Flow sensoring ...

[Jethro_uk]

With our combi DHW still playing up, I finally pulled the flow sensor apart.

It's a plastic impeller with a ring magnet set into a groove. The one I extracted had an incomplete (I am guessing broken from looking at the photos on eBay ?) part circle of plastic around the magnet.

So the idea is as the impeller spins so does the magnet and some sort of hall-effect goes on in the housing that is translated into pulses for the control board. (Based on the YouTube videos where a helpful chap insists you need a "Hertz" multimeter ...).

All of which seems an ingenious and over complicated way to solve the problem with as few moving parts as possible.

No idea WTF the magnet wasn't embedded into the impeller. I guess they have to sell spares.

But I keep thinking there must be an easier way to determine flow ? Surely if you have a venturi, or series of venturi in a housing of known dimensions, then you could determine flow from pitch of water passing through. Even if you had to shape a baffle to give it a hand to whoosh ? Combine that with temperature and you'd have a pretty decent sensor that would be more reliable, surely ?

Returning to the failed sensor in hand I wonder where that tiny fragment of plastic ended up ? It wasn't anywhere in the housing.

[Steve Walker]

There are many ways of determining flow, but a turbine and hall-effect sensor is simple, reliable, cheap and can be used to detect flow/no-flow or variable flow.

A simple flap operated switch will prove flow, but is not too reliable and only gives flow/no-flow.

A spring and free moving venturi will allow the venturi to move against the spring, proportionally to flow and can trigger a hall-effect or reed switch for flow/no-flow or a chain of sensors for variable flow rate.

For a fixed venturi or the cruder orifice plate, you'd use a differential pressure transmitter or switch, but such transmitters are more costly and the switches less reliable - especially with low flows and little differential pressure.

Other methods are more complex and expensive still (and overkill for the application) - thermal mass flow (two temperature elements, one electrically heated and a comparison of temperatures); vortex shedding (an obstruction in the pipe that causes vortices, which naturally shed alternately from side to side, causing the obstruction to vibrate, varying frequency with the flow); Coriolis effect mass flow (two parallel tubes, formed into an Omega or other shape, mechanically excited and the flow calculated from the phase shift between them.

There are other ways too, but open channel flow sensing, gear flow devices and the like are not relevant.

Hopefully sitting out of the way, at the bottom of a radiator, where the flow is low enough not to disturb it. However sod's law dictates that it'll migrate to jamming in a motorized valve, the pump or the replacement turbine, at some inopportune moment.

[The Natural Philosopher]

well what is 'easier'?

if the fluid is conductive, shove a magnet outside the pipe and put some electrodes inside.

Could use a pitot tube and pressure sensor...

but rotating windmills or anemometer cups are well known to be very accurate, and a rotating turbine and magnet and a coil outside is as good as.

[Brian]

We had the flow sensor fail on our CH a month or so back.

We have a service contract and the technician turned up within a few hours. He fixed it temporarily by applying a magnet to the flow sensor - just for a moment. The company replaced the sensor about 10 days later, having ordered one.

I looked at the sensor, it seemed to be a simple flap in the pipe which the flow moved. I assume our old one had got stuck an the magnet helped free it.

[Paul]

I used one of these for an oceanography project.

"How Magnetic Flowmeters Work

Magnetic flowmeters use Faraday?s Law of Electromagnetic Induction to determine the flow of liquid in a pipe. In a magnetic flowmeter, a magnetic field is generated and channeled into the liquid flowing through the pipe. Following Faraday?s Law, flow of a conductive liquid through the magnetic field will cause a voltage signal to be sensed by electrodes located on the flow tube walls.

When the fluid moves faster, more voltage is generated. Faraday?s Law states that the voltage generated is proportional to the movement of the flowing liquid. The electronic transmitter processes the voltage signal to determine liquid flow.

In contrast with many other flowmeter technologies, magnetic flowmeter technology produces signals that are linear with flow. As such, the turndown associated with magnetic flowmeters can approach 20:1 or better without sacrificing accuracy. They represent about 23% of all flowmeters sold.

Plusses and Minuses

Mags are intermediate in accuracy therefore not commonly used for commodity transfer except for some special cases where the fluid is not expensive like water. Can be adapted for sanitary uses. They have large line sizes available. No pressure drop induced. Dirty liquids and even slurries OK. Very reliable.

On the other hand, don?t work on nonconductive fluids such as oils.

Steam or gas flows don?t register.

Electrodes can become coated. <=== "

That impeller scheme sounds better already.

The signal level on the magnetic flowmeter is pretty small, and I would guess the electronics on the probe use synchronous detection for the sensed voltage. Only the signal at the frequency the probe operates at, is amplified and passed through, so that stray signals (electrochemistry) does not affect the result. If the mag field is a sine wave, the electrode signal should have a strong sine wave to it as well.

While working on the project, I had no instructions on cleaning the probe.

As sensors go, the probe is also a power-pig. I think the probe used more power than the microprocessor board taking the readings. You need sufficient mag field, to get a measurable signal on the electrodes.

Paul

[Clive Arthur]

<snip>

If it's a ring magnet, then it likely has multiple poles so giving increased resolution compared to a single magnet.