YSI 5239 Dissolved Oxygen Probe - Manual (Page 11)

7
P RINCIPLES O F O PERATION
T HE C LARK O XYGEN S ENSOR
The 5239 probe is a Clark type polarographic (voltammetric)
dissolved oxygen sensor. The sensor is made of a silver anode
and a gold cathode (see Figure 2), and is separated from the
measured medium by a semi-permeable Teflon membrane.
The temperature sensing element (thermistor assembly) is
mounted next to the oxygen sensor vertically, providing
temperature readings for the DO system.
The membrane selectively allows oxygen to permeate into the
sensor, but prevents most interfering molecules and fouling
materials from entering. Upon permeating through the
membrane, oxygen is reduced at the gold cathode. The current
resulting from this reduction is diffusion-limited and is
proportional to the partial pressure of oxygen in the sample.
The counter reaction is the oxidation of silver at the
Figure 2
Temperature sensor
Anode (silver)
Cathode (gold)