Don’t trust that dome thermometer

 *Updated. Post originally published June 7, 2013.  

Most BBQ grills and smokers come with a dial thermometer installed in the “dome” or hood of the cooker. Virtually all of these are of the cheapest construction possible. None of them are accurate. But, even if you replace yours with a higher quality and more accurate dial gauge, will it even give you a temperature that is useful?

Look at the stem of your dial thermometer when it’s installed in your cooker. You see 1- 2 inches of stem positioned right near the top of the lid or dome. Temperature is averaged over the length of that stem and read on the dial. Even if it’s accurate, it gives you the temperature of the air space well above the food you are cooking. Heat is rising from the coals beneath and dissipating through the metal or ceramic of your cooker’s top. Frequently the dome, where your factory installed gauge measures temperature, is as much as 50°F cooler than where your food is cooking.

Several variables affect the readings of the dome thermometer. The longer you leave the dome closed, the more closely the dial thermometer will eventually reflect nearly the same temperature at the food’s location. However, every time you open the cooker heat escapes and you have to start over. The dial thermometer will be wrong for some time.

If you use a deflector plate or drip pan under the food, heat currents come up the sides of the cooker into the top producing a greater temperature at the dome thermometer’s location than you have at the food’s location.

In virtually every recipe, the recommended cooking temperature is the temperature at which your food should be cooked. If you’re using the dome temperature, you’re likely to have your heat setting anywhere from 35°F to 50°F too high (or too low). This variation can cause you to overcook and dry out your food. Accurate temperature is important in long cooks of larger cuts. It’s even more critical when searing and grilling steaks.

The best alternative to the dome’s dial thermometer is reading the temperature from a probe next to the food. ThermoWorks recommends using a thermometer probe as close to the food on the grill as practical and about 1” above the grill surface. Some probes may have an integral clip to grasp a grate rod.

Other probes could be clamped in place using metal office clips or clips from the local hardware store. Professional BBQ teams seem to prefer a ball of aluminum foil. This ingenious method is flexible, cheap and easily repositioned during a cook.

Fashion a crumpled ball and tighten it around the stem or cable of the probe in such a way that the probe tip extends 2-3 inches from the end of the ball and rests about 1 inch above the grill surface. To move the probe during a cook, simply grasp the foil ball with your tongs and put it where you’d like. When done, the foil ball is easily removed from the probe.

Other alarming thermometers sold for home BBQ or oven cooking employ thermistor probes with a very limited range and less moisture sealing, not to mention lower accuracy. Consumer cooking alarms suffer from a very high rate of probe failure in BBQ use.

Due to flames and hot coals, a BBQ cooker will expose your probe’s cable to temperatures well in excess of your target dome or grill temperatures. Even if you’re using a low heat of 250°F, the coals will be much hotter. If your cable comes close to those coals, or to a flame, you will burnout the interior probe insulation and short the probe. BBQ cookers are producing a very humid environment. Moisture is the other most common reason for probe failure. Professional thermocouples built for the commercial smokehouse industry use higher temperature materials and superior sealing techniques to protect the sensor assembly.

The ThermoWorks ThermaQ accommodates two commercial thermocouples. One probe can be used in the food, with an alarm setting. The second probe can be used to read the grill or smoker temperature near the food’s location. Over time, you spend less on probes by using the professional probe solution from the beginning. Thermocouples also come in a greater variety of probe types allowing many different measurement solutions.

So what should you do with that dial thermometer in your dome? We recommend leaving it in place. If you take it out, visitors will think your BBQ is broken. The dial plugs the hole! You can even replace it with a more expensive after-market dial unit so it looks like you have everything. Just don’t depend on it.

When you’re cooking, read the temperature next to the food. After the temperature of the food itself, that’s what really matters.