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I've recently bought a single-ring induction hob (portable cooktop).* I rather like it, but I noticed that, in common with many induction hobs, it has both power levels and a temperature control mode. It's just a cheap thing and only has 20°C steps, but setting 100°C seems to maintain a good simmer with a couple of different diameters and at a range of depths.

But it's a glass top, with no temperature sensor visible. The glass itself doesn't get very hot at all of course, only by conduction from the pan. Its temperature would also change too slowly given the rate the heating appears (from the bubbles) to pulse on and off.

So what's being measured? It feels like the pan, but in that case is there an infrared thermometer looking through the glass? Normal glass is opaque to thermal IR, after all.

* I've got new solar panels, so electricity is free, unlike gas. Eventually I'll get paid for the power I generate and don't use, but not yet.

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    Unlikely to be "normal glass" in that application. Somewhere I've seen a picture of some induction cooktop in pieces and there was a thermal sensor basically in the middle of the coil, but I don't know exactly what mode of sensing it was using, and there might well be different schemes in use by different manufacturers, anyway.
    – Ecnerwal
    Commented May 15 at 19:43
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    I've wondered this myself, hopefully you get an answer.
    – FuzzyChef
    Commented May 15 at 19:50
  • @Ecnerwal I'm pretty certain it's not normal glass. And given the thickness I doubt you could do IR thermometry (even really imprecise) through borosilicate, which would be a more realistic option. Infrared spectroscopy is pretty much what I do for a living. IR thermometers can be really cheap, so I suspect that's what they use. I paid £6 for one including all the electronics and display and the actual sensor is around £1 ($1US/€1) in small industrial quanitities.
    – Chris H
    Commented May 16 at 7:00
  • I'm itching to write a scathing response here, but I'll hold back, because don't actually know what it's measuring, and even if this is measuring something, as opposed to using a badly calculated heuristic. In any case, this "temperature" display doesn't have anything to do with any temperature in your pan, and I have found it useless for cooking. It doesn't even keep a constant temperature in the pan during a single cooking. So, in practice, you can simply forget it.
    – rumtscho ♦
    Commented May 16 at 8:09
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    I've used an expensive model where I'd say that the temperature measurement was low, because we were doing a lot of simmering, and I found I needed to set it to 85, not 100. So I've wondered the same for a while.
    – FuzzyChef
    Commented May 16 at 18:25

1 Answer 1

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They use a thermocouple to measure the temperature, a join of two different types of metal wire that generates current when heated (when exposed to a temperature differential i think but it's been decades since i studied physics). The current is what gets directly measured. for an induction cooktop i'm assuming that its measuring the base of the cooking vessel not it's contents. I don't know how accurately this translates to cooking temperature of the contents but the more expensive models claim really precise control.

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  • Welcome to SA! We appreciate your taking a try at this question, but if you look at the question, your answer doesn't quite fit it. The OP isn't asking how temperature probes work, they're asking precisely what is measured.
    – FuzzyChef
    Commented May 21 at 23:46
  • @FuzzyChef my bad, and you're right, I got side-tracked by the mention of the infra-red thermometers in the post and discussion in the comments below it. Should I move it to the comments below the post as I think it fits the discussion there? I'm unsure of the etiquette. Thanks
    – user13716
    Commented May 22 at 0:03
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    I'm sure this can't be right. The glass top of the cooking surface is smooth, so a thermocouple would have to measure the back of the glass. This is insulated from the pan (glass is a poor thermal conductor) so thermally further from the pan base than the contents. It's possible some use a dipping probe - I'm familiar with the concept from lab hotplates. Whether that's actually a thermocouple or a thermistor is another question but not one for here.
    – Chris H
    Commented May 22 at 5:59

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