The Original Dwyer Magnehelic gage for high accuracy guaranteed within 2% of full scale and for the wide choice of 81 models available to suit your needs precisely. Using Dwyer's simple, frictionless Magnehelic gage movement, it quickly indicates low air or non-corrosive gas pressures either positive, negative (vacuum) or differential. The design resists shock, vibration and over-pressures. No manometer fluid to evaporate, freeze or cause toxic or leveling problems. It's inexpensive, too. The Magnehelic gage is the industry standard to measure fan and blower pressures, filter resistance, air velocity, furnace draft, pressure drop across orifice plates, liquid levels with bubbler systems and pressures in fluid amplifier or fluidic systems. It also checks gas-air ratio controls and automatic valves, and monitors blood and respiratory pressures in medical care equipment. (Standard model shown in picture.)