Not sure what the capacity is at upper fitting, I only used it as a boil kettle. It will be awhile before I can check myself if you wrote the folks at more beer they may be able to give you an answer faster. If I had to guess Id say around 5 gallons. I have noticed with mashing in other kettles a one point temp (if thats what your getting at) reading isnt an accurate assesment of the entire vessle temp it varies a degree or three around the kettle I use a lab thermometer in conjunction with the pot thermo to get an average reading. As for the dip tube question the 1/2" female threads are available from inside the kettle to rig a dip tube.