Electrochemical Oxygen Sensors – Are they obsolete?

I am still a huge fan of Electrochemical (EC) Oxygen sensors. They’re the best choice for many applications, and because they’re analog and not digital, they provide a real-time signal that’s free from digital noise.

Just like optical dissolved oxygen sensors, electrochemical dO2 sensors measure the partial pressure of oxygen. That number is then used in conjunction with Henry’s Law to calculate the solubility of oxygen in the liquid. For a more complete explanation, see the blog post at this link. The difference between an optical and an electrochemical sensor lies in the mechanism for determining the O2 reading.

While optical sensors use a fluorescent coating and light to measure the quenching properties of oxygen, EC sensors use a polarized cell that measures the electrochemical response of oxygen.  An electrochemical oxygen sensor, also called a Clark-type or polarographic detector, uses electrochemistry to reduce oxygen. This reduction of oxygen creates a current proportional to the partial pressure of the O2 content of the sample being measured.

At its core an EC sensor is a pair of dissimilar metal electrodes bathed in a conductive electrolytic solution.  A membrane covers the electrodes, and oxygen from the sample permeates through the membrane to the electrode where it is reduced.  The electrodes are polarized with about 0.6 volts of electricity, but no current flows between them unless there is oxygen available to carry out the electrochemical reaction.

What could be the downside of an EC sensor? Over time, the electrodes may get fouled due to by-products of the reaction. If this happens, the response time of the sensor gets slow until it reaches the point where resistance builds up in the cell and the measurement becomes inaccurate.  As a general rule, when EC sensors fail they fail with low readings.

So what about areas where EC sensors excel? I really like them in applications where I need wide dynamic range.  I can use the same analyzer to tell me whether I’m reading 0.001 ppm or 20.0 ppm. If I need to track changes quickly in situations where the noise from an optical probe would be too great, or if I’m in the gas phase and reading less than 0.001% O2, I prefer to use EC sensors.

If you’re measuring TPO using the Hach 6110 TPO analyzer, then an EC probe is the only way to go. The 6110 only measures in the gas phase, first giving a discrete headspace O2 and then giving a discrete dO2 concentration. An optical sensor just won’t work in this situation, because the response is not fast enough to properly track the constantly changing sample as gas is released out of the package.

My final thought is that for most applications, optical sensors are usually the best option. But when you want a wide dynamic range or continuous measurement, electrochemical sensors can’t be beat.

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