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:
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.