The RH-3 Series Differential Pressure Switches monitor low-air pressure, vacuum, or differentials of air pressure. These switches are commonly used to prove blower operation, to sense dirty filters, and to monitor pressure drop across coils. Both the Model RH-3 switch and Model RH-3-2 switch are shipped with 1/4 in compression fittings, suitable for use with 1/4 in OD copper or plastic tubing, and a 12 in (30.5 cm) length of 1/4 in plastic tubing.