Delta F Non-Depleting Coulometric

Zirconium Oxide

Paramagnetic

Polarographic

Delta F Non-Depleting Coulometric

Principle of Operation
The polarographic oxygen analyzer incorporates an electrochemical sensor that responds to changes in the partial pressure of the oxygen in a sample.  The sensor cell consists of a cathode, or sensing electrode, an anode, or reference electrode, and an electrolyte, usually aqueous potassium chloride.  The cathode is separated from the sample gas by a permeable membrane that permits the diffusion of oxygen from the sample gas into the measurement cell.  A predetermined voltage selected to make the sensor specific for oxygen is applied between the two electrodes.  When the sensor is exposed to a sample gas, the oxygen in the sample diffuses through the membrane to the sensing surface of the cathode, where it is electrochemically reduced and produces a linear output current that is proportional to the partial-pressure changes of the oxygen in the sample.  The output current is amplified and is indicated as percent oxygen by readout instrumentation.

Advantages and Disadvantages

The polarographic electrochemical oxygen sensor is insensitive to shock and vibration.  It can be mounted in virtually any position without changing its sensitivity.  The rate at which the sample gas flows over the sensor is not critical since sampling is diffusion-controlled through the membrane. The sensor can be stored for long periods of time; the anode is not consumed until power is applied to the sensor.

Some disadvantages are inherent in polarographic instruments. They are generally slow in operation because the oxygen has to diffuse through the membrane. Since a membrane is used to control the rate of diffusion, these instruments tend to be quite pressure- and temperature-sensitive unless some kind of compensation is provided. Regular usage results in gradual degradation of the anode material and electrolyte solution, causing drift and erroneous low readings and requiring frequent calibration. Because the sensor anode and electrolyte solution are consumed in making oxygen measurements, the sensor requires replacement or refurbishing at intervals of six to twelve months, depending on usage.

Typical Usage

The polarographic oxygen analyzer is used for the most part to measure dissolved oxygen in liquids.