Calibrating My Oven

Every modern baking recipe specifies at what temperature the food is to be baked. A typical cookie or bar recipe specifies some temperature falling in the range of 325 to 375 F. There are exceptions, but that's 90% of the recipes. A difference of twenty-five degrees is huge. If you are following a recipe and want your cookies to come out right, you need to have the temperature set correctly. That does not necessarily mean setting the oven dial to the temperature specified! In fact, that's the point of this post. The temperature dial on your oven sets an oven thermostat, which controls the temperature. Like your wall thermostat, the thermostat can be out of calibration, hence inaccurate. Unlike your wall thermostat, there is probably not a reasonable way to calibrate it unless you have some super-duper fancy oven - which I sure don't. Instead of adjusting the thermostat, usually we have to adjust the oven temperature dial so that the visual setting you want is "off" the mark just the right amount to have the oven wind up at the right temperature.

The only way you know if your oven thermostat is off is by buying an oven thermometer, preheating your oven to a given temperature with the thermometer inside it, and then looking to see what the thermometer says. If you have a fancy oven with a digital reading of the temperature - if they actually make ovens like that - you should still check the oven temperature with a thermometer at least once to verify the temperature sensor is working correctly. It's only telling you the temperature is at X degrees; that doesn't mean the oven is actually at X degrees! The only indication of the actual temperature of the oven that us folks with "normal" ovens have that's independent of the dial that sets the temperature is the "it's at temperature" light - but who knows if that's working right? When you check your oven temperature, assume your oven thermostat is going to be off by some amount, and don't be surprised if it's off by a lot. I've checked the thermostats of ovens at three places where I've lived, and they've all been off.

The oven thermometer I used twenty years ago has found itself some randomly selected home, so I went to the local Williams-Sonoma store and bought a new one - the Taylor Elite Oven Thermometer, described here and presently listed on the Williams-Sonoma Web site for $18. It has a simple dial face and a clip that lets you fix it to one of the oven racks in a position where you can read it. If you have a nice window in your oven door so you can see it with the oven closed, you're lucky. I have a cheap apartment oven and had to open the oven door to read it. I also used a flashlight because there is no oven light, always a plus. At least I could find my flashlight right away, which has got to be a first. But just to make sure things didn't go overly well the batteries were of course worn out and I had none in my battery stash at home. I could probably have done without the flashlight, but I didn't want to be messing about sticking my head into a hot oven to read the thermometer - various scenarios flashed before my eyes and none were very pleasant, although one or two might have been vastly amusing to my friends - and so a trip to the local drug store for batteries ensued as well. (In fact, one of my friends has quite a little head-in-the-oven story of her own, but I couldn't possibly do it justice here, so we're all out of luck. In any case, I wouldn't recommend anybody going that route, especially if you have long hair. Or any hair.)

I was a little disconcerted at first because I set the oven dial to 350, waited until the "preheating" light went off, then checked the thermometer and found the oven was only at 275. What??? I closed the door again and waited longer. The next time I checked it was at 370. Whew! I was glad my oven wasn't off by seventy-five degrees! There are a few likely reasons for my initial reading of 275:

1. The thermometer takes a little while to register the temperature, and as the oven steadily heats up, the thermometer lags behind.

2. The preheating light goes off way to soon before the sensor actually says it's at the right temperature.

3. The temperature at the thermostat sensor reached the correct temperature and shut the light off, but the interior of the oven did not reach temperature as quickly as did the part of the oven where the sensor is located.

I've been thinking it was the first reason, because the thermometer does not in fact register temperature changes quickly, and the second reason really doesn't seem likely because the preheating light seems to be closely linked to the thermostat and the oven on/off electrical current; and as for the third reason, who would design an oven so that the sensor doesn't sense what the internal oven temperature is more closely than seventy-five degrees? But I actually don't know why it still said 275 when the preheating light first went off for 350 degrees. In any case, the main take-home lesson is to wait a good long while after turning on the oven before measuring the temperature with this thermometer. The secondary take-home lesson is give your oven more time to preheat than the preheating light says, if the temperature is critical. And when would it be critical? When the in-oven cooking time is short, as for cookies. If the cooking time is long, only a small part of the cooking time will be at an incorrect lower temperature after preheating, but if the cooking time is short, you might do an entire batch of cookies before the oven is finally at temperature.

