Oh No, my high Tech thermometer stopped working

I got this very high tech brass thermometer from a boiler in the basement of the Plaza hotel about 20 years ago, but it needs a little calibration to be good as new. It has been on my tank for about that long and I can read it from across the street. I don't have one of those little Sissy thermemeters



You can see it on the wall here near my high tech skimmer