"Stormin Mormon" writes:
Total submersion is the only accurate test.

Only for thermometers that are intended to be used submerged. I have a
number of glass lab thermometers (mercury and alcohol) and at least
half of them have a line on the body to mark how far they are supposed
to be submerged. The part above that is expected to be in air.

In addition, many bimetal dial thermometers are not waterproof and will
be ruined if you submerge them. The bimetal element is only in the
lower inch or so of the hollow steel shaft, and that's all that needs
to be submerged.

Then there are electronic thermometers that are not waterproof and will
be ruined even faster when submerged. In these, the measuring element
is a small thermistor bead, and it's at the end of the metal shaft.

Dave