HI 9142 is a rugged, waterproof meter that solves the common problems of field use, such as cold, rain, snow and dust, that can damage a meter, rapidly deteriorating its performance and life. It is very simple to use: calibration is performed with HI 7040 zero oxygen solution, while 100% calibration is done in air. The polarographic probe (HI 76407/4) is provided with a membrane covering the sensors and a built-in thermistor for temperature measurement and compensation. The thin permeable membrane isolates the sensor elements from the testing solution, but allows oxygen to enter. When a voltage is applied across the sensor, oxygen that has passed through the membrane reacts causing current to flow, allowing the determination of oxygen concentration.