> I would consider clipping a thermocouple probe to the top edge

Unfortunately the paddle arrangement makes any penetration into the
contents impossible(?) with a fixed probe arrangement. The liquid level can
vary so the probe either needs to go all the way deep or be easily variable
between batches.

>  around 200C you will almost certainly need a thermocouple as I don't
think PRT probes for that temperature are too readily available

That was sobering, but ebay shows plenty that are rated to 400degC. A
thermocouple is fine otherwise but considering I'm going to tinker with the
slip ring first it would have been a deal breaker.
--=20
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