Accuracy of the OxyGuard CO2 Analyzer

Around 2005 the Danish company OxyGuard released a submersible dissolved CO 2 meterthat measures the partial pressure of CO2 using a gas permeable membrane and infra-red measurement cell. The meter was intended for the aquaculture and water treatment industry, which deal with much higher dissolved CO2 levels than those associated with ocean acidification. The CO2 partial pressure is converted to a concentration (mg/L) 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.

Unlike most aqueous CO2 probes, the OxyGuard CO2 Analyzer utilizes an infra-red CO2 measurement cell.

Unlike most aqueous CO2 probes, the OxyGuard CO2 Analyzer utilizes an infra-red CO2 measurement cell.

Measuring CO2 in aquaculture or water treatment systems

What was the problem with traditional methods of measuring dissolved CO2 based on pH and alkalinity? In aquaculture and water treatment applications, dissolved carbon dioxide (CO2(aq)) is typically quantified by measurement of pH and alkalinity (and often expressed as mg/L CO2). 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 HCO32- and CO32-. Using published data on the dissociation constants (K), the alkalinity and pH data can be used to back-calculate the CO2(aq) concentration.

CO2 reacts with water to form carbonic acid (H2CO3) and carbonates.

CO2 reacts with water to form carbonic acid (H2CO3) and carbonates.

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.

How well does the OxyGuard CO2 Analyzer work?

In the early years after its release, 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 (see below).

The response time of the OxyGuard CO2 Analyzer at different water flow velocities over the probe. Span level represents the point at which the meter was reading 80%, 95% or 99% of the true value.

The response time of the OxyGuard CO2 Analyzer at different water flow velocities over the probe. Span level represents the point at which the meter was reading 80%, 95% or 99% of the true value.

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 (see graph below).

Accuracy of the OxyGuard CO2 Analyzer. Known CO2 concentrations were generated using a gas mixing pump. 

Accuracy of the OxyGuard CO2 Analyzer. Known CO2 concentrations were generated using a gas mixing pump. 

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

This research has been published:

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.

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