A Magnehelic gauge works by using a sensitive diaphragm that responds to changes in pressure. When pressure is forced on the diaphragm the dial of the Magnehelic gauge points to a number representing the amount of pressure.
A Magnehelic gauge is an a measuring instrument that measures the static pressure in a heating, venting and cooling or HVAC system. The pressure measured can be positive, negative or differential. The instrument can be used to measure fan and blower pressure, air velocity, furnace draft, pressure drop across orifice plates, filter resistance, liquid level of bubbler systems and pressures in fluid amplifier or fluidic systems.
In order for the diaphragm to function accurately, the positioning of the Magnehelic must be correct. The Magnehelic needs to be level and positioned vertically, otherwise the diaphragm sags, producing shifts in pressure leading to inaccurate readings. Magnehelic instruments are available in a variety of scales and various pressure units varying from pascals and inches of water to pounds per square inch. Because many technicians work on a variety of systems, a technician needs to buy multiple instruments to cover the variety of systems. Due to the shape of the Magnehelic lens, the pressure reading must be done by looking straight at the dial and not at an angle because the shape of the lens will distort the view resulting in inaccurate readings, also known as a parallax error.