Model 621 Discharge Unloader Valve will divert the totaloutput of the air compressor to the atmosphere, while the checkvalve prevents back flow from the air receiver. With the compressorproducing air at just above atmospheric pressure, the motor orengine horsepower draw is greatly reduced and also has a coolingeffect on the compressor. Detail Features: Brass Construction, ASTM B16. Every safety valve is set and tested at the factory. Heavy duty construction for lasting performance. Top quality design, engineering and precisionmanufacturing. Precision machined for smooth operation. Stainless steel spring. Tested in National Board Approved Quality AssuranceProcess. ModelName InletSize OutletSize Min Temp Max Temp PressureRange Height Width Thickness Approx. ShippingWeight 621-4-4 1/2" 1/2" -250 350 5" 2-27/64" 1-13/16" 1 lb. 15 oz.