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Which thermometer do I trust?

My brew sculpture has a Ranco pid controller with a temp sensor and thermowell in the MT. The MT also has a Fermentap dial thermometer. The two readings almost never match. Last batch the dial read 149, about what I was shooting for. The pid read 155, I would be OK with that but a little higher than I wanted. My experience with using a traceable hand held scientific thermometer is that if you stick it in five different spots in the top of the mash, you get five different readings. I recirculated and stirred BTW.

With the thinking that heat rises the temp in the middle where the dial and thermowell are would be the place to measure. Filling it with ice water, reading then the same with boiling sounds good to zero both but I don’t think the pid reads that low since there is really no need for it. So trust one, trust the other, take an average or DWHAHB.

The beer is currently fermenting nicely.

My thermometers all read within a degree or two which is close enough for me. It does sometime bug me though when they don’t all agree.

My initial reaction: Pick one location to measure consistently from batch-to-batch and pick one thermometer going forward. Ignore the other one or find another use for it.

It would drive me crazy to use more than one thermometer, because they would probably never match. As a batch sparger who was obsessed with mash temp in the early days, here’s the solution I came up with: viewtopic.php?f=5&t=109024. The probe goes into the center of the mash about 4 inches above the bottom of the cooler. The location may not be perfect, but I am able to use a consistent process and obtain consistent results.

Thanks guys. I do tend to trust, or maybe like the Fermentap dial thermometer most. For one thing it is simple and I have had them on many MTs. The one on my kettle agrees with boiling at 212 or so. I will have to compare the PID on the HLT with the Fermentap on it next batch since it is the exact set up as the MT.

The beer tastes good and that is the bottom line.

I occasionally check my thermometer against another one just to be sure it hasn’t gone out of calibration.

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