Issue 154 May 2003
Automatic Temp Controller
Data Logger for Slow Cooker

MEASUREMENT ACCURACY

There are several sources for error in this process, including the inherent inaccuracies of the thermocouple and cold-junction temperature sensor, how isothermal the cold junction and sensor really are, the accuracies of the A/D converter and voltage reference, and gain errors in the amplifier. A good reality check is to test the temperature of boiling water, because 212°F is actually within the range of the typical barbecue-cooking temperatures. The circuit and setup described here yielded a boiling water temperature of 213°F, which is good enough for barbecue purposes.

DESIGN LIMITATION

The circuit has a design limitation. When the thermocouple probe is at a lower temperature than the cold junction, the voltage produced is negative. However, the circuit, as configured, is single-ended, so it treats a negative voltage as if it were 0 V.

Consequently, if the probe is colder than the cold junction, the computed temperature will be the cold junction temperature, not the actual probe temperature. There are several ways to correct this, but none were used, because I was only concerned with high temperatures, not low ones.

AIRFLOW CONTROL

Now you’re capable of measuring the cooker’s temperature, but how can you control it? Adjusting the lower or upper draft openings to regulate the airflow would require a motorized mechanism. A simpler approach is to use a small fan.

Little airflow is required for barbecue-cooking temperatures. The 40-mm fan that I used only puts out 10 cubic feet per minute, but can produce cooker temperatures higher than 400°F. Although bigger fans can produce higher temperatures, they’re unnecessary for barbequing, and you don’t want to cook the electronics, which are mounted only a few inches from the firebox. The fan speed can be controlled by the microprocessor using one of its PWM outputs from fully on to off in 255 steps.

So little airflow is needed that air leaks can prevent low temperatures from being reached! There must be a good seal between the cooker’s lid and base. Also, you must prevent air from leaking in through the opening when the fan is off. The simple flapper valve, which is shown in Photo 5, accomplishes this; it’s constructed from a piece of aluminum pie pan that’s epoxy-glued to a small metal hinge. When the fan is on, it opens the aluminum flap. The flap shuts when the fan is off, sealing the opening.