thermometer calibration

My good buddy and fellow homebrewer Curt loaned me his very precise mercury thermometer a while back, and I attempted to calibrate the 5 Blichmann thermometers using water in a pot on the stove.  This was no problem with the water settled at room temperature and comparing the mercury thermometer to the Blichmanns. four of the five were off by either 1F or .5F, and I was able to dial them in nicely at 69F.  However, when I heated the water to boiling, the Blichmanns were way off.  I can't remember exactly how much off they were, but  I seem to recall as much as 5 or maybe 7 degrees.  I tried to let the water cool down to 152F (a midpoint temperature between the ranges favored by alpha amylase enzymes and beta amylase enzymes during mashing) and calibrate them  at that point, but I never could hold the water right at 152 long enough to do the calibration... too small a volume of water and no ability to fine tune the heat applied to it.

Isotemp Immersion Circulator

Later, as I was telling Curt about my experience, he said he had just the thing in his lab, and here's a pic of his assistant showing me how to set up their Fisher Scientific Isotemp Immersion Circulator.  It took about 20 minutes or so to get the constantly recirculating water temp up to 152F, but once it was there, it held the temp steady as long as I needed it.  Calibrating to 152 was really easy this way, and that has me thinking about how I can build my own do-it-yourself style Isotemp Immersion Circulator.