My wife has recently complained that the oven seemed “off” - that it was not as hot as it was set to be. We had a cheap thermometer that seems to agree but I know they are notoriously inaccurate.
I mentioned this to an appliance repair guy who was there for our washer and he said to first try to calibrate it per the owner’s manual.
The first step was to accurately measure, so I bought a laser thermometer and put a grill plate in the oven and set it to preheat to 350.
When the bell went off - indicating the desired temperature had been reached, I pointed the laser and it read 280 - aha, it is wildly off. Then I upped the desired temp to 400 and again, when the bell rang it was around 325 - so again, off, more or less by the same amount. But when I tried 475 it was correct (although this time I was distracted and didn’t check right at the bell).
Then I decided to cool it down and try again, but measure at the bell and then 5 minutes after, and at each level, five minutes after the bell it was exactly at the temperature I set.
I found the instructions on how to recalibrate but it seems like it just allows you to add or subtract some degrees to make the math work, but my problem doesn’t seem like the scale is off (and not by 70 degrees).
So the question - is calibration measuring the displayed temp or the one that sets the bell off? Will recalibrating help my problem, by holding off on the alert until the desired temperature is actually reached? It feels like not.
Or is it the sensor that tells the bell to tell me the oven is at temp broken?
In the meantime we will just wait 5 minutes after the bell to put things in the oven
The oven’s thermostat is measuring the temperature of the air in the oven. This is in fact what the heating element heats, and so the temperature of any given solid object being heated by the air inside your oven (such as a grill plate) will understandably lag behind the temperature of the air. This is consistent with your low readings of the grill plate immediately after the oven air reaching the targeted temps of 350 and 400, and your reasonably accurate readings when you waited for some time after the oven display indicated the air had reached 475.
For better results, you might want to measure the air temperature directly, rather than the temperature of a massive object that’s being heated by the air. This video shows exactly that:
Note that they advise measuring the lowest and highest air temperatures experienced some time after the oven first indicates preheating is done, and then using the average of those readings as an offset for selecting the desired oven temp in the future.
TL, DR: don’t try to calibrate your oven based on a single temperature reading of a large thermal mass immediately after your oven indicates preheating is done.
I’m pretty sure that the oven’s alarm will only tell you when the oven’s set point is reached. After that, the actual temp in the oven is going to go higher than the set-point. Then lower.
What you are calibrating is the average temp of the oven.
Oven temps oscillate around the actual set-point on the front panel (called hysteresis), but you can apply an offset to most (all?) ovens in order to average those oscillations out. You really need an accurate thermometer to calibrate properly. I used a ChefAlarm temp probe, which is pretty accurate.
Here’s some instruction. for calibrating your oven. Using the readings that you collect over time, you can average the high and low temps, then apply an average offset to the oven’s temp control so that the actual temperatures go an equal amount above and below the set-point.
This wouldn’t be a good tool for measuring your oven temperature unless you knew what the emissivity of your grill plate (or other target) was. And if you’re picking metal targets, their emissivities can be all over the place, most often quite low, because they’re reflective. Especially in the thermal infrared. Your thermometer has an adjustment someplace for emissivity. Turn it to 0.1, and take a reading, and turn it to 1 or 0.95, and take another reading. The temperatures will be very different.
A tray loaded with charcoal briquettes or coal (enough that you can’t see the bottom) would be a much better target. That would have an emissivity of about 0.95 or 0.97. Note, at some high temperature these things would ignite, and I don’t know how high.
Better to use a thermocouple placed in the middle of the oven, with the wire run out through the door gasket. You measure with the door closed.
By the way, “thermometer” properly refers only to instruments where the instrument, or the sensing part of the instrument, is at the temperature being measured. What you have there is a “pyrometer”. Just sayin’.
We calibrate our environmental chambers at work using a thermocouple and thermocouple readout. Simply stick the thermocouple in the chamber and run the wires out to the readout. It’s not necessary for the thermocouple junction to “touch” anything inside the oven, but the reading will be more stable if it is in thermal contact with something that has more mass than the junction alone.
On edit: instead of a thermocouple + thermocouple readout, you could use a thermistor + thermistor readout. But I am a bit dubious on how well thermistors maintain their accuracy at higher temperatures. By contrast, thermocouples seem to “love” high temperatures. (At least up to a point.) Carefully check the specs.
When I approached the same issue, I found two things:
My oven really hit the desired setpoint about two heating cycles (maybe 10-15min) after it told me that it had reached the desired temp, and
My analog oven thermometer responds slowly
When I used a remote-reading digital range/grill thermometer, I verified the above. Now, I just habitually wait about ten minutes after the beep before putting anything in the oven where time/temp really matter.
Crafter_Man knows more about oven calibration than I do.
About thermistors, I think most of them have a lower maximum temperature limit than home cooking ovens. I’m thinking something like 200 or 250 F is typical for a maximum thermistor temperature, right?
An RTD or platinum resistance thermometer (these are the same thing thematically if not always in practice) would be even better than a thermocouple, though it’s hard to imagine the degree or two of error in a good thermocouple system making that much of a difference. Crafter_Man knows more about platinum resistance thermometry than I do, any day.
Omega.com is a good place to buy any temperature measurement device you are likely to want. They are the Sears of temperature. If you really want to knock yourself out, go hunting for an SPRT and a resistance bridge, instead of buying your next car.
Another thing that could be different between ovens would be the heat transfer coefficient. If it’s a forced convection oven, it will heat things faster even though it’s at the same temperature, and they will take less time to cook, or be more cooked (burnt) in the same time.
Well, I’m certainly not an expert in the field. Though I did spend a few years working in a temperature metrology lab. Fun times.
Agree, an RDT (and especially a PRT) would be overkill. Those things are easily good to ±0.1 °C, and ±0.02 °C can be achieved without too much effort. That type of measurement uncertainty isn’t required for home cooking, especially when you consider the oven’s heating element is being modulated, and thus the temperature in the oven is cycling. Plus they’re expensive and very delicate.
Yep, getting down to a few microkelvin ain’t cheap. Bridges are veryexpensive. Same with standard resistors, along with an oil bath to stick them in. I think an SPRT is around $6K, which is nickels & dimes compared to the other stuff. Oh, and you’ll need a triple point of water cell. Add it all up and you’ll find a new Ferrari is cheaper.
A sheet of aluminum foil sprayed with high-temperature flat black paint would be close to optimal, I think. Advantages: almost no thermal mass, so comes up to temperature immediately; emissivity close to 1; cheap and easy enough to place several targets around the oven, to get a sense of temperature variation.
This will have little heat capacity – the “almost no thermal mass” you mention – and will cool down a great deal while you are opening the oven door and going from one to the next.
BTW if you have a window in your oven door, it won’t be transparent to the thermal infrared. Unless it’s made of gallium arsenide, or barium fluoride, or zinc selenide, or some such.
As an aside, grab a copy of Harold McGees The Curious Cook. (If you don’t have his seminal On Food and Cooking, Food and the Lore of the Kitchen, get that too.)
He goes through calibration for making a perfect soufflé guaranteed every time, and the physics behind the calibration.
