Oven Thermometers

From Season 3: Beef Burgundy

Have you ever baked a cake or pie that was only half-done after the suggested cooking time? We have, and the reason was a poorly calibrated oven. In fact, our research indicates that two different ovens set to exactly the same temperature can differ in actual temperature by as much as 90 degrees. That's why we often use an oven thermometer to tell us what's really going on. We rounded up ten popular models to test.

But First, a Word About Your Oven

Cooking in an oven is not as straightforward a process as you might think. In fact, three different dynamics of heat transfer are simultaneously at play. Radiation is the heat energy (generated by the heating element) carried through the air inside the oven cavity. Convection is the movement of the hot air itself; think of the fan inside a convection oven. Last is conduction, which is the transfer of energy from one hot surface to another; think of a piece of meat browning while in contact with the surface of a hot pan. Of those three dynamics, radiation from the heating element is responsible for the lion's share of browning that occurs when you bake or roast in the oven.

The average oven designed for home use does not simply heat up to the temperature set on the dial and then stay there. An oven's heating elements are either on at full power or off—with no middle ground. To maintain the desired temperature, the heating elements cycle within a manufacturer-determined tolerance, heating up and cooling down to temperatures just above and below the desired temperature. The precise temperature tolerances and timing of the cycles vary from manufacturer to manufacturer. This cycling process is regulated by an internal temperature sensor located in the oven cavity.

Uneven Heating

We also confirmed the common assertion that the heat within an oven cavity is not consistent; that, in effect, there are hot and cold spots. Though we might have suspected otherwise, we found that the bottom of our electric test oven tended to run hotter than the top, usually by between 5 and 15 degrees. We also found that the rear of our oven ran hotter than the front by roughly 5 to 10 degrees. There was also a stunning difference from right to left in our oven, with the right side sometimes running up to 50 degrees hotter than the left!

The uneven heat is the reason why many cookbook authors suggest rotating pans in the oven when you are baking. We ran a simple test of baking sugar cookies to confirm this advice. Sure enough, the cookies were browned a little less evenly from one side of the pan to the other when we failed to turn the cookie sheet part way through the baking time.

Curious as to whether there was any truth to the common kitchen wisdom that electric ovens heat distribute heat more evenly than gas ovens, we repeated our tests on a gas range in the test kitchen. The temperatures recorded in our tests bore out some validity in this axiom. For instance, the temperature differential between the bottom and top of the cavity was closer to 50 degrees, where it had been just 5 to 15 degrees in the electric oven.

In an attempt to explain all of these temperature variations, many of our sources pointed to numerous factors that affect the way in which an oven heats up. Included among them are the size of the cavity; the number and position of the heating element(s); the size, shape, and position of whatever you are cooking; and, in the case of double wall ovens, whether you are heating just one oven at a time or both simultaneously. Without performing extensive tests on different brands of ovens, we cannot comment on the desirability of different models; we can only strongly suggest that you use a good thermometer to investigate the performance of your own oven.

The Thermometers

An oven thermometer will give you a fighting chance in the guessing game of temperatures inside your oven. Widely available in stores from the local supermarket right up to fancy kitchenware emporia, we wondered if price really mattered.

As awareness of mercury's toxic properties has increased, the mercury-based thermometers that were once common have become rare. In some cases, blue-dyed alcohol has taken the place of mercury, but most of the other models we tested were based on a bimetal coil. The bimetal coil, mounted inside thermometer and attached at one end to the pointer on the dial, comprises two types of metal, bonded together, which have different rates of expansion and contraction when subjected to changes in temperature. When the temperature increases or decreases, the coil changes length, which causes the pointer on the dial face to rotate to indicate the temperature.

According to our tests at moderate to high oven temperatures, most of the thermometers where pretty accurate, with one model spot-on. On the other hand, three models produced readings that were more than about 18 degrees off, which was the limit of our tolerance.

During the testing, we also noted that three models in particular could be difficult to read owing to small dial faces that are overstuffed with graphics or oversized dials that span almost 25 degrees. When you shop, look for an uncluttered thermometer face with bold numbers and thin dials to help with readability.

The last issue that affects everyday use is stability. All of the thermometers can either sit on the oven shelf or hang from it. In both cases, unfortunately, it is easy to knock over the thermometer while maneuvering a pan in or out of the oven. Even more irritating is when the thermometer falls through the wires of the rack to the bottom of the cavity. Both instances made for a clumsy retrieval process with an oven mitt-shod hand. Two of our thermometers minimized such antics; they were exceptionally stable in our knock tests. Also they were each only 2 inches tall, so it was easy to maneuver pans up and over without disturbing them.