Ion-selective Electrode (ISE) and Optical Infrared Sensors Compared

By Ray Hicks, President,

Reliable carbon dioxide and measurement and control is important in algae and bio-fuel production. When comparing CO2 measurement devices, vendors offer both ion-selective electrode (ISE) probes and optical (Infrared, or IR) sensors. Having a general understanding of the difference can help you better evaluate which is right for your application.

ISE Probes

An ion-selective electrode probe is designed to be submerged in liquid. The probe contains a gas permeable membrane, and an electrode solution surrounding a pH sensor. Partial pressure of the carbon dioxide in the liquid causes it to diffuse through the membrane, which causes a pH shift in the electrode solution. The change in pH is measured by an internal pH sensor, which is proportional to the carbon dioxide level.

The advantages of ISE technology is that it has been a reference standard for measuring CO2 in aqueous solution for many years. Name-brand gas measurement and control systems specify ISE probes as add-on sensors to their devices. As researchers moved to scale up algae production, it was easy to use the tools they already knew and had on hand.

The disadvantage is that ISE CO2 sensors must be regularly “recharged” by regularly adding electrode solution. In addition, over time the ISE probe membrane will be fouled with algae, requiring either disassembly for cleaning or membrane replacement.

Optical Sensors

Optical (Infrared, or IR) sensors have also used for years to measure CO2 levels in the atmosphere or indoors. IR sensors use Infrared light that is passed through a sample of air in a small tube with inlet and outlet ports. CO2 molecules absorb the IR light of the same wavelength that is characteristic of the gas. The difference between the amount of light radiated by the IR lamp and the amount of remaining IR light received by a detector is measured. The difference is proportional to the number of CO2 molecules inside the tube.

The advantages of IR CO2 sensors is that except for periodic calibration, they require no maintenance. To measure CO2 in liquid, instead of drawing CO2 through a gas-permeable membrane, they rely on the fact that the CO2 level in the headspace of an enclosed system will equalize with the CO2 level in the liquid. The gas is then sampled from the headspace, and a measurement is taken. There is no fouling, and no maintenance requirements.

The disadvantages of IR sensors is that over time, the IR lamp changes, and the sensor must be recalibrated with a reference gas, typically once a year. In addition, IR sensors require that the sample be taken from an enclosed space, so they are not practical for open algae ponds. Finally, IR sensors cannot function at > 99% humidity, although this can be remedied by using Nafion tubing to dry the gas samples in real-time.

As algae-related projects ramp up to full production, system designers will look for best-in-class tools to measure and control CO2 levels. Both ISE probes and optical sensors have their place in projects depending on requirements.

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