A magmeter with 30% error?

How to correctly measure flow with a tee union directly on an electromagnetic flowmeter.

Inventing and making specialty firefighting vehicles is what Innovative Fire Fighting Solutions (IFFS) is strong in. They focus mainly on systems for extinguishing water and hose systems. Fire trucks all have their own requirements regarding the amount of water on board, the flow and the distance over which water must be transported. For measuring the water flow, IFFS uses magnetic flow meters from Mecon. These are known for their accuracy. So why did they have a 30% deviation?

A characteristic of fire trucks is the high flow rate of about 10m/s, which is higher than the average industrial system. This makes magmeters a good choice. But due to the incredibly compact construction of these vehicles, there is hardly any room for straight feed or discharge lengths. And that's exactly where the problem lies in this case. The flow meter is mounted in the outgoing system, and the customer had a requirement that there be connection points on both sides of the vehicle. Therefore, the flowmeter is mounted just in front of the rear bumper, followed by a tee union. That tee union disturbs the flow to the point where there is a 20% error. This sounds like a huge design error, but it is not that bad. After this was discovered, tests were done with several flow rates to determine whether the disturbance was linear over the entire range. It was and therefore such an error can be compensated for in the flow computer. The remaining 10% measurement error turned out to be a conversion error in the software of the car's PLC.

Photo: the electromagnetic flow meter and the tee.

Fire truck where the electromagnetic flow meter is installed.

Magmeters vs Ultrasonic flowmeters in measuring water

For this type of application, ultrasonic flowmeters may also be an option, due to their high turndown and full pass. But in this case, the fire engine has to be inspected and shipped to the customer as soon as possible after assembly. So there is little time to test and set up the entire system. This makes the magmeter preferable to an ultrasonic flowmeter: they are easier to install and faster to commission. In addition, ultrasonic flowmeters are more sensitive to disturbances in the flow and are less easy to compensate for. Another disadvantage of ultrasonic flowmeters in this application is also that it is a relatively expensive flowmeter for measuring water. Finally, magmeters are fairly easy to exchange for another flowmeter.

Magmeter applications

Magmeters are quite often associated with measuring electrically highly conductive liquids, but it's application isn't limited to that. Mecon's flowmeters are capable of measuring liquids with a conductivity of 0.3 μSiemens. It is also a meter that can handle well a high solid fraction in the liquid. All in all, it makes the magmeter the choice of choice for example:

Dosing of viscous additives in water
(Bio)chemistry: edible oils, biofuels, non-edible oils (even with low conductivity)
The ultra high accuracy makes magmeters very suitable for settlement measurements at transhipment companies.
Larger industrial water treatment
Dredging and sand extraction
Measuring high flow rates

Photo: placement of the flow meter under the vehicle, just before the tee and rear bumper.

Photo: field test of the entire system. In the background, the calibrated test cart, which serves as a reference.

Photo: field test of the entire system. In the background, the calibrated test car, which serves as a reference.

The role of Teesing

With these kinds of specialized applications, the component selection is very critical. And because it concerns a few pieces or small series, this product selection must be done right the first time. That is why IFFS likes to work together with our flow specialists at an early stage of the design. These are project-based systems that often have similarities with previous projects. We have been working together for a number of years and both parties focus on a long-term relationship, because Teesing builds up application-specific knowledge with each project.

5 tips for electromagnetic flow meters

Finally, a few tips. Use them if you are designing a system with a magmeter:
1. Choose the diameter such that you keep the nominal flow rate between 3-5m/s. The turndown is between 0.5-10m/s, but 3-5m/s is the optimum and gives room for outliers.
2. Straight inlet and outlet lengths make measuring a lot easier. Although flow disturbances often give a linear deviation so it is fairly easy to compensate for.
3. Grounding is very important. Make sure there is a good galvanic connection between the flowmeter and the piping.
4. Choose the lining (inside of the orifice) with care. For water, rubber is best. If chemical resistance is important: choose PTFE. For sand and dredge applications you can choose: either very soft which moves easily or very hard (ceramic lining).
5. Air entrapment is something you want to avoid, so never build in the highest point.

Straight feed length is important for the accuracy of electromagnetic flowmeters

Figure: straight feed length improved accuracy of magnetic flowmeters.

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