YSI Dissolved Oxygen, Conductivity, Salinity

YSI Dissolved Oxygen, Conductivity, Salinity. Dissolved oxygen is the measurement that decides whether living things in water survive.

The models differ by 1M, 4M, 10M, 20M and 30M.

Optical or membrane

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.

What moves the number

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.

Where it is used

• aquaculture and fisheries
• wastewater aeration control
• river, lake and coastal surveys
• process water monitoring

Practical note

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 5 models.