Considered the standard instrument for measuring the breathing effort of dive regulators, this Magnehelic Gauge has dual, 5" scales to sense both positive (exhalation) and negative (inhalation) pressures. Breathing resistance is measured in inches of water column, equivocally the respiratory effort required to draw water one inch up a straw, etc. Used with the #48101 Regulator Probe (not included), this is an essential addition to the dive facility repair department.  Order 43302 for +/- 2 inch scale.