Make sure your water quality readings are accurate

April 6, 2011

In the week that water quality hits the headlines – for all the wrong reasons in Japan: radioactive iodine levels in seawater near the Fukushima Daiichi nuclear plant reach a new record – 4,385 times the legal limit, the highest reading since the quake on 11 March 2011. And closer to home, the Environment Agency’s decision to put profiles for bathing water quality across England and Wales online in detailed profiles of the cleanliness of about 500 sites, it is clear that accurate data relating to water quality is vital.

Both these situations, very different but equally important to those affected, are reported upon, the former making national headlines around the world, the latter eagerly read by those in the water industry, or who live close to a monitored beach or who are planning a ‘bucket and spade’ holiday in the UK. If the readings, on which this data is based, are wrong it could have, in the case of Japan, life threatening results.

And, as Surfers Against Sewage campaigns director, Andy Cummins, added: “The new bathing water profiles are a significant step forward in the provision of public information about important coastal environments.

“These bathing water profiles will not make the water cleaner, but they will allow beach users to make more informed decisions about when is best to use the water and the information to know when it might be better to give it a miss due to pollution incidents.”

Now at Van Walt we’re not in the business of monitoring radiation levels in water but we do know a thing or two about water quality meters and there is a universal truth relating to the accurate collection of data using a water quality meter – make sure your meter is calibrated correctly.

The accuracy of water quality meters and when and how often to calibrate a meter are both subjects which seem to be somewhat misunderstood and our view is: all water quality meter manufacturers quote the accuracy of the meter itself. So, for example, for pH this will usually be 0.1 or 0.2, but don’t be fooled because no one uses a meter on its own as it will be connected to a cable and an electrode or a cluster of electrodes. Most manufacturers, with the exception of YSI, do NOT quote an accuracy of a system i.e. meter-cable-electrode, which will be much lower than that of the meter alone. The comparison of accuracy between systems should, in our view, only be made for a total system which we call the SYSTEM accuracy.

While on this subject, it’s worth mentioning that FIELD meters will generally have a lower accuracy than LABORATORY meters and it is well worth considering the SYSTEM accuracy when writing or reading a report.

In addition, yearly calibration of a meter is nonsensical because all you can do is calibrate an instrument AT THAT MOMENT IN TIME and as soon as it is been calibrated then taken somewhere else or stored for a few days, weeks, months before being used again, the calibration could be upset.

It is our recommendation that for accurate water quality readings your water quality meter should be calibrated or, as a minimum, a check against a “confidence” solution is done before any important batch of measurements is taken.

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