Southern Water standardises on cost-saving, stable pH systems from ABB

Southern Water has cut the annual cost of measuring the pH of potable water from its treatment works by a considerable amount thanks to ABB's Series 7600 pH / Redox (ORP) sensing system. What's more, the systems are so robust that they reliably maintain a stable and accurate pH reading for the time between routine calibrations.

All industrial pH measurement systems include three essential components - a pH measuring electrode, a reference electrode that compares the pH of the process fluid against a standard electrolyte and a temperature sensor. The 7600 Series is extremely cost effective because key components, such as the electrodes, liquid junction and the electrolyte can each be replaced separately. This contrasts with combination electrodes, where the pH sensor, reference electrode and temperature sensor are all housed together in a sealed unit. This usually means the entire system must be replaced if any single component fails.

"Combined probes usually last a year. With the 7600s we typically get three years out of the pH probe and even then it may be only one of the sensors that needs replacing," says Paul Bicknell, ICA Technician for Southern Water at Hardham Water Supply Works near Bognor Regis. "In fact, the main annual maintenance cost for the 7600 systems is replacing the reference salts, which cost around 20 pounds."

The other big benefit for Southern Water is the long-term stability of the pH measurement itself. All of the company's pH monitoring systems are calibrated quarterly, but when alternative systems were checked they were regularly found to be giving readings that were outside acceptable error limits.

It is vital to maintain an accurate pH of around 7.4 for water entering the distribution network, so Mr. Bicknell knew they needed a more stable measurement system: "The readings from the 7600 probes are almost always within acceptable limits when we come to recalibrate them and sometimes we don't even need to make any changes other than toping up the reservoir."

This exceptional stability is largely possible thanks to a reservoir of electrolyte solution used to constantly feed the reference electrode. In contrast, combined probes rely on a gel-based electrolyte, which is sealed into the unit during manufacture.

Mr. Bicknell joined Southern Water around seven years ago, at which time the use of combination electrodes was on the increase: "Contractors were installing the combined units because they were cheaper to buy up front, but it was a false economy and they were unreliable," he says. "I invited ABB in and asked them to come up with a better solution and the 7600 system was it."

The 7600 probes were subsequently fitted at water treatment works throughout West Sussex, where they have now been operating for around five years. This was followed around three years ago by deploying more of the units throughout sites serving East Sussex.

The 7600 Series is available in flow, in-line and dip versions to suit most applications.