Dwyer Instruments Magnahelic. For differential, positive or negative pressures. Particularly useful for low pressures. Can be scaled in inches of water, psi, or air velocity. Can be flush, surface or pipe mounted. Metric scales available. Accuracy is 2%, except for gauges with less than 1" total scale, in which case the accuracy is 4%. Exclusive magnetic linkage provides exceptionally responsive and consistently accurate readings. These gauges have a dead read out without tapping. Gauges have 4" dial with plastic face. Sapphire bearings provide friction free motion of the helix which actuates the pointer. The helix is precision milled from an alloy of high magnetic permeability. Damping is with silicone fluid. A zero adjustment is on the rear of the case. Case is die cast aluminum.