How to Choose Flowmeters

The conductivity of the water is an important factor. As a rule-of-thumb, mafglos will not work below 50μS/cm, regardless of the cost difference.

I suggest you visit a few flowmeter manufacturer websites and read up on their products. You can get tips on how to select flowmeters from some of them.

Alternatively, you can go to an online magazine like ControlGlobal. They've got lots of articles. Here's one on different flowmeter technologies.

Personally, I've never had much problems with electro-magnetic flowmeters. The one attraction I see for ultrasonics, however, is that they can come as clamp-on types. This saves on installation and, if ever, replacement. Accuracy is quite good but, of course, dependent on how it is installed.

I am just a guest, but I have the following comments:

There are two types of Electromagnetic Flowmeters, full bore (or full diameter) magmeters, and insertion magmeters. Full bore magmeters are very popular due to lack of line restrictions, very high accuracy, reliability and theoretically no calibration requirements. Their cost was prohibitive (we use rule of thumb of US $1000.00 per inch diameter), but we've seen signs of it dropping. They are less expensive in original design than retrofit, because of requirement to cut a section out of the pipe, flange or couple it, and install the full bore magmeter.

Insertion mags require a tap (nominally 1.5 or 2-inch) on the pipe through which the magmeter portion is inserted. The advantages of retrofit installation are apparent - also one shaft length can serve many diameters, providing you have head space above the unit. They are inherently cheaper after maybe 6 " in diameter, but are less accurate, and also sensitive to buildup on the inside of the pipe.

Ultrasonics can be mounted as a full-bore configuration, with the same considerations as a full bore mag. Ultrasonics however have become useful as strap-on flowmeters, which do not require intrusion of the pipe. This portability makes them popular for short-duration studies. The tradeoff is accuracy, tolerance to changes in the flow (entrainment, for example), and sensitivity to buildup inside of the pipe.

My personal best solution on a fully-filled, 200 mm line would be a full-bore magmeter during the original design. The cost is now not much higher, if at all, than a mechanical meter, and the life span may be a lot longer.

<...ultrasonics also have problems with "clean" fluids...>

Oh yes, quite. One ultrasonic added to the outside of a 65mm Ø PVDF pipeline carrying ultrapure water (18.2MΩ.cm @ 20degC) suffered a reading error from a static electricity effect introduced by the speed of the pump rotor a few tens of diameters upstream.....

The info at hand differs between manufacturers and models. You may have to contact suppliers and have them list what they have.

Magnetic is stated as robust and easy with minimal head loss. Some claim pipe empty and direction sensors.

Disadvantages listed for some

max cable length of 100m between converter and tube.

pipe must be full.

conductivity 5 micro S/cm

The Doppler seem to be capable of higher flows.

disadvantages listed on some.

Physical impurities 100 micron max.

Rust must be removed.

Placing it level is critical.

Cannot be placed in direct sunlight.

In 1975 we had a magnetic flowmeter on a site next to a electric rail line and it was influenced. Maybe technology did improve since then.

Go through www.forbesmarshall.com and find out their office in Baroda. Contact them, they can answere you.

Regarding calibration in next post: Magmeters also do need calibration. No flow meter can be used without calibration.