Instrumentation is a critical component in water and wastewater treatment plants, since the measurement and control of the various processes determines the quality of the water we drink, recycle or discharge into the sea. One such critical instrument is the electromagnetic flowmeter, or magmeter, which is now commonly used to measure instantaneous and cumulative water flow in water and wastewater treatment plants.

Given the widespread use of these instruments, there are now over 15 manufacturers offering a variety of models. Consequently, the technology and features bundled into magmeters have evolved in recent years with companies like Endress + Hauser developing leading edge products.

Choosing a good magmeter requires a clear appreciation of the important characteristics, qualities and features of models currently available. An example is the innovative Promag family from Endress + Hauser, which offers - at no additional cost - features and benefits not available from most other makers. Understanding the following six key attributes will help purchasers and engineers select the best magmeter for the job.

Transmitter housing

Instrumentation installed at water treatment plants, especially along the coast, often faces extremely harsh environmental conditions. Housing of the magmeter is therefore key, and a variety of housing materials is on offer ranging from hardened plastic to metal. Plastic has a tendency to crack in harsh Australian conditions, unlike the diecast powder-coated aluminium housings on Endress + Hauser magmeters. These IP67 rated housings come as standard on the Promag range and ensure protection against dust and moisture. Moreover, the terminal compartments in this range are physically separated from the electronics to avoid exposure of the PCBs to moisture and damage during termination. Commissioning the Endress+Hauser meter is also a more secure process since captive screws on the transmitter prevent them from getting lost.

Demanding environments

In some treatment plants, magmeters are installed in sumps and ‘buried’; in such cases, a model with a suitably high ingress protection rating would be a logical choice. The Endress + Hauser IP68 specified flowmeter models come with the sensor cable fitted, terminated and potted at the sensor making it possible to pressure test the housing and the cable termination for possible water ingress. This setup guards against possible sensor failure during operation that may be caused by insufficient potting or incorrect cable termination. Apart from saving installation time, this also leads to more reliable operation.

Accuracy at low-flow velocities

Some older or poorly designed electromagnetic flowmeters have a limited turndown. As a result, accuracy suffers at low flow velocities which can occur during off-peak periods. Modern flowmeters, such as the Promag family, have digital signal processing and the amplifier gain adapts automatically as the flow velocity varies. This leads to more precise measurements since accuracies can be as high as 1 per cent of the measured value at velocities lower than 2 cm/s.

Earthing

All magmeters require a solid connection between the measured fluid and ground. The traditional engineering solution was to install earth rings between the flanges of the meter and the pipework. Apart from being unwieldy, complicated and costly, this procedure significantly increases the risk of application problems due to inefficient earthing. Twenty years back, Endress + Hauser devised a cost-saving innovation – the reference electrode1– which removes the requirement for earth rings. Ever since then, this has been standard on their magmeters. The reference electrode not only decreases costs, but increases reliability and makes for easier installation.

Empty pipe detection

Magmeters work on the assumption that the pipe is full of liquid, meaning that a pipe that is not full can negatively affect their performance. Some manufacturers use the measuring electrodes mounted on the sides of the measuring tube for empty pipe detection. With this design, the pipe must be half empty before an alarm is raised, resulting in the possibility of large measuring errors. Endress + Hauser flowmeters have an additional empty pipe detection electrode fitted at the top of the measuring tube. An alarm will be raised if this electrode is even momentarily not covered by liquid, so that the user can manage the behaviour of the outputs and avoid excessive measuring errors. This feature can be used, for example, to monitor the correct functioning of bleed valves.

Service tools

Post-installation service and measurement verification is of course important and manufacturers offer a host of services and tools to check the transmitter and sensor. Endress + Hauser, for one, offers a fully automated, user customisable test cycle to verify the performance of any Promag magmeter onsite. The results are stored in FieldCheck, a robust, battery-powered field device, and can be supplied to the user for their own use. In the workshop the stored data can be downloaded to a PC and verification certificates printed to satisfy ISO 9000-or similar-quality requirements. All tolerances of the tested variables can be predetermined by the user. The comprehensive report shows a brief performance summary as well as the detailed deviation and tolerance violations.

Increasingly stringent environmental, health and safety legislation requires high quality water in our taps and non-polluting wastewater that is adequately treated prior to discharge. The selection of a magmeter model is therefore crucial to the successful operation of any water or wastewater treatment plant.