Polarographic oxygen sensors, also called as Membrane oxygen sensors or Clark cell oxygen sensors, are widely used for the measurement of oxygen in natural water. Polarographic oxygen sensors are used on oceanographic CTD (conductivity, temperature, and depth of sea water) measurement instruments, and are routinely taken to great depths of ocean. Polarographic Oxygen Sensors are the popular underwater oxygen sensors.

Polarographic oxygen sensors are often referred to as a Clark Cell. In this type of sensor, both the anode (typically silver) and cathode (typically gold) are immersed in an aqueous electrolyte of potassium chloride. The electrodes are separated from the sample by a semi-permeable membrane that diffuses oxygen into the sensor. The silver anode is typically held at a potential of 0.8V (polarizing voltage) with respect to the gold cathode. Molecular oxygen is consumed electrochemically with an accompanying flow of electrical current directly proportional to the oxygen concentration (based on Faraday’s law). The current output generated from the sensor is measured and amplified electronically to provide a percent oxygen measurement.

Advantages of Polarographic Oxygen Sensors

  • Longest operating life, typically 300,000 hours
  • Polarographic oxygen sensors are available either in disposable or refillable/reusable
  • While inoperative, there is no consumption of the electrode (anode)
  • Almost indefinite storage times
  • Not position sensitive
  • The sensor of choice for dissolved oxygen measurements in liquids

Disadvantages of Polarographic Oxygen Sensors

  • Polarographic oxygen sensors typically exhibit differences in output between upcasts and downcasts, the quality of calibrations are often depth dependant
  • Relatively high frequency of sensor replacement
  • Sensor membrane and electrolyte require maintenance
  • For gas phase oxygen measurements, the sensor is suitable for percent level oxygen measurements only