Kamado-style ceramic cookers are famous for their ability to retain heat. The heavy ceramic shell absorbs and holds heat, as well as reduces the amount of heat that escapes from the cooker. However, this ability to absorb and hold heat is a bit of a two-edged sword when it comes to achieving a truly stable cooker temperature. A question you will see a lot regards stabilizing the temperature of kamado-style ceramic cookers. Typically, someone will post about how they thought they had the temperature stable for thirty minutes, came back in two hours and found that the temperature had slowly crept up. We'll take a quick look here at why this happens, focusing on one important component of temperature stabilization: the temperature of the cooker's ceramic shell.
A ceramic cooker won't achieve a stable temperature until everything is stable. This includes the air temperature, of course, but it also includes the temperature of the ceramic shell. And when does the temperature of the ceramic stabilize? When the amount of heat that it is absorbing from the fire is equal to the amount of heat it is radiating into the outside air. If your fire is putting out enough heat to keep the air at a certain temperature AND heat up the ceramic shell, what happens when the ceramic shell finally gets to a stable temperature? The ceramic shell no longer soaks up as much heat from the fire as it did when its temperature was rising. So all that extra heat now begins to raise the temperature of the air inside the cooker. To maintain a stable temperature, you need to slowly reduce the size of the fire as the temperature of the ceramic shell increases.
For our first test, we wanted to demonstrate how long it takes the ceramic shell of the cooker to achieve a stable temperature. The approach was relatively simple. Using a large Big Green Egg, we attached a thermocouple to the stem of the dome thermometer to measure the interior temperature of the cooker. We attached a second thermocouple to the exterior surface of the ceramic lid. We then used a temperature controller to bring the cooker up to 300°F and keep it there. The following graph shows what happened.
Graph 1. Temperature of the cooker interior and outside surface vs. time.
What we see is that the cooker's interior (air) temperature stabilized after about 37 minutes. The temperature of the ceramic shell took about 2 hours, 15 minutes to stabilize. So, the ceramic shell was soaking up heat from the fire for about 1 hour and 40 minutes after the air temperature had stabilized. But once the shell reached a stable temperature it didn't need nearly as much heat to remain at that temperature as it did while the ceramic was heating up. The only reason the air temperature didn't keep creeping up is because we had a temperature controller constantly reducing the airflow to the fire. The fire gradually shrank in size, there was no excess heat, and the air temperature remained stable. But the main point of this test was to demonstrate that it takes a long time (2 hours 15 minutes) for the temperature of the ceramic shell to stabilize.
So, our first test showed that the ceramic shell takes a lot longer to reach a stable temperature than the interior air of the cooker. And as we said, we used a temperature controller to keep the interior temperature stable while the ceramic heated up. Now we'll demonstrate what happens when you don't have a temperature controller to reduce the size of the fire, rely instead on manually adjusting the airflow, and then walk away after 30-40 minutes thinking you have a stable cooker.
For our second test, we manually stabilized the cooker at 300°F and kept it there for about 40 minutes, tweaking the vents as necessary. We then let it ride in order to show how the interior temperature doesn't actually stabilize until the ceramic temperature stabilizes. The following graph shows what happened this time:
Graph 2. Temperature of the cooker interior and outside surface vs. time.
The cooker's internal temperature hit 300°F at about 7:30 into the test. We kept a very close eye on the cooker and made several tiny adjustments to keep the cooker at this temperature. At about 46:30 into the test, or after about 39 minutes of tweaking, we stopped making adjustments to the vents and just let nature take it's course. The red arrow shows on the graph at which point in time we stopped making adjustments.
The green arrows show the approximate time that the ceramic and the air inside the cooker stopped heating up. From the time that we stopped tweaking the vents until the ceramic finally reached a stable temperature, you can see the interior temperature of the cooker slowly rose about 50°F above our intended target. During this time, as the heat needed to raise the temperature of the ceramic was reduced, the resulting extra heat went into raising the temperature of the air. Only when the temperature of the ceramic stabilized and the temperature of the air rose enough to use the extra heat from the fire did the cooker stabilize.
So, what do we learn from this little experiment? Essentially that your cooker is not going to reach a truly stable temperature until the ceramic is heat soaked. This can take 2 to 3 hours from the time you start your fire, so you need to keep an eye on your vent settings and slowly reduce the size of your fire for a lot longer than you think if you are truly shooting for a particular temperature.
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