Temperature Probe placement on brew kettle

Another issue with temperature probe height; if you are using your kettle to heat your mash water, placing the probe too high will prevent you from doing thick mashes (where the volume of water to grain ratio is low) as the probe will be above the water line.

For example, if you are brewing a mild, with a boil SG around 1.030 and you want a mash thickness of 1 qt / pound, you will only have 2.125 gallons of water for your mash. In my 8 gallon 13 1/2 inch diameter kettle, that's only about 3 1/2 inches, well below the probe.

If you are too close to the bottom you'll pick up more heat from the heat source itself (burner or element if electric) than as a measurement of the wort. You want to be about 1/2 or a third of the way up from the bottom to somewhat get away from that effect. The longer the probe is helps with that issue too, however too long of a probe will interfere with hops sacks or an immersion chiller if you use those.

You definitely want to be just under that lowest rib on a keggle. Mine is above it and it never worked well for 5 gallon batches. I use that keggle now as a mash tun and I tend to use a higher water to grain ratio than most inorder to get it into the mash. If you can get a metal worker to professionally flatten a small square of that first rib, then weld a coupling in at that spot I think that would be pretty ideal.

Ideally you want your thermometer in the midline of your average batch. In a keggle you want it slightly below that first rib if you ever do 5gal batches.

Biggest thing is to make sure it doesn't get in the way of your immersion chiller if you use one.

They don't have much use on a boil kettle other than for cool down, hops rests, maybe estimating when boil will start etc. A turkey fryer style thermometer works just fine for me.

Why do you need a thermometer in a brew kettle? Water boils at 212°F (100°C) at sea level everywhere on the planet.