A 30 Year-Old Love Process Controller Saves Christmas Dinner

If we assume necessity to be the mother of invention, then it follows that looming disaster is the architect for "MacGyverisms." For those who may be too young to remember, MacGyver was a late 80's television series that revolved around one Angus MacGyver, a mysterious secret agent from some murky US government organization. Macgyver's strength was his inventive use of common items to improvise solutions to negate impending disasters. It was common for MacGyver to construct sophisticated devices from items such as a half-empty coffee can, duct tape, flashlight batteries, metal coat hangers, yesterday's newspaper, paper clips, and eyelets from a pair of shoes. What made the show so popular was that all of his clever solutions were based on sound, accurate science.

Recently, I had a personal need for a "MacGyverism."

Late in the afternoon on the day before our family holiday dinner (and to my dismay), my wife decided to bake holiday cookies. Knowing that the next day was destined to be very busy, I wished she had chosen to relax and enjoy a holiday movie. Instead, she was in the kitchen making cookies.

After carefully preparing the dough, she loaded a cookie sheet and slid it into the oven. I was trying to remain un-seen, yet near, as I wanted to be the chosen taste-tester at the exact moment they came out of the oven. Unfortunately, roughly 3 minutes later, we both smelled something burning. Peering into the oven and through the smoke, we saw mounds of what was now charcoal. Wow---only three minutes from fresh batter to black charcoal---what was that temperature setting?

After a short discussion and a check with our oven thermometer, we determined that the temperature controller on our 20+ year old oven had suddenly failed; in this particular case, it had failed in a manner that did not turn power off to the element, so the oven continued to climb well above the 375 degree set point. 

I have no idea if this type of failure is common, or if most controllers fail such that the oven simply doesn't heat. Either way, the family dinner was supposed to be tomorrow, and here we were with a refrigerator full of food, and no oven with which to prepare it.

Another short discussion uncovered the reality that microwave ovens are nice, but that we have never mastered the art of cooking with one; ours is strictly a re-heat device for our family. We needed another plan. In an effort to maintain hope, I jokingly referred to our oven as "the incinerator” and suggested that we search through cookbooks for recipes that require a minimum 700 degree baking temperature.

It was then that I realized I was the only one smiling; suddenly it became very chilly in the room, even with the incinerator continuing to spew heat-laden smoky fumes.

I recognized the opportunity and then quickly explained how happy I was that my wife decided to make cookies; had she not, we wouldn't have known of the failure. Sometimes, timing is everything.

After some thought, I remembered that I had a process controller and some sort of thermocouple that were take-offs from a job I did for a friend many years ago. That recall quickly led to the next challenge: find them both and try to cobble together a temporary control system for the oven to get us through our holiday dinner.

I began trying to solve the problem by back-dooring specs by searching for parts on-line; I knew we only needed the baking feature (no broil), so I was focused on controlling only the lower element; the specs on that replacement part indicated that it was rated at 9 amps at 240 vac. So, in addition to the temperature controller and thermocouple, I now needed a relay or contactor that could handle a 9 amp 240vac load.

A few hours later, I assembled the temporary control system using a 10 amp relay found on a very dusty shelf of a local hobby store. I used the original temperature controller's failure mode to my advantage, as it allowed me to use the original control as the oven's master power switch, and use my 30 year-old process controller as the temperature-maintaining device.

At about 11:00pm on the eve of our family dinner, I tested the temporary control, and I was surprised how quickly it reached the 375 degree set-point. And I was even more surprised as it went past the set-point and continued to climb---EXACTLY as the original did.

Believing I mis-wired the temporary controls, I checked my work twice and found no obvious error. I then looked into the oven and noticed that the upper element was glowing. I immediately remembered seeing a note on one of the Appliance Parts websites describing how some ovens use the upper element to reduce the pre-heat time. After these ovens reach a temperature within 50 or so degrees of the set point, the upper element turns off, and then allows the lower element to maintain the baking temperature.

Armed with that subtle, yet very important tidbit, I allowed the oven to cool, and I then happily neutered the upper element by disconnecting it.
Round two of testing the now-modified and semi-neutered former-incinerator now began; after 25 minutes, the oven reluctantly arrived at the set-point, but it maintained the temperature nicely, with minimal overshoot. Thankfully, the process controller's self-tune feature did the job, and I found no need to tweak gains or settings in the controller. So, at about 2 am I finally went to bed, after turning off the oven, of course.

Later that day, we did have dinner, prepared in our own oven. It was excellent, as my wife is an outstanding cook. During dinner, it was noted that although designing machine controls may not give us much to talk about at parties, it does—on occasion—come in handy for things one might least suspect.