Magnehelic® differential pressure gauges provide a direct reading of pressure and/or air velocity. These gauges are used to measure filter resistance, air velocity, and furnace draft. Magnehelic® gauges capable of measuring just pressure or both pressure and air velocity are available. Single-scale pressure models, calibrated either in inches water column (in. w.c.) or psi, are common. Dual-scale gauges are normally calibrated for pressure in in. w.c. and for air velocity in feet per minute (FPM). Several models with dual-scale gauges are available. They cover pressures from 0.0 to 10 in. w.c. and air velocity ranges from 300 to 12,500 FPM. Generally, Magnehelic® gauges are permanently installed in the equipment, but portable models like the one shown in Figure 1-41 are also available. Like manometers, pitot tubes and/or static pressure tips are accessories frequently used with portable Magnehelic® gauges to make air pressure and velocity measurements in air distribution system ductwork.