While calibrating my oven, I found that it doesn't maintain a very even temperature over time. The "now heating" light, or preheating light - I don't know what it's officially called - goes on every so often, when the oven temperature drops a certain amount below the setting. Then the heat goes on and heats the oven again, and at some point the oven gets plenty hot again and the electricity (this being an electric oven) goes off, at which point the temperature starts to drop again. This is surely how most ovens work. What surprised me was the amount of variation, and that the amount of variation was much greater at higher temperatures. At 300, the temperature seemed to go up and down by fifteen degrees after it was preheated. At 400, the temperature seemed to go up and down thirty degrees. It would make sense that an oven would have a harder time maintaining a higher temperature, but apparently the simple operation of this inexpensive oven causes increased variability at high temperatures. How does that impact cookies? Well, if a batch would only take five minutes to bake at 400 degrees, I might catch the oven when it's at the high temperature for the 400 setting, or at the low temperature - and so the temperature at which the cookies bake could vary by thirty degrees based on the exact moment I put them in the oven. That's not so great! Luckily most cookies bake at lower temperatures where the temperature is somewhat more stable and bake long enough so that nearly a full heating-cooling cycle occurs while they are in the oven so they actually get baked at the average temperature. However, if one has this sort of problem with their oven, I can think of two things it makes sense to do when baking cookies in order to avoid problems due to vagaries of oven temperature:

1. Bake longer at lower temperatures rather than shorter at higher temperatures, if possible.

2. Use baking pans made with thicker metal that will help even out the metal's temperature when baking at higher temperatures.

The point of using the oven thermometer to calibrate the oven is to help make the results of our cooking efforts match the results the originator of the recipe achieved. I keep asking myself, "Well, it says 350, but did the originator of the recipe have an oven that was really at 350 when it said it was 350?" But that way lies insanity! We have to assume the recipe has the best temperature and cooking time, make sure our oven is at least reasonably well calibrated, and go with it. If we don't like the results we can always adjust the temperature or baking time to something we like better, but we should start with what it says.

If the temperature goes up and down, what's the right temperature to set at? For example, if you set the oven to 350 and the oven thermometer shows the temperature varies between 355 and 370, where should you set the oven in order to have it be at the best temperature for a 350-degree recipe? I bet there is some answer most likely to give the best results most of the time, but I sure don't know what it is! But here's what I did. I assume the temperature goes up at a steady rate, and comes down at some other steady rate. If I set at 350 and the temperature actually ranges between 355 and 370, then it's an average temperature of 362, which is twelve degrees above 350. If I turn the oven dial down to 338, then (350 - 12 = 338), the thermometer should then show the oven temperature ranges between 343 and 358,with an average temperature of about 350.

To sum it up, more generally speaking, I calibrated my oven this way:

1. Set the oven for 350.

2. Wait a while, then read what the thermometer says over about 40 minutes' time. Don't check more than once every five or ten minutes if the oven door has to be opened to read the thermometer, and make sure the oven door gets shut again quickly.

3. Calculate the temperature that's half-way between the highest and lowest reading.

4. If the midpoint temperature is higher than 350, set the dial lower by the difference every time I want the oven to be at 350; for example, if the midpoint measured temperature is ten degrees higherthan 350, set the dial ten degrees lower (at 340). If the midpoint temperature is lower than 350, set the dial higher by the difference.

Hopefully the math isn't beyond anybody who wants to follow that procedure. If it is, sorry!

I calibrated separately for 300, 350, and 400 degrees. In theory a thermostat could be off different amounts at different temperatures. However, it appeared that my thermostat was off by about the same amount at all three temperatures. The picture with this post shows what my calibrated oven temperature dial looks like, marked with green Sharpie to show the correct spot on the dial for the three calibrated temperatures. I could hardly mark the dial without cleaning off the grease and grime that had accumulated on the oven's control panel, now, could I? My oven might like it if I calibrate on a regular basis. But I probably won't. :-)