I think I know the answer(!) but there just might be something out there which I'm not aware of.
I'm trying to adjust the automatic by-pass valve on my heating system so that it opens when the pump is running with both zone valves closed (i.e. in pump over-run conditions) but stays shut when water is circulating normally.
Problem is that it's very difficult to tell when it's open. It's a Pegler USV 16/22-PE which has a knob to turn to set the opening pressure, but no external indication of what the innards are doing. It's in the airing cupboard, close to the pump and zone valves, and a long way from the boiler. Listening to its outlet pipe with a stethoscope or feeling how hot the pipe gets are both inconclusive because sound and heat are transmitted from pipe to pipe even when there's no flow.
What I really need is something akin to a clamp-on ammeter which will measure the flow in the by-pass circuit without having to be plumbed in. Anyone know if anything like this exists - at consumer-level prices, of course!?