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

Lifehacks Asked by Dalton Bentley on September 1, 2021

I purchased a $10 Relion 2-second digital oral clinical thermometer and it appears to be completely unreliable. It is off 0.5 – 1.0° F on successive measurements (even with proper delay) and differs from another digital thermometer by 0.5 – 1.0° F.

Is there something commonly available around the house that generates a precise temperature to calibrate this oral thermometer?

Typically you can calibrate a digital thermometer using an ice bath, but clinical digital thermometers typically only operate in the 90 – 110° F (35 – 42 C) range. I see that the melting point of gallium at 85° F, but I don’t have any gallium around the house and the boiling point of water is too high for this thermometer. I thought about throwing together a resistance heated reference but quickly discovered that gets complex. Unfortunately, I do not have a mercury-filled glass thermometer for comparison.

5 Answers

There isn't a high precision temperature reference in the range you want. Wood's Metal and Field's Metal (melting points 158F and 144F respectively) melt too hot, and mercury melts much too cold (-40F). Gallium is readily available (among other things, in the form of a gag spoon that melts in your hand), but very expensive for what should be a single use.

The best option I can suggest is to buy a second digital thermometer "to keep the first one honest". If they agree, you can be pretty confident the reading is correct. Worth noting that there are few instances where as little as a half degree F in body temperature is significant -- for a 98.6 "normal", you'd have to be above 100 F to be considered to have a fever.

Since you already know by comparison that the $10 unit is questionable, I'd recommend just returning it to the place of purchase for a refund, as "defective".

Correct answer by Zeiss Ikon on September 1, 2021

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.

Answered by ratkiley on September 1, 2021

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.

Answered by Spring Ford on September 1, 2021

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:



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,


Compare that to what the medical thermometer says.

Answered by MICHAEL MICHALSKI on September 1, 2021

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.

Answered by John Wang on September 1, 2021

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