Optical oxygen measurement is state of the art and provided by Centec and several of our competitors. However, many sensor manufacturers offer Clark electrodes to determine the oxygen content in the product. Therefore, you should know the principle of both technologies and the advantages of the optical method.
Optical oxygen sensors contain a small glass component (“optical window”) installed in the measuring head. On the surface of the optical window there is thin film of fluorescent molecules. A polymer optical fiber transmits excitation light (blue-green-light) from the light source to the fluorescent molecules. When the molecules absorb this blue-green-light they are promoted to a higher energy state. After a certain time they convert back into a lower energy state during which a detectable red light is emitted. The same polymer optical fiber transmits this red light to the measurement device.
If oxygen is present the energy is transferred from the excited fluorescent molecules to the oxygen. Thus, the detected red light decreases according to the oxygen content. Centec offers an in-line oxygen sensor (OXYTRANS TR) and a portable device (OXYTRANS M). Both of them use the optical method.
Clark sensors contain two electrodes which are linked by a potassium chloride solution (KCl) serving as electrolyte. One electrode consists of platinum (Pt); the other one consists of silver (Ag). When a small constant voltage is applied across the two electrodes the Pt becomes negative (> cathode) and the Ag becomes positive (> anode). The electrolyte and the product are separated by a thin membrane which is permeable to oxygen. The partial pressure difference of oxygen between both sides of the membrane is the force “pushing” the oxygen from the product through the membrane into the electrolyte.
At the cathode, electrons are “consumed” by the reaction with H2O and O2 which leads to OH-. At the anode, silver reacts with Cl- from the electrolyte to produce AgCl and free electrons. The silver chloride (AgCl) deposits on the surface of the silver electrode. The OH- produced at the cathode combines with the K+ from the electrolyte, so the electrolyte changes from KCl to KOH. The free electrons generated at the anode flow through the circuit to the cathode resulting in a current that is proportional to the concentration of oxygen (see attachment).
Disadvantages of the Clark method
Due to the consumption of Cl-, the electrolyte needs to be replaced frequently
The membrane is sensitive to pressure shocks and fouling, so its life time can be short
Removing the AgCl deposits from the anode is a time consuming procedure
Frequent calibration results in high upkeep and labor costs
Oxygen is consumed from the product during the measurement
There has to be a constant product flow to avoid inaccurate readings
After disconnecting and connecting again, the sensor can need hours for polarizing
Clark sensors are less accurate compared to optical oxygen sensors