Hays Cleveland D-06100-00 Single Pointer Gauges are simple, inexpensive flat-face gauges. These gauges are intended for use with combustion air only. They are dependable, maintenance free, and long-lived in any application requiring an accurate continuous indication of combustion draft, pressure or differential pressure. A special beryllium spring provides permanent, unvarying tension. A three-way selector valve is standard, for resetting the gauge to zero and cleaning sample lines without disrupting factory-set calibration.