Broadly speaking, instruments for measuring Dissolved Oxygen are generally deployed as dip-probes and are split into two main measurement types:
The ‘traditional’ method has been to use electro-chemical membrane probes with replaceable end-caps, but in recent years there have been a number of optical systems introduced. In brief, the optical option offers increased accuracy (especially at low levels) and less maintenance down-time, however the optical systems tend to be more power-hungry, and may not respond as quickly. Some of these optical systems still require a membrane, others use solid-state technology to reduce the maintenance time and costs.
The electro-chemical versions basically fall into two distinct methods often referred to as Galvanic and Polarographic. These systems require an electrolyte and a semi-permeable membrane. The optical versions use Luminescence technology (sometimes referred to as Fluorescence) – a blue LED emits lights which reacts with a dye. Membrane versions have the dye in a membrane, these are degraded over time and the membrane will need replacing. Solid state versions do not need a replaceable membrane. To prevent fouling of the sensor, an air-cleaning system is often used – naturally readings are suspended whilst air is blasted over the sensor.