Model DFS- 221 is an airflow proving switch designed for duct heater, oven, and other HVAC or Energy Management applications where an open-mounted, nonadjustable switch is desirable. It can be used to sense positive, negative, or differential air pressure. The plated housing contains a diaphragm and a snap-acting SPDT switch that operates on pressure rise of 0.05″w.c., ± 0.02″ w.c. The snap switch has ¼” 90º quick connect spade terminals. The DFS–221 has two sample line connectors (¼” OD slip-on fittings) that accept flexible tubing.