The conventional wisdom is that temperatures measured at the lid will be higher than those measured at the top cooking grate, and that temperatures at the top grate will be higher than those at the bottom grate. Is this true? If so, what are the temperature differences?
In 1999, I conducted two "experiments" by measuring temperatures in an empty cooker and during a cook of two pork butts. I concluded that the lid temperature averages 12-15°F higher than the top grate temperature, and the top grate averages 4-10°F higher than the bottom grate.
Of course, two "experiments" cannot adequately explain the complex dynamic of temperature inside the WSM, and they don't take into account different types and quantities of meat or water pan variations like water, sand, or an empty pan. Obviously, further tests are warranted.
To continue this effort, I conducted a series of cooks to measure cooker temperatures using an empty water pan, a water-filled pan, and a sand-filled pan. The test method and results are described below. As I conduct more tests, I'll post the results here.
Click on any of the pictures for a larger image.
Water Pan Test Method
In September and October 2003, I conducted three temperature "experiments" to test water pan variations in the WSM.
Each session consisted of cooking 2 whole chickens. All chickens were fresh, natural fryers, not previously frozen, and weighed 3-1/2 to 3-3/4 pounds each (Photo 1). Each chicken was prepped as follows:
Each session had a different water pan treatment:
To measure temperature inside the cooker, a k-type bead probe thermocouple was attached to the center of each cooking grate (Photo 4). The bead was positioned 1-1/2" above the grate surface. The ETI 1305 data logging thermometer (Photo 5) recorded the temperature from each probe at 5 minute intervals.
To measure temperature at the lid, I checked the Trend Model 33 industrial-grade bi-metal thermometer at 15 minute intervals, manually recording the temperature in a cooking log. This thermometer is mounted in the lid opposite from the lid vent (Photo 6).
For each session, the cooker was fired with a 10-pound bag of Kingsford charcoal briquettes using the Minion Method:
Each cooking session began at 7:45am and lasted 4 hours. Weather conditions were similar on each day: clear skies, calm conditions, outdoor temperatures starting in the mid 50's and going no higher than 70°F by 11:45am. The WSM was shaded so that direct sunlight would not affect cooker temperature.Water Pan Test Data
Graphs and data tables for each cook are shown below. Time represents the number of hours/minutes elapsed during each session. Bott Vent % represents the amount open a single bottom vent was set at the time indicated.
Water Pan Test Statistics & Conclusions
Here are some statistics based on just the last three hours of each session, when the cooker was maintaining the 225-250°F target temperature. Given that there are far fewer data points for lid temperature, I have not included lid measurements in these statistics.
Here's what I take away from the experience of conducting these three cooking sessions and looking at the raw data and summary tables above.
Obviously, these statistics and conclusions are just a snapshot based on three cooking sessions, and I caution readers to not extrapolate these results to other cooking situations, including other cooker firing methods and fuels, different varieties and quantities of meat, longer cooking sessions, and different weather conditions. Still, I hope that in some small way, they give you some insight into what is happening inside your Weber Bullet when you fire it up. And as I said at the beginning of this article, I'll do more tests along these lines and post the results here in the future.
I invite readers to let me know if I've screwed up in any way with this these statistics and conclusions. If I've drawn conclusions that are not supported by the data, or I've overlooked conclusions that are supported by the data, please let me know so I can correct this article.
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