Electromagnetic flow meters: six tips to picking the right one
INSTRUMENTATION is a critical component in water and wastewater treatment plants since the measurement and control of their various processes determine the quality of water we drink, recycle or discharge into the sea. One such critical instrument is the electromagnetic flowmeter (magmeter), which is commonly used to measure instantaneous and cumulative water flow.
Given their widespread use, there are over 15 manufacturers offering a variety of models. Consequently, the technology and features bundled into magmeters have evolved in recent years.
Choosing a good magmeter, such as the Promag from Endress+Hauser , requires a clear appreciation of its important characteristics and an understanding of the qualities and features available. The following six key attributes will help purchasers and engineers select the best magmeter for the job.
1. Transmitter housing
INSTRUMENTATION installed at water treatment plants, especially along the coast, often faces extremely harsh environmental conditions. The type of magmeter housing is therefore crucial. Housing materials range from hardened plastic to metal. Plastic however has a tendency to crack in harsh conditions, unlike diecast powder-coated aluminium housings. These IP67 rated housings, which come as standard on E+H’s Promag range, protect against dust and moisture.
Also, ensure that the terminal compartments are physically separated from the electronics to avoid exposing the PCBs to moisture and damage during termination. Commissioning is also more secure where captive screws on the transmitter prevent them from getting lost.
2. Demanding environments
IN some treatment plants, magmeters are installed in sumps and “buried”. In such cases, magmeters with a suitably high ingress protection rating are a logical choice. Such models should come with the sensor cable fitted, terminated and potted at the sensor, making it possible to pressure test the housing and cable termination for possible water ingress. This setup guards against possible sensor failure during operation, which may be caused by insufficient potting or incorrect cable termination. Apart from saving installation time, this setup also leads to more reliable operation.
3. Accuracy at low-flow velocities
SOME older or poorly designed electromagnetic flow meters have a limited turndown. As a result, accuracy suffers at low flow velocities, which can occur during off-peak periods. Modern flow meters have digital signal processing, and the amplifier gain adapts automatically as flow velocity varies. This leads to more precise measurements, since accuracies can be as high as one percent of the measured value at velocities lower than 2 cm/s.
4. Earthing
ALL magmeters require a solid connection between the measured fluid and ground. The traditional engineering solution was to install earth rings between the meter’s flanges 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 ago, E+H devised a cost-saving innovation - the reference electrode - which removes the need for earth rings. This electrode not only saves cost but increases reliability and simplifies installation.
5. Empty pipe detection
MAGMETERS work on the assumption that the pipe is full of liquid; hence, a pipe that is not full can negatively affect meter performance.
Some manufacturers use 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.
E+H flow meters have an additional empty pipe detection electrode fitted at the top of the measuring tube. An alarm is raised if this electrode, even momentarily, is not covered by liquid. The user can then manage the behaviour of the outputs to avoid excessive measuring errors. This feature can be used, for example, to monitor the correct functioning of bleed valves.
6. Service tools
POST-installation service and measurement verification are important. Manufacturers offer a host of services and tools to check transmitters and sensors. For example, E+H offers a fully automated, user customisable test cycle to verify the performance of Promag magmeters onsite. The results are stored in FieldCheck, a battery-powered field device, and can be supplied to the user. 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.
* Commentary by Endress+Hauser Australia
12-Sep-2005