Trial identifies clearer pH detection method

Low conductivity in Cornwall's soft waters was posing South West Water a problem in measuring pH. Charlotte Unwin of Endress+Hauser explains how a trial helped identify the best way to improve accuracy

Taking accurate pH measurements in a low conductivity environment was proving such a challenge for South West Water (SWW) that instrumentation control & automation specialist Robin Lennox decided to set up a trial rig to investigate how best to improve the accuracy and reliability of measurement. He chose to locate the trial at DeLank Water Treatment Works on Bodmin Moor in north eastern Cornwall where some of the softest raw water in the UK can be found.

Taking a low conductivity pH measurement requires thought and forward planning. Many river sources in the south west, coming straight off the moor, can have conductivity values as low as 30-40ìS/cm, in which even the most sensitive electrodes can struggle to maintain a stable measurement. They also require the use of low conductivity buffers that can only be used once and are easily contaminated, potentially leading to erroneous results and additional expense.

With a wide variety of possible solutions available, Lennox's team wanted to establish the general principles of the different measurement equipment and, more importantly, to highlight the individual benefits each could offer. The trial was conducted over a period of six months using a selection of transmitters and electrodes from a variety of manufacturers.

The team quickly concluded that the established measurement method using flowing references brought considerable benefits. The flow introduces a small amount of conductivity into the sample, making accurate, stable measurement possible. Also, by having a very small amount of electrolyte continuously flowing through the reference electrode, ingress of sample is prevented and operation is significantly improved in the low conductivity water. By maintaining a continuous flow of KCl through the reference electrode there was no buffer ingress into the electrode during calibration.

"We also investigated sample presentation in order to determine which configuration gave the most accurate and stable measurements," explains Lennox. "We found that when there was a cross flow across an electrode there was a tendency for the pH electrode measurement to change with the rate of flow.

"This is because changes in flow rates can cause a change in reference junction potential as ions are pulled away from the reference. This effect is pronounced in low conductivity applications."

The SWW team was able to stabilise the measurement by setting up a dual validation tank assembly that uses a weir and baffle plate to produce a gentle, uniform upward flow around the electrode. Another innovation assessed in the trial was the use of Endress+Hauser's Memosens digital sensor technology.

Instead of high impedance cables, this system uses simple twisted pair cables that are both cheaper and longer lasting. An added benefit of using Memosens is that the non-contact, inductively coupled head makes the cables insensitive to damp and contact. This allows a pH electrode to be placed directly into a process without any problems from moisture.

"We have also reduced the return to service time of the electrodes," Lennox continues. "While this used to be between 45 to 60 minutes, the flowing reference electrodes can be returned into service almost immediately.

"This greatly improves the availability of the measurement and reduces the calibration time on control applications. For applications where the pH measurement is used to control the process, we always use two electrodes. Before this trial we would control in the mornings from electrode A while electrode B was being calibrated, then swap them over in the afternoons. Now we can do both at once."

Having completed the trial, Endress+Hauser's Memosens sensors consistently produced the most stable, accurate measurements. "The Liquisys CPM253 transmitter teamed with Memosens sensors have continued to perform well in situ," Lennox confirmed. "We have now installed the Ceraliquid CPS41D digital pH electrodes across the region and have recorded a factor of 10 improvement in measurement accuracy. While we used to get reference readings within 0.2 or 0.3 pH units, we now get readings within 0.03 of each other."

He concludes: "We intend to upgrade all pH measurements at our water treatment works to Memosens with flowing references and are using Memosens at some wastewater treatment works. We are impressed with the way Endress+Hauser has worked with us to provide the best solutions and with how they've responded to the challenges we've faced."

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