Doing a bit more digging about the current clamp tells me something I realised was obvious in retrospect – the clamp needs to be around a live wire on it’s own. Clamping it over the multi-core mains cable won’t work because the EM field from the live wire and the return path through the neutral wire cancels each other out.

That’s why the clamp last night (and today, with the proper value components around it) shows no change whether the heater is on or not. OK. There’s some high-end sensors that can sense current in multi-conductor cables but the only accessible one I can see is this Modern Device one (https://moderndevice.com/product/current-sensor/). It looks like it’ll work great and isn’t expensive ($14), but it’s not a nice tidy clamp.

Options:

1. Build a device that can be placed inline with the power cable that will allow the sensor access to only one of the wires in the cable. This could just be a customised power cable, quite easy to make but liable to raise an eyebrow from the estates people at work. This means having to understand the difference between the heater running and the hotplates.
2. Mount the current clamp inside the coffee machine, over the wire that goes to the heater. This is non-invasive-ish, but I’m a little nervous about proximity to the heater and also having trailing wires hanging out of the coffee machine. This would give a nice unambiguous and accurate signal of when the heater is running though.
3. Tap or sense the wire that the float switch is on. The float switch is in the water reservoir and is what triggers the heater to run. This may well be low voltage, and DC so I think a current sense clamp won’t make anything of it. The cable is terminated with plain plugs and sockets that look fairly standard so I could just make a special cable that has a sensor on it to pull a pin high or low when it’s closed or opened.
   I still don’t like the idea of having cables trailing out of the coffee machine. Perhaps I can make something discrete in the base – there is plenty of space, and routing for a cable out the bottom (the cable for the hotplate uses it). Maybe even some pogo pins on the bottom that would integrate with a little board mounted on the scales that the whole machine sits on.

I think option 3 is my favourite. It’s likely to avoid working with high voltage or current and that cable assemble looks easy to do something with in a reversible way. I need to see how to use a pin to sense the open or closedness of that switch. I feel like that should be easy.

Some machine parts

Docs for parts: https://doczz.pl/doc/430837/bravilor-bonamat here’s an excerpt that mentions the parts for the Novo:

• PCB is 402650 or 6.101.153.000 – keypad PCB L 82mm W 65mm countries GB/IRL buttons: 2
• Float switch is 347388 or 6.101.071.000 – magnetic switch NO connection plug-in connection cable length 160mm L 45mm mounting ø 6mm – what’s that connector on the end?
• Heater is 417420 or 6.101.061.000 – flow heater 2000W 230V ø 58mm H 120mm connection male faston 6.3mm

I can’t find any description of the connector on the float switch. It looks like a KK-396 housing https://uk.rs-online.com/web/p/pcb-connector-housings/6795066/ except measuring the pins looks like the pitch is 3.6 or 3.7mm rather than 3.96mm.