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Some pH electrodes like it hot

Edited by the Engineeringtalk editorial team Mar 28, 2005

A novel electrode supplied by Yokogawa UK has played a key role in pH monitoring under difficult environmental conditions in British Sugar's plant at Newark, Nottinghamshire.

A novel pH electrode supplied by Yokogawa UK has played a key role in pH monitoring under difficult environmental conditions in British Sugar's plant at Newark, Nottinghamshire.

The use of a robust, temperature-resistant Pfaudler electrode not only allows pH measurements to be carried out accurately in water at temperatures approaching boiling point: it also minimises calibration and maintenance requirements and improves process efficiency, resulting in considerable cost savings.

The requirement for pH measurement occurs during the process of extracting sugar from beet.

The beet is sliced into strips and soaked in water to extract the sugar prior to converting it into a form that can be transported for treatment and refining.

During this process, the solution is alkalised by adding milk of lime, and it is important to monitor the pH at this stage.

Monitoring with traditional pH probes presents problems because the temperature of the solution is approximately 92C +/-2C.

At these temperatures, the lifetime of a pH probe falls dramatically to around 20% of its life at 20C.

The result is that probes have to be replaced more often and calibrated more frequently, so the cost of installation and maintenance rises.

A further problem is that the lime tends to precipitate out of the solution and form a coating on the electrode, which prevents it from functioning properly.

When Yokogawa engineers were presented with this problem, they suggested that British Sugar should consider a novel type of pH sensor manufactured in Germany by Pfaudler, a German company represented in the UK by Yokogawa.

The Pfaudler pH electrode differs from the conventional type in that, instead of being based on a blown glass tube with internal fluids, it is made of mild steel coated with glass enamelling, and has no internal fluids.

The Pfaudler design results in a sensor that can operate at temperatures of up to 140C.

It is also physically robust enough to be located in fast-flowing and turbulent areas of the fluid under test - conditions which do not lead to any build-up of deposits.

The sensor is very stable in operation and gives accurate and repeatable readings, needing recalibration only once a year instead of once every one or two weeks with traditional sensors.

A further benefit results from the cyclic nature of sugar-refining processes.

Because of the limited season when beet is produced, the plant is typically only in operation for nine months of the year.

This presents a disadvantage for normal pH sensors, which have to be kept wet to ensure satisfactory operation.

When the plant is shut down, these electrodes dry out and stop operating.

The Pfaudler electrode, on the other hand, can be allowed to dry out and will work instantly the plant is started up again.

As a result, the customer has a solution which ensures minimum downtime, produces accurate and repeatable readings, and ensures ease of maintenance and hence overall efficiency.