The fickle nature of interference might mean that’s not significant.
OK, leaving the 1 kΩ in place, can you connect a capacitor in series between pin 6 & GND, and a series resistor in the wire to the switch - like dBC’s circuit. The capacitor wants to be quite large - 470 nF up to I’d guess 4.7 µF - start small, and the series resistor 150 Ω or larger (to limit the current into the switch when it closes).
Is the body of the meter metal? If it is, is there a connection between the body and the GND terminal at the meter?
So no discernible difference in the number seen?
No discernible difference.
Is the body of the meter metal? If it is, is there a connection between the body and the GND terminal at the meter?
The body of the mechanical flow meter section is metal. This (along with the temp sensors) is wired to a detachable plastic “integrator” from which the pulse and GND wires emerge. There are no external connections and both the flow meter and the integrator wiring connections are very well sealed with resin or the like. I imagine I would need to sever the wiring and/or completely dismantle everything to determine any body connection.
I will try and dig out the capacitor and series resistor you suggest and connect tomorrow. Not even sure I have the required sizes.
I wouldn’t suggest that - but can you put a multimeter on the Ohms range across GND - the metal body? My thinking is - the body might be acting as an aerial and picking up the interference.
Don’t worry about that too much - as near as you have should show whether it makes a difference or not. The resistor value must not be too small, as you must limit the current out of the capacitor as the transistor switch turns on to 25 mA.
can you put a multimeter on the Ohms range across GND - the metal body?
0 Ohms.
OK, leaving the 1 kΩ in place, can you connect a capacitor in series between pin 6 & GND, and a series resistor in the wire to the switch - like dBC’s circuit. The capacitor wants to be quite large - 470 nF up to I’d guess 4.7 µF - start small, and the series resistor 150 Ω or larger (to limit the current into the switch when it closes).
Did eventually get round to trying this configuration but with a 100nF capacitor and 220 Ω resistor - nearest values that I had to hand. No apparent difference.
In the absence of a quick fix I think I should put this particular problem on the back burner. Less of a capitulation than a need to get on with other things! I have the TH pulse working fine and I need to make this permanent and uninterrupted with the heating season imminent (for us anyway). Collecting this data is essential for a wider, future heating mix project.
Although the current meter needs to be located where it is I think any further meters I install can be positioned to give shorter (and shielded) cable runs with much better separation from power circuits as the pipework is “accessible” on the other side of the “power centre”. Whether this will remove the problem of spurious pulses is , of course, questionable but I can always revisit the issue at that time if need be.
Many thanks for the help with this issue. Even if no solution at the very least I have a little more understanding of the problem and a bit more practical experience.