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How hot is the oven?

By Brian Geiger, contributor

May 21st, 2009


Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.


Friend of The Food Geek Jennifer asks via Twitter:

Hi, Jennifer,

Yes, there is an easy and free way to know if your oven is calibrated properly, within 5 or so degrees. There are some limitations, but you get what you pay for.

There is an intrinsic problem in knowing if a tool that you have measures properly. How do you know that your ruler is exactly a food long? What if humidity or temperature warps the wood? How much does a pound really weigh, and compared to what?

On the scientific end of the scale, many smart people have thought about such things and come up with ways of making that determination. Perhaps there's a material that is known not to change that acts as a baseline, which is fraught with danger because, as time goes on, individual items will decay. A meter, for example, was compared to an iron bar way back in the day, and eventually upgraded to a platinum bar because, you know, iron rusts. Still, as I said, everything decays eventually.

The better way is to take something that has to behave in a certain manner by the laws of physics. Heavier-than-normal science: In the case of the meter, it has to do with how quickly light travels in a vacuum compared to time measured by an atomic clock. While I couldn't easily measure all of that, there are people who can, and more importantly, they can do it repeatedly and exactly with different devices and materials, and they can show you why it works.

Fortunately, measuring temperature is much, much easier. You just have to find something that you know happens at a certain way at a temperature that your oven is made to produce. The easiest way to do this is by boiling water.

At sea level, water boils at nearly 100°C or 212°F. My oven goes down to 170°F, so all I have to do is half-fill a pyrex dish with water, set the oven to 210, and the water should not boil. Set it to 215 and I should see a full boil. If the water boils at 210, then we know the oven is hot, and if it doesn't boil at 215, we know it's cold. Then it's just a matter of adjusting to suit.

Now, if you're above sea level, then you'll have to make some adjustments for that, because water boils at a lower temperature above sea level. The opposite is true if you're below sea level (which is rare, but certainly possible). 

Okay, I tell a lie. The boiling point of water is technically correlated with altitude, but that's something of a coincidence. Really, what you want to look at is barometric pressure. It just so happens that the force that air exerts against us and our water goes down as you get higher, generally because there's less air sitting on top of you pressing down. Still, weather can make a difference, but not really enough of one for you to care.

In order to save you lots of guess work, you can use this boiling point calculator which will let you input either a height above sea level or barometric pressure if you have a barometer that you trust handy, and you can learn your proper boiling point.

The downside of this approach is that it's only one data point. Ideally, you'd want another, higher temperature to compare against. I'd be interested in hearing in the comments suggestions from the readers for things that can be used to calibrate at a significantly higher temperature. I will say that there are some options mentioned in past Kitchen Mysteries that might be usable.

posted in: Blogs, food geek, water, temperature, thermometer, science, oven, boiling, calibration, meter
Comments (4)

Remle writes: I baked some bread in a 350 degree oven. The oven also contained an aluminum frying pan with two cups of water in it. The water never boiled even though the bread baked properly. Why? Posted: 10:02 am on October 7th

TheFoodGeek writes: It's always these little last minute flourishes that get me in trouble. I'll do some testing, but I suspect the problem is my suggestion of the pyrex dish, which makes perfect sense on the one hand because you could actually see the water boiling, and isn't that nice?

The problem is that most of the heat transfer in an oven is radiant, so a transparent liquid in a transparent dish is going to get very little heat transfer. Though the air is transparent as well, it need much less heat to get to temperature than water does.

One option is to use a non-transparent dish. Another is to add a pizza stone and put the dish on the pizza stone. The pizza stone would not only transfer heat via conduction instead of convection, but it would smooth out that temperature cycling that jfield mentioned.

Jeff Potter suggest in Cooking for Geeks to use sugar instead of water. Sugar melts at 367°F. His procedure is to set the oven at 350°F, ensure that the sugar hasn't melted, then to 375°F.

If you wanted to be a bit more careful about it, you could start at 325°F, then move to 350°F, then 375°F. If you are really keen to know your electric oven, you could pay attention to when the relay kicks on (that's the loud clicking noise) which would start the heating process, and then again when it turns off. If the sugar starts melting only just before or as the oven stops heating, then you can get a good idea of how much higher the temperature is at the top of the heating cycle than what you've set.

Incidentally, if you buy a new oven and have it installed, it is the duty of the installers to calibrate your oven, but they might not do it unless you request it. You should always request a calibration. Posted: 9:47 am on October 11th

EngineerJim writes: I too would have thought that water should boil in a 215 degree oven, bot now I'm not sure. I found your post as I was trying to check the calibration of an oven, a brand new GE Profile. I placed 1/4 cup of water in a Pyrex container, set the thermostat to 220F, and waited 30 minutes after the oven announced that it had reached its set point. A second (uncalibrated) thermometer confirmed the oven's digital readout. No boil- just a few bubbles clinging to the sides of the cup: gas coming out of solution. During the next two hours I stepped the temperature up to 250, and still no boiling.

I removed the water, filled the container with an additional 3/4 cup of tap water, and raised its temperature to boiling in a microwave. I removed it and used a candy thermometer to verify its temperature was close to 210, having cooled slightly. I then replaced the Pyrex container with the water and the candy thermometer in the 250 F oven, and watched in horror as the temperature dropped to 170F. A meat thermometer confirmed this reading. After ensuring that the temperatures were stable , I removed the meat thermometer and left it in the oven. Its indicated temperature quickly rose above it's 190F maximum, while the candy thermometer (still immersed in the water) remained at a steady 170F.

This convinces me that the average oven temperature was in fact well above 220F, and that the water temperature was at equilibrium well below 212F. Perhaps the water was being cooled by evaporation. Bottom line is that the boiling point of water may not be a useful calibration point for oven temperature.

Please try boiling water in a 215 F oven and let me know what happens. Posted: 12:00 am on October 9th

jfield writes: Since ovens don't maintain a constant temperature, but rather cycle 20-25 or so degrees above and below the target temp, it's kind of hard to really pinpoint the exact temp. Here's a thought: hang a probe thermometer in the oven, set the temp to (whatever), and then take a reading every couple of minutes (after pre-heating) for ten-fifteen minutes and then take the average. Posted: 9:40 pm on May 26th

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