordered to close or open at the set pressure on either an increasing or decreasing pressure. Pressure switches may be either field-adjustable or preset at the factory. This type of pressure switch is dependable and has good repeatability.