Model AFS-222-137 Air Pressure Sensing Switch is a general purpose proving switch designed for 24 V ac Energy Management and HVAC applications where a switch with DPDT contacts is needed. It can be used to sense positive, negative, or differential air pressure. The plated housing contains a silicone diaphragm, a calibration spring and a snap-acting SPDT switch with a DPDT relay installed on the snap switch at the factory. The DPDT feature consists of a relay pack circuit board assembly containing a power connection terminal block and a DPDT slave relay with two form-C 8-amp contacts connected to two terminal blocks. The sample connections located on each side of the diaphragm accept ¼” OD metallic tubing via the integral compression ferrule and nut.