Zirconium Oxide

Paramagnetic

Polarographic

Galvanic (Fuel Cell)

Principle of Operation
The primary element of the galvanic sensor oxygen analyzer is an electrochemical cell consisting of two electrodes in contact with a liquid or semisolid basic electrolyte (usually potassium hydroxide).  This type of sensor is commonly called the "Hersch cell," after its discoverer.  The cell electrodes are made of dissimilar metals, such as silver and lead.  When a gas sample is introduced into the cell, it diffuses through a membrane usually made out of teflon. The oxygen in the sample contacts the silver cathode and is chemically reduced to hydroxyl ions.  The hydroxyl ions then flow toward the lead anode, where an oxidation reaction occurs with the lead.  This oxidation/reduction reaction results in a flow of electrons proportional to the oxygen concentration of the sample.  The electron flow (current) is measured by an external metering circuit connected to the cell electrodes.  This current is proportional to the rate of consumption of the oxygen and is indicated on a meter as a percentage or parts per million of oxygen in the sample.

The cathode reaction involved in the reduction of oxygen is:

The anode reaction is:

The overall reaction is:

Advantages and Disadvantages

The galvanic sensor oxygen analyzer is essentially a battery that produces energy when exposed to oxygen and, hence, is consumed by exposure to oxygen.  It is rugged and insensitive to shock and vibration.  The cell can be mounted in virtually any position without changing its sensitivity.  The sensor can be packaged as a relatively small, self-contained, disposable cell.  It can then be used as a fairly inexpensive means of oxygen measurement in small portable devices.  The sensor measures percentage or trace levels of oxygen directly.  When properly calibrated, it can provide reliable and accurate measurements.  Some sensors can be refurbished rather than replaced by replacing the sensor anode.

Galvanic sensors have several major disadvantages.  Because they operate on a battery principle, their life expectancy is a function of usage.  Furthermore, as these sensors age, they have a tendency to read low due to a loss in sensitivity.  For most process control applications, false low oxygen readings can produce dire consequences.  As a result, analyzers that use battery-type sensors must be recalibrated on a frequent basis, sometimes as often as once per day, depending on the criticality of the application.

Another major drawback of battery-type sensors, particularly when used for trace oxygen measurements, is their susceptibility to "oxygen shock."  If exposed to a large concentration of oxygen, these sensors can take several hours to recover.  The combination of false low readings, frequent recalibrations, susceptibility to oxygen shock, and  relatively short life span greatly lessens the value of galvanic sensor oxygen analyzers in many critical applications.

Typical Usage

The galvanic sensor oxygen analyzer is typically used to validate the quality of semiconductor grade gases; monitor the quality of breathing air; measure oxygen content in annealing furnaces or other heat-treatment operations; measure oxygen levels in potentially explosive or hazardous atmospheres or environments; and monitor oxygen levels in food processing and food storage, and similar applications where product purity can be affected by the presence or absence of oxygen.