Introduction
Dissolved Oxygen (DO) is an essential measurement parameter in aerobic bioreactors. The growth of all cells is heavily dependent on DO because it acts as a terminal electron acceptor in aerobic respiration. However, if excessive amount of DO is added to the process, it may limit the growth of the culture and promote undesirable organisms. Consequently, the measurement of DO is critical to effective operation of systems. Today, a variety of sensors are available in the market, each with its own advantages and disadvantages.
Optical Sensors
Electrochemical Sensors
Electrochemical DO electrodes are divided into two separate types: polarographic and galvanic. These electrodes are constructed with an anode and a cathode submerged in an electrolyte solution. A teflon or silicone permeable membrane is used to confine the components. When the cathode is polarized with negative voltage, dissolved oxygen molecules diffusing through the membrane is reduced at the cathode. An electrical signal produced by the cathode travels to the anode and then to the instrument. The oxygen tension versus the electrode current can be calibrated since the diffusive flux is a function of the partial pressure of oxygen in the flow. Instead of measuring the DO concentration, the electrode senses DO activity (or tension).
As in the case for the polarographic electrodes, a voltage is applied externally while an internal potential is generated as in the galvanic electrodes.