Why Your Dial Thermometer's Reading
Is Different Than Your Digital Probe

In the good old days, we all judged the temperature of our cookers using a dial thermometer stuck through the side of the cooker. But now, we have thermometers galore. Analog, digital, Bluetooth, Wireless, thermocouples, thermistors, RTD Platinum, and on and on and on. So is it any wonder that people constantly asking why they are getting different readings on different thermometers?

Well one of the most frequently asked questions is "Why am I getting different readings when I clip my digital thermometer to the stem of my dial thermometer?" To answer this question, you have to know what type of dial thermometer you are using and understand how it works.

Dial Thermometer Construction
Almost all dial thermometers use a bimetallic strip which converts a temperature into mechanical displacement. Bimetallic strips consist of two strips of different metals having different coefficients of thermal expansion. The metal strips are connected along their length by fusing them together. The strips are fixed at one end and free to move on the other end. There are two types of bimetallic thermometers, helical and spiral, meaning the bimetallic strip is either wound in a helix or in a spiral. Most of the dial thermometers used for barbecue cookers are helical, meaning the bimetallic strip is wound in a helix and located in the stem of the thermometer:

So it should become obvious that the temperature-sensing element in these dial thermometers is not a single point, but rather extends almost the entire length of the thermometer's stem. Why this is important is that this means that the thermometer is not sensing the temperature at one small location in your cooker, but is instead sensing the average temperature across several inches. This begs the question, "How much does the temperature vary from one end of the thermometer's stem to the other?" Let's answer that question.

Temperature Variation
To measure the temperature variation along the length of the stem of a dial thermometer, we inserted a bamboo skewer through the hole in a large Big Green Egg cooker and mounted small thermocouples at 1" intervals, starting ½" from the inside surface of the dome:

We then heated up the cooker to a moderate temperature and let the cooker stabilize. The following graph shows what temperatures were recorded at those four positions along the stem of our bamboo thermometer when the cooker temperature as measured at the grate was 350°F.

So, depending on where you measure, there could be an average temperature difference as great as 28°F and a maximum difference of 40°F. Or to put it another way, depending on where you clip your digital thermometer's probe, you could get readings as much as 40°F apart.

So, here is what we have learned from our testing:

So, test your probes and your dial thermometers in boiling or freezing water. If they are close, they will still be close when you put them in your cooker. Any difference you experience is caused by where you clip your probe.

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