Result 1C: Evaluating the accuracy of a common DO meter

After a break, we are back to testing after the monsoon season. To recap, we had tested some easily accessible conductivity meters against the lab. Additionally, we also proposed a quick screening method for selecting conductivity meters by using standard solutions with known conductivities.

In this study, we examined the performance of a common DO meter against a certified lab. Unlike the conductivity testers that can cost a few $, a reliable DO meter will start from several $100s. The one that we procured is DD-5509 from Lutron.

 

Dissolved oxygen meter

Calibration steps were followed based on the manual provided by the manufacturer. The oxygen concentration of ambient air was used as one of the calibration points.

Results

5 samples were collected from Moira/Mapusa rivers 

They were tested with the DO meter that was internally calibrated

Samples were also sent to ITALAB for comparison purposes

The DO values ranged from 7 to 7.4 mg/ml

Based on standard tables for saline water and temperature, the DO values were corrected to account for dissolved salts in water. The tables can be accessed here. https://water.usgs.gov/software/DOTABLES/.

DO meter performance comparison

The mean error for the portable DO meter was 2.89% and the correlation was 0.65. This implies that the DO meter can be used for quantitative testing but the testing needs to be carefully done by stirring and the values needs to temperature and salinity corrected. The time difference between internal testing and lab testing, user errors could could have caused the variations.

 

Observations

The DO values did not vary significantly across the samples. Interestingly, the DO of the samples near the sewage discharge was not very different from cleaner areas of the river. This is in contrast to the conductivity that changed drastically and declined in sewage infested samples. Hence, we conclude that the biological oxygen demand (BOD) is a better measure of the pollution of the river. However, this test typically takes 5 days to complete and needs to carefully monitored.

Conclusions

A common DO meter (DO-5509) is accurate for conducting water testing. However, salinity and temperature corrections need to done to achieve a better accuracy.