how does a photohelic gauge work
Dwyer Photohelic Pressure Gauge & Switch. 3-in-One Indicating Gage, Lo-limit and Hi-Limit Control. Photohelic Switch/Gages function as versatile, highly repeatable pressure switches combined with a precise pressure gage employing the time-proven Magnehelic design. The Photohelic measures and controls positive, negative or differential pressures of air and compatible gases.