Calibration in a wastewater treatment plant

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August 2001 Maintenance, Test & Measurement, Calibration

Ken West, Fluke (UK)

Just one sewage treatment works, run by Thames Water near the famous Twickenham Stadium to the south-west of London, is capable of handling 540 million litres of fluid per day and serving two million people.

Add another five treatment plants across London - imagine all the pumps, the thermocouples, the water level meters, the flowmeters, the pressure meters, the dissolved oxygen, turbidity and toxicity measurements - and you get an idea of the requirements for process control. Then think of the problem of calibrating all of these instruments, and producing certificates to show they are within the required tolerances.

Are the meters on these devices accurate today, and will they still be accurate tomorrow? ISO 9002 demands regular checking. "Process calibration is just part of my work, and something that our customers demand," says Lyndon Hamill, lnstrumentation Control and Automation (ICA) Technician at the Mogden site. "With the continuous pressure to improve efficiency, any tool that can help us to meet that goal is valuable."

Reliable calibration tool

Over 1000 process 'loops' need to be calibrated at the Mogden Lane site alone on an annual basis. To meet ISO 9002 (BS 5750), the ICA team needs a reliable test tool that can do the job quickly and efficiently. With previous experience of the ruggedness and reliability of Fluke meters in the field, the team purchased the Fluke 702 documenting process calibrator. This one handheld device can source and measure all the essential processing parameters, eliminating the need to carry two devices - one to generate signals and a digital multimeter to take the readings.

During calibration, the user of the Fluke 702 can select 3, 5 or 11 test points across the span of a process transmitter so that the simulated output is automatically stepped at 50, 25 or 10% intervals. At each stage, a measurement is automatically taken and the full-scale error is calculated and stored along with time, date, TAG ID, instrument serial number and operator number.

All this information can be uploaded at a later stage to a PC (via Fluke's PLINK' Windows-based software) for printouts or export to databases. This allows hard-copy certificates to be produced or stored on disk, as part of an automated procedure rather than as a time-consuming manual task.

Uniform testing procedure

Using the Fluke 702's owner-generated procedures, with prompts downloaded from the Fluke-produced software, ensures that a uniform procedure is followed every time. The simple procedures, with on-screen guidance, also allow nonspecialised personnel such as electricians to handle many common calibration tasks.

Troubleshooting is also among the 702's capabilities. A recent problem with an RTD temperature sensor in a sludge digester would have proved a long job, requiring the use of a transmitter and a DMM. However, thanks to the Fluke 702's ability to source and send at the same time, the problem was quickly tracked down to a faulty transmitter, which was then replaced.

What about calibrating the calibrator? Fluke has a long history of expertise in this field, and offers a full NAMAS-accredited service from its network of calibration labs. For the 700 Series calibrators, calibration intervals can be one or two years, depending on the user's required working tolerances.

And the end of the story, producing an average of 4,4 MW of power by burning the methane from the sewage digestion plants, Thames Water sells the excess not used at its site to the UK's National Grid. This makes the Mogden Lane site a net producer of energy. Not bad, when you think what comes in at its front door!

This article was supplied by Spescom MeasureGraph, sole supplier for Fluke in South Africa.

Spescom MeasureGraph

(011) 266 1500

salestm@spescom.com

www.spescom.com

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Maintenance, Test & Measurement, Calibration

Calibration in a wastewater treatment plant

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