Accuracy of the
OxyGuard CO₂ Analyzer

In aquaculture and water treatment applications, dissolved carbon dioxide (CO₂) is tpically quantified by measurement of pH and alkalinity. The pH measures the concentration of H⁺ ions resulting from the dissociation of carbonic acid (see the reactions below), while alkalinity is used to approximate the total concentration of dissolved inorganic carbon. Using published data on the dissociation constants (K), the alkalinity and pH data can be used to back-calculate the CO_2(aq) concentration.

You can see how the calculations are done on my calculator page, or take a look at the nice carbonate chemistry overview given by Andrew G Dickson. The pH/alkalinity measurement protocol is relatively cheap and easy compared to other methods, but is prone to error in waters high in salts or dissolved organic substances. A small error in pH measurement (e.g. 0.1 pH) can mean a large error in estimated dissolved CO₂ for the concentrations found in aquaculture or waste water. An example of the pH/CO₂ relationship for sewater is given below.

Around 2005 the Danish company OxyGuard released a submersible dissolved CO₂ meter that measures the partial pressure of CO₂ using a gas permeable membrane and infra-red measurement cell. The CO₂ partial pressure is converted to a concentration via a thermistor and salinity-specific calibration method. The direct measurement of dissolved CO₂ avoids the problems of accurate pH measurement and non-carbonate alkalinity agents in the water.

There was scepticism amongst many users about the precision of this meter, so along with John Fleng Steffenson and Bjørn Tirsgård at the Helsingør Marine lab, we set out to test whether the OxyGuard CO₂ Analyzer gave correct CO₂ concentrations. Water flow over the probe membrane was an important factor in determining response time.

The commonly reported under-reading of this meter may have been due the slow equilibration of the gas space inside the probe with the CO₂ gas pressure in the water at low water velocities. However, provided the probe is given time to equilibrate, the meter will give an accurate reading.

The precision of measurement is 1 mg/L CO₂, so this meter is not useful for atmospheric CO₂ concentrations (about 0.8 mg/L).

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This work was made possible by a postdoc fellowship from the NZ Foundation for Research Science and Technology

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This research has been published and is available as follows:

Moran, D., Tirsgård, B., Steffensen, J.F., (2010). The accuracy and limitations of a new meter used to measure aqueous carbon dioxide concentration. Aquacultural Engineering 43, 101-107

doi:10.1016/j.aquaeng.2010.07.003 ------- get PDF of paper

Accuracy of the OxyGuard CO2 Analyzer

Accuracy of the
OxyGuard CO₂ Analyzer