There are two DO (dissolved oxygen) technologies - diffusion and equilibrium.
DO measurement based on the rate of diffusion measure the diffusion of O2 across a membrane. The anode needs to be cleaned, recharged or replaced.
Equilibrium is where the DO measurement based on the partial pressure of O2, and the reading is generally independent of fouling and the anode is not consumed because any O2 consumed at the cathode is regenerated at the anode. Hence, equilibrium technology doesn't consume itself like diffusion does over time.
Considering the relatively high investment in industrial quality DO instrumentation, keeping the cost of ownership down can be a factor in looking for a DO probe.
Because DO probes use excitation and low level high impedance internal circuits, DO probes are always sold with a DO analyzer or transmitter which conditions the DO signal to a conventional industrial output. The DO probe does not connect directly to a PLC analog input.
Dan