Issue
154 May 2003
Automatic
Temp Controller
Data
Logger for Slow Cooker
by
John Moyer
Most
chefs would argue that barbecuing is more art than
science. Nevertheless, John introduced ATmega8-based
technology to the procedure. The result? An automatic
temperature controller that eliminates the need
for constant vigilance when slow cooking.
Start
•
Project Goals • Thermocouple
Properties
• Measurement Accuracy
• Controller Electronics
•
On/Off Control •
Sources and PDF
A traditional barbecue
typically involves cooking tough cuts of meat
(e.g., pork ribs, pork shoulder, and beef brisket) over
a wood or charcoal fire at a low temperature (200° to
250°F). The meat is cooked for long periods of time
until it’s tender (e.g., 5 to 6 h for ribs and 10 to
20 h for brisket or pork shoulder). Usually, steel offset
cookers, modified 55-gallon drums, and even in-ground
pits are used.
Another
barbecue tradition requires someone to be present for
the 5 to 20 h that the meat is cooking. This person
must add fuel so the fire doesn’t go out and regulate
the cooking temperature. Although modern technology
can’t shorten barbecue-cooking times, it can eliminate
the need for someone to be present to maintain the cooking
temperature.
CERAMIC
COOKERS
Photo
1 shows a different kind of cooker. The cooker’s thick
ceramic walls do such a good job of insulating that
a single load of lump charcoal fuel can maintain barbecue-cooking
temperatures for more than 24 h. This neatly solves
the problem of having to add fuel during a cooking session,
but there’s still the issue of temperature regulation.
The firebox is at the bottom of the cooker, and the
temperature is adjusted using the lower and upper draft
openings. Additional air moving through the cooker produces
a bigger fire and higher temperature.
|
(Click
here to enlarge)
|
Photo
1—The door at the bottom opens and closes, and the
cap at the top turns to regulate the cooker temperature,
which is monitored with a dial thermometer. |
After
it’s set for a given temperature, a ceramic cooker does
a remarkable job of maintaining that temperature. However,
if internal or external conditions shift too much (e.g.,
changes in the weather or wind, or ash buildup in the
firebox), the draft settings must be adjusted in order
to keep the temperature from changing. To have true
hands-off temperature control, you need a closed-loop
process control system. In such a system, the actual
cooker temperature is monitored and used to adjust the
airflow to maintain the desired temperature.
