The 499A TrDO is a membrane-covered amperometric sensor. The sensor consists of a gas-permeable Teflon1 membrane stretched tightly over a gold cathode. A silver anode and an electrolyte solution complete the internal circuit. During operation, oxygen diffuses from the sample through the membrane to the cathode. A polarizing voltage applied to the cathode converts all the oxygen entering the sensor to hydroxide ions. The reaction produces a current, which the analyzer measures. The current is directly proportional to the rate at which oxygen reaches the cathode, which is ultimately proportional to the concentration of oxygen in the sample.