The old style thermometer in the vent can be calibrated. I have a vise for my drill press with a v channel machined in it. I place the thermo in boiling water for the initial test, then clamp the stem in the groove firmly, then twist the dial by hand. Then retest in boiling water.
Just be sure not to squeeze it to tight in the vise. If you do it will not work at all. Just tight enough not to spin.
The newer style I am unsure if you can.
But now I want to try it [emoji848]
I just remembered that I have one from a newer Genesis that I added to my old OTG black. It's in the attic in storage. I may need to dig it out and try.