YSI Pro-BOD - Manual (Page 4)

2
O
PERATING
P
RINCIPLES
The 5905/5010/200-BOD/Pro-BOD probe is a polarographic
dissolved oxygen sensor. An oxygen permeable membrane covers
an electrolytic cell consisting of a cathode and an anode. This
membrane acts as a diffusion barrier and an isolation barrier
preventing the fouling of the cathode surface by impurities in the
environment. The cathode is the gold electrode, and the anode is
the silver electrode which completes the electrolytic cell and acts
as a reference electrode.
Upon entering the cell through the membrane, oxygen is reduced
at an applied potential of -0.8 V referenced to the silver electrode.
The reduction current at the cathode is directly proportional to
the partial pressure of oxygen in liquid (expressed as %-air
saturation) which is also proportional to the concentration of
dissolved oxygen (in mg/L) at a particular temperature. Thus the
same partial pressure of oxygen (%-air saturation) in liquid gives
different concentrations of dissolved oxygen (mg/L) at different
temperatures because of the different solubilities of oxygen at
different temperatures.