If you check Richard Henry'sbook, "Controls in Black and White Photogaphy" (he was a research chemist with a PhD. and an M.D.) he says almost all thermometers are inaccurate. It takes avery expensive, certifiied one to be accurate. But all you need is one that is reliable (i.e. consistent) then work out your processing times according to your inaccurate thermometer. When the negative is right, the time and temp. are right!
Tp paraphrase O.J's lawyer.