After a broken thermometer on a recent brew day, I replaced it with a spare “Lab Grade” glass thermometer I had for just such an emergency. I started having high final gravities (1.020 vs. 1.012). It turns out this so-called lab thermometer read 32º F in an ice bath, but only 198º F in boiling water. I estimate my mash was at 162ºF vs. the target 150º. I’ve never had this problem with a lab thermometer, and used them for a quick check on my digitals. Has anyone had this inaccurate a lab thermometer?
One thing I don’t like about the glass thermometers is how long they take to read. My mash tun has a dial therm and a thermowell with a probe from a PID controller. Since they are always in contact there is no wait or very little. I also bought a “lollipop” traceable scientific therm because the PID and dial never agreed, The lollipop and the dial do so that is what I go by.
Calibrating your thermometer is a good idea. With a good freezing reading and bad boiling I would not know how to gauge how far off it may be at mash temps. A couple of other thermometers might be the fix. Even a cheap dial one from the grocery would be something to compare to.
Buy a digital thermapen and be done with it. I’ve found with thermometers, you can spend $100 over a few years, wind up with a drawer full of junk and a bunch of undermodified worts and bad information, or spend $100 once. Great for bbq as well.
You can also purchase the Thermoworks Dot with silicon coated cable for moist environments. Costs less than the Thermapen. I have read that acidic mash can damage the silicon cable with long term immersion.
I have both the Thermapen and Dot and like them. The Dot also has heat resistant cable probe for use in BBQ grill, oven, and smoker.
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