New Thermometer, two results

I've been relying on an analog thermometer that I picked up from the brew store around two years ago, and right now it's measuring 120˚f while my new electronic (Fisher Sci Digital) thermometer reads 112.5. After stepping up to AG, I was getting around 65% efficiencies consistently (3 batches).

That was probably the time that I decided to order a new thermometer since all of my temps were (supposedly) dead on and I've brewed on other peoples systems and always had 80-85%.

I just want to be sure, I should trust the new thermometer right?

Thanks!

Check both in boiling water. I check every new thermometer, even the expensive ones. And I re-check them every few months, too.

If your old analog one is the dial type, it may be possible to recalibrate it by twisting the nut behind the dial.

Boil a quart of water and have a quart of water at freezing (filled with Ice cubes) stirred well for 2 or 3 minutes and the ice removed. The one should be boiling (212), the other should be freezing (32) mix both in a large boil quickly and it should be about 122 degrees. (A little cooler do to heat loss to the air and bowl, but within a few degrees) If it reads high on any of those then it has issues. Quantities have to very close for this to work, and the bowl in question can not be a huge heat sink.

Remove the ice? Why? That sounds like a really bad idea. People think ice water is 32F but it isn’t. Here’s a proper ice bath:

The water/ice must be pure, as in distilled.

Use slush (crushed ice) with the water level slightly below the ice. When determining melting point remember that water is densest at 4C so the ice/water mixture must be continuously well stirred or the water at the bottom will be 4C and the rest of it somewhere between 0C and 4C (magnetic stirrer works best). A steady well-stirred temperature is 0.0C.

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There’s a simpler, but still pretty good method on Thermoworks.
http://www.thermoworks.com/learning/thermapen101_creating_an_icebath.html

The big thing is to have a lot of ice, 1/2" over the water and stir.

That is just it, if you want to hit a mid point temp to test your thermometer, you need to remove the ice, as it will lower the temp of the mix of boiling and freezing water. There is nothing that says the center of a block of ice is 32 degrees, if you put a gallon jug of water in the freezer for a week, the center of that block will be the temp of the freezer, even if it is 100 below zero. All I was suggesting was a way to test a single midpoint temp.

Ah, the midpoint. I didn’t consider that, I was thinking about the endpoints. I see what you’re saying. You’re right about the ice, it’s going to start out at the temperature of the freezer.

It’s going to be hard to get much accuracy mixing boiling and near freezing water into a third container. There’s the thermal mass of the bowl and you’re going to have to get the volumes near perfect. If you do that you might want to go with three parts hot to one part cold to get 75C (167F)

I check my thermometers against the strike water to ensure they’re in general agreement. If they’re within a degree Fahrenheit I’m happy.