pH is the potential measure of hydrogen ion concentration in the solution, expressing the acidity or alkalinity of solution on a Logarithmic scale on which 7 is neutral, lower values are more acid and higher values are more alkaline. SANSEL make online ph 641 indicator is a precise, rugged and weather proof instrument carefully designed for the most accurate measurement of pH/ ORP in all industries. This instrument is a panel mountable with a compact standard size of 48 x 96 x 65mm with front end pot meter type easy adjustable 4pH & 7pH buffer calibration, zero & SPAN adjustment for ORP. This instrument operates from 90V to 270V AC / DC ,50Hz.