Water flow rate pulses into KNX binary inputs?

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I need a bit of advice on how to get water flow rates/total-use into openhab.

These relatively cheap hall-effect sensors that look like they mount inline with the water inlet.

I found a similar project on github using an auduino. That said, I’d prefer to avoid the Auduino route if I already have a knx binary input.

It looks like they put our a rising and falling pulse. My understanding is that this could be collected by a KNX binary input for floating contacts (there are also 24v and 240v options). If I can can count the pulses in something like the MDT BE-04000.01 then I can use the KNX binding to report water use.

The flow sensor (claims to) put out the following signal. Does this count as a “floating contact” in KNX-speak?

Can anyone with experience of KNX binary inputs comment on this working?


I use a couple of these in various places.

They give out (very roughly) 420 0v pulses for every 1 litre that passes through them.

I use a pulse input of a Velbus VMB7IN, but I assume there must be KNX pulse input device???

420 pulses/liter! Nice to know @MDAR. That said, I’m not sure that putting 420 pulses (per flowmeter) onto the knx bus is a good idea…

Indidentally the binary input modules that look like they would make sense are these two:


Any ideas on which makes the most sense given the 0v/5v/signal wiring?


Unfortunately I can’t help with that bit, I’m strictly a Velbus man.

But what I can tell you is that the Velbus module I’m referring to has a pulse counter onboard, so it only submits to the bus a counter value, rather than an update for every pulse.

I think you’re right, ~420 pulses x >11 litres, every minute anyone is using water will bring your KNX bus to its knees.

UK water delivery speed is supposed to be around 24 litres per minute, so that’s one heck of a lot of pulses.

However, the 080000.1 diagram looks like a 0v switch input device to me :wink:

I think you are right about needing something infront: the binary input is rated for 30ms S0 pulses. (which look identical to the output from the hall sensor). Assuming household flow of ±60l/m * 400 pulses per liter = we get about 400 pulses per second. Which will never fit into the 30ms pulse specification. Looks like the future is something like this: https://www.arcus-eds.de/impuls_2.html

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Look spot on to me :slight_smile:

Who knows what you could use the second input for :slight_smile:

My home is setup with…

Electricity pulse - simple LDR over the LED on the meter

Cold water flow rate (to whole house)

Cold water flow into the on-demand water heater (house cold - on-demand flow = cold only consumption)

Gas flow from a reed switch style pulse output from a secondary gas meter. (I’m told there are gas meters out there now with proper 0v pulse outputs, just not in the UK)


60 litre per minute flow into your property!!! Blimey, do you live at the bottom of a water fall?

That’s exactly the plan!

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