therm calibration doesn't make any sense

In ice water it reads 39f, so it seems to be showing temps as higher than they really are.

In rolling boiling water it shows 205 f. Lower than it really is.

What the heck is going on?

Bi- metal therm?? Is it a cheap one? Even the expensive ones can be off, but the cheap ones can make you pull your hair out. If it's new I'd return it.

Someone else can probably explain the science, I only know the real world frustration. Best bet is a quality digital IMO>

It's a 10 dollar tru-temp from Target.

Update: I retested it in boiling water and I got 212. However, ice cubes in water still reads 40f. I guess I'll try it again tomorrow with crushed ice.

I use this exact thermometor for mashing. It is the tru-temp and I got it from target lol. It's got the "rounded L-shaped" probe. I've tested it to death like you have and I can also confirm that it's off by different amounts at different spots.

I only use it to measure about a 10 degree temperature range (149-158 mashing temp range). In this range it reads 2 degrees hot for me. I just know this after using it enough times. The first 10 brews I used it on I used both the tru-temp and a floating thermometer to compare till I was comforatble enough to just use the true temp.

I've used it for about 15-20 brews and still use it. I like it bc it has the probe and cord and I can keep my tun closed and still know what's going on inside without opening my lid. I just know that it's 2 degrees hot so if it says I'm mashing at 157 it's really 155 etc.''

I do use the floating thermometor for everything else (measuring strike temp, heating sparge water, etc.). The tru-temp is strictly for seeing if I hit my mash temp and monitoring the temp over the course of the mash and only because it has the probe and is convenient.

Stick your floater or other thermometer in there with it till you get a feel for it.

Hmm, I think I'll get another one just to be sure. Last time I brewed I had very low efficiency (expected 1.059, got 1.050) and I suspect my mash wasn't right... I'm brewing again tomorrow and I want better results this time.