Starter 400D Dissolved Oxygen Meter with STDO21, one model. Dissolved oxygen is the measurement that decides whether living things in water survive.
ST400D-G
Starter 400D Dissolved Oxygen Meter with STDO21. Dissolved oxygen is the measurement that decides whether living things in water survive.
A membrane sensor consumes oxygen in order to measure it, so it needs the sample to move past it, its membrane fouls, and its electrolyte has to be replaced. An optical sensor measures by luminescence quenching: it consumes nothing, needs no flow, has no electrolyte and holds calibration for far longer.
Membrane sensors remain cheaper to buy and are still the right answer for occasional bench work. For anything left in the water — a pond, an aeration basin, a survey run — an optical sensor costs less over a year, because the cost of a DO sensor is mostly maintenance.
Oxygen solubility falls as water warms and as it gets saltier, and it falls with altitude. A meter that does not compensate for temperature, salinity and barometric pressure is reporting something other than saturation, which is why those three inputs appear on every serious instrument.
Take the reading with the sensor in moving water and let it stabilise; a membrane sensor held still reads its own consumption and drifts steadily downward.
Supplied in one model.