I had the chance to take apart an old mechanical flowmeter and I was a bit puzzled by the way the pulse output works.
In electronic flowmeters the pulse output is normally of the open collector type but this mechanical one obviously has to use a different method since the flowmeter isn't powered.
There is a sensor which has two small coils and a disc which turns in between these coils. The pulse input of the PLC has a active 24V DC supply connected to the pulse output. When disc turns there is a pulsing current flow proportional to the volume flow through the measuring device.
Can someone enlighten me on how this arrangement is working?