500 gallon propane tank - what's inside?

If you're looking for the temperature at the top of the tank then come off the top of the tank with a tee rather than an elbow and put your well down into the tee. Hot water will rise up into the tee and will give you top of tank temperature.

Sounds like tube to bottom would be used for filling. The port should work well enough for a temperature well if you want to go that route, might be a little slow to react.

Or just tape all your sensors to the outside of the tank under the insulation (with a plan for servicing if it comes to that).

A lot of rust and debris. I had one in a butane tank that I'm using.

A bad smell.

On the ports, I wouldn't do any more cutting/welding on the tank than absolutely necessary. Surface mounted probes & sensors work very well without the possibility of compromising the tank.

Medman said:

I could also use a dremel or other small cutting tool to cut the tube at the top just below the threads and then drop the tube to the bottom of the tank. It would save me from having to have anything welded.

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there should be a level gauge on the tank, held in place with 4 machine screws. You can adapt into that connection and add a temperature well with multiple sensor wells.

Here is one I built to measure four levels in a tank.

Why not just tape a sensor to the tank ?

I found this inside one of mine.

Nice find!