What are some reasonably reliable ways of detecting that a water pump (electrical motor based) is "dry running", i.e. no water is passing through the pump either because there is no water available at the inlet, or due to "air-block" ?
I'd prefer mechanisms that are least intrusive, i.e. ideally – do not require re-plumbing. However, if all practical mechanisms involves some plumbing work, then it might as well.
An indication regarding possible cost of sensors, installation skill required, would be very useful.