Thermometer woes

I have two thermometers and a digital temperature controller. I have been using an ordinary lab thermometer the entire time I've been brewing, but recently bought a dial thermometer with the intent of installing it into my MLT, so I decided to check the two against each other (and the controller) to verify that they at least corroborate each others' measurements.

In a cup of ice water, with the tips of all three located very close to each other, I got:

Lab: Between 34 and 35 F
Dial: Between 33 and 34 F
Controller: 33.8 F

I didn't want to put the controller probe into boiling water, but I tested the other two, and got:

Lab: 212 F
Dial: 209 F

Okay, so suppose the ice water truly was 34 F, and both lab and dial thermometers gave a correct reading within 0.5 degrees. And suppose the boiling water truly was 212 F, the lab thermometer measured it correctly within less than 0.5 degrees, and the dial thermometer read 3 degrees low. If both instruments are linear from freezing to boiling, that should mean that the lab thermometer is trustworthy for all temperatures between freezing and boiling, and the dial thermometer reads something like 1 degree low for every 60 F over 32.

Now, we get to the confusing part.

I stuck the lab and dial thermometers into my pre-heating mash tun, stirred it up thoroughly, and gave them a minute to reach their verdicts. I was expecting to read about 2 degrees lower on the dial than the lab. I didn't write down the exact numbers, but I got about 174 on the lab and 166 on the dial. I decided to go with the lab for deciding when to add my grain, and mashed-in when the temperature dropped to about 165. The mash settled out to 152-154 according to the lab thermometer, and the dial continued to read about 8 degrees lower, in the mid-140s. This is with both thermometer tips submerged to the same depth and adjacent to each other, in a thoroughly-stirred mash.

So, clearly at least one of these two thermometers has a non-linear error between 32 and 212. They match at freezing, and are close at boiling, but somehow the discrepancy widens drastically in the ~140-180 range. I've been using that lab thermometer for mashing for a long time, and I usually have gotten 70-80% efficiency, so I'm inclined to believe it's not reading 8 degrees high. The dial thermometer has a nut you can rotate to calibrate it, but that just offsets the whole scale and is pretty worthless if the thermometer has a non-linear error.

Any ideas, thoughts, suggestions, comments, recommendations?

Buy a Thermapen. My .02.

Best investment I have made.

If you ar looking for advice on how to fix your thermo that is reading wrong, then I say trash it, regardless of the calibration nut.

IMO, the glass lab thermometers are the most reliable simply because there are no moving parts to go out of whack. Providing that it is of decent quality and properly calibrated when manufactured, it should stay that way. OTOH, the bi-metal dial thermometers have a number of moving parts and I've found them to be dicey as to reliability. So much so that I no longer use them at all when brewing. Your controller probe is likely a thermistor. These are generally very reliable and accurate and should be consistent throughout their specified operating temperature range. My advice would be to forget the dial thermometers altogether as I did. Wish I had done it much sooner.

I'm gonna take the dial thermometer back to the LHBS and try to get a refund of store credit, which I'm pretty sure they'll be willing to do.

I just got a Thermoworks Themapen for xmas. It has a certificate with it, that says it's good to a much higher degree of accuracy than I care about... my other thermometers did not have that, and I always wondered what the temp was... I think I'll trust that little certificate from now on. They're like $100 though, but I think in this case you get what you pay for. http://www.thermoworks.com/products/thermapen/splashproof_thermapen.html

It is super fast too. Just walking around the house hitting hot and cool air pockets it changes quickly. Pretty amazing.