Thermal sensing and transmitting wirelessly seems a mostly solved problem.
You can buy a gizmo to monitor a barbecue grill for example. A bit of
google-fu
produced this: https://www.bbqguys.com/bbq-accessories/bbq-grilling-
tools/thermometers
If you insist on "rolling your own", your favorite flavor of
microcontroller, any of several types
of sensors, and wireless module. A bit of googling will turn up many
options here.

It would seem the more troublesome issue is the encapsulating the device,
with a power source, in something considered food-safe, while maintaining
chemical
compatibility with your liquid. If I was trying to hack together a quick
and dirty option,
I'd probably look to encase it in a bit of copper tubing with one end
closed off.

If I was solving this professionally (i.e time=3Dmoney, processes other
people
can follow, available repair parts, etc), I'd spend some time on Omega
Engineering's
website (www.omega.com) They have a _lot_ of options and will do customs
for
quite reasonable prices, but this page has some that look "about right"
https://www.omega.com/pptst/TJ36-ICIN.html (probe 1/4" dia, 24" long, type
K,
stainless steel case - about $50 US)
They have some wireless solutions as well on this page:
https://www.omega.com/pptst/UWTC-NB9.html


Hope that helps
-Denny




On Thu, Mar 22, 2018 at 5:15 PM, K S <kresho@gmail.com> wrote:

> > I would look at using a PRT stuck to the outside of the bowl, you could
> even have a bunch of them around the outside
>
> The bowl itself is double skinned and oil filled with heating elements at
> the bottom. It does have a sensor and a PID temperature controller, and
> maintains it's set temperature quite well. To test I painted a matt black
> strip on the outside and checked with an IR sensor and FLIR - there is a
> small temp profile across it's surface but it's quite good. But the
> contents inside take time to heat and can be at very different temperatur=
es
> to the bowl itself/
>
> For example, if I fill the bowl with 20l of water I need to set the bowl =
to
> well over 130degC for the water to even boil within an hour even though t=
he
> bowl get' s there in 15 minutes. The time taken for heat exchange from th=
e
> bowl to the contents is significant and varies according to their mass,
> volume (contact with the bowl?) and type, but since the contents are bein=
g
> stirred they are quite uniform in temperature. Now if I want something to
> be determined as "cooked" at 110degC there is no way (that I know of)
> without actually measuring the actual contents.
>
> Of course I could measure the temperature of the submerged inner surface =
of
> the bowl which should be the same as the contents touching them, but that
> is no easier than the contents themselves.
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