Establishing a known gas partial pressure at the reference electrode is key to the proper operation of a Nernstian solid electrolyte sensor. In a traditional approach, gas of a known composition may be delivered to the reference electrode from a gas cylinder, or from ambient air in the case of an oxygen sensor. As well as being cumbersome  and inconvenient, the flow of cold gas onto the reference electrode can cause errors due to thermoelectric effects. More importantly, a gas phase reference electrode results in inherently poor thermal shock resistance due to the difficulty in forming hermetic seals between the sensor and probe components at high temperature.

EMC sensors employ state of the art solid state reference technology to eliminate the need for a gas reference.  The sensor is completely self contained,  and may be incorporated into a probe without forming a rigid mechanical joint, resulting in unrivalled thermal shock resistance. The benefits of a solid reference are summarised below:

 No need for a gas cylinder

 No reference air supply needed

 Miniaturised sensor design (a few mm in size)

 Highly portable

 Excellent thermal shock resistance

Principle of operation

A solid reference electrode may be understood by considering the Gibbs’ phase rule which may be stated as  F = C−P + 2 where C is the number of components, P is the number of phases, F is the number of degrees of freedom the system has. The constant 2 implies that both temperature and pressure are variables. A solid oxygen reference material generates an oxygen potential via the equilibrium between a metal and it’s oxide:

In this system there are 2 components (metal and oxygen) and 3 phases (metal, metal oxide, and oxygen gas). From the Gibbs phase rule the number of degrees of freedom is given by F = 2−3+2 = 1. Temperature is the one degree of freedom left, thus at constant temperature the partial pressure of oxygen is fixed. A solid hydrogen reference may be formed in a similar manner, through the equilibrium between a metal and its hydride.




© Environmental Monitoring & Control Limited 2010