Household reference for measuring accuracy of an oral (digital) thermometer?

Most probe-type (<$20) digital medical thermometers are called disposable thermometers because they get out of calibration and there's no way to calibrate them, so they come in 10 to 100 packs, for medical professionals. They get out of calibration if dropped, if they get too hot, or after 2 years. Analog liquid-filled glass thermometers don't need calibration, but where can you find one?

They sell a calibratable digital thermometer with a separate probe attached with a wire for $200. You have probably seen them in doctor's offices: because there is a slot in the top to hold a box of protective sleeves.

If you want an accurate digital thermometer you can calibrate yourself, buy a scientific or food service thermometer, because they can measure boiling water and 32F.

I observe that the boiling point of Dichloromethane is 39.8 C.

"A dense, non-flammable colourless liquid "

250ml is available for £12 ish.

It is toxic to some degree or other and it may dissolve your plastic digital thermometer but I suppose you could protect it with something. I wouldn't boil this up in the house but maybe outside with a following wind?

There is of course Diethyl Ether (34.6 C) however it is HIGHLY INFLAMMABLE and you wouldn't catch me boiling that up. Let me know if you plan to try that one and I will bring my sunglasses and come and watch from a distance.

I'm an analytical chemist, here's how I would do it. Take a cooking thermometer, calibrate it and check the medical thermometer against it. This is why chemists (or students taking college chemistry) need to get good at algebra. It's not too difficult. But it's non-trivial. I'm going to work in Celsius so set your thermometers to Celsius mode. (It makes the calculations a tiny bit simpler)

Boil some water and check with the cooking thermometer. Record the temp. Now take some ice and stir it with the cooking thermometer until the temp stabilizes. Now take the linear equation:

At+B= T

where A and B are unknown constants your solving for, T is the temperature of the water bath and t is the reading on the thermometer.

You will end up with a system of two equations, in unknowns A and B. One for 100 degrees Celsius, the booking water, and 1 for 0 degrees Celsius, the ice bath. Let's call The the reading in the hot water and Tc the reading in the ice bath. So, 100A+B=Th

0A+B=Tc ---> B=Tc We now have pinned down Tc

100A+Tc=Th ----> (Th-Tc)/100= A Now we know A

(Th-Tc)t/100+Tc=T Now we substitute A and B into the original equation, and we have and equation for the measured temp in terms of the actual temp. But we need the actual temp in terms of the measured temp. So we have to rearrange things a bit.

T-Tc=(Th-Tc)t/100 We just move subtract off the Tc moving it to the other side

100(T-Tc)/(Th-Tc)=t Now we have the actual temp in terms of T the measured temp.

So the final equation for calibrating the cooking thermometer is:

100(T-Tc)/(Th-Tc)=t

Where

T is the temperature you measure, or , read off the cooking thermometer.

Th is the temperature you read off the cooking thermometer in the he boiling water

Tc is the temperature you read of the cooking thermometer when you are stirring the ice bath (make sure you stir it with the thermometer)

t is the actual temperature of whatever the cooking thermometer is measuring, corrected by the calibration you just did.

Now we put it all together.

Take a lot of water and bring it to a good boil.

Measure the temp with the cooking thermometer. This is Th . Write it down, even if it's exactly 100.

Now take a cup, of crushed ice.

Stir the ice and water with the cooking thermometer.

The temp you measure is Tc. Write it down, even if it's exactly 0.

Now take a cup and put some tap water in it. styrofoam cups works best as the temp will stay more stable. Heat the cup in the microwave in 30 second bursts until it's between 37 and 38 degrees C. If you get it to hot you can stir in some cold water until it cools down enough.

Now, measure the temp with the cooking thermometer. This is T, write it down.

Now measure the water with the medical thermometer. Write that down too.

Now, plug those numbers it into the formula we derived,

100(T-Tc)/(Th-Tc)=t

Compare that to what the medical thermometer says.

It would seem to me that if you kept a daily log of the temperatures measured, you could graph the temperatures to determine if your temperature has become abnormal regardless of how inaccurate the thermometer is. Other than that would be to research the temperatures of supercooled solutions when they shift to the solid state due to impact or sound, unfortunately the ones I can think of do this at too high a temperature for such thermometers.