How do you test a Thermometer's accuracy?

[BradKW]

So I'm looking at buying a 4th thermometer because the 3 I have don't agree with each other. It's quite possible that one of them is telling the truth, but the constant lies from the rest make me distrustful. Maybe when I finally get two that agree, I can quit. 

But possibly not...getting 3 to agree would probably make me feel better.

I remember finally figuring out how to check a new level for level...surely there's some simple way to figure out which cheap chinese POS thermometer is accurate? 

I don't have access to a nuclear clock tho, so if that's part of the answer, I need a different one.

 

[rvpopeye]

Go to the local school and ask if the science/chem teacher has a calibrated one?
Do you have an infrared gun thermo? Point it into a pot of boiling water to see if it's right.
Then use that to test the other ones.

 

[highdesertranger]

are we talking a regular fluid tube thermometer, or digital, or the IR type? well water freezes a 32 and boils at 212 at sea level. or you could compare to a known good thermometer. like at a doctors office or an official weather station. or you could buy an official guarantied scientific thermometer. boy are they expensive. highdesertranger

 

[highdesertranger]

just a hint, IR thermometers do not work on liquids. if you point it into a pot of boiling water you are measuring the temp of the bottom of the pot. highdesertranger

 

[Ballenxj]

just a hint,  IR thermometers do not work on liquids.  if you point it into a pot of boiling water you are measuring the temp of the bottom of the pot.  

Interesting, didn't know that.

 

[rvpopeye]

Really? Interesting ,Never tried it on water.
Just figured boiling water was a known temp and he IS at sea level fer sure !
I've been using mine to check hot oil for deep frying at 375 and it seems to work pretty good
Or is oil different? IDK
Well the school idea should get the job done!
Unless they don't do the same stuff there that I did in school?

Maybe the ice water thing would work with the thermometers in question ?

 

[highdesertranger]

the instructions that came with mine said they will not work on liquids. the way they explained it is that it needs to reflect of what you are pointing it at, and with liquids it's not a reliable temp reading. highdesertranger

 

[rvpopeye]

You read instructions ?????
Now I'm worried about you !

 

[Dennis]

Regarding the IR thermometer---if you point that at a window that has a cloth/other type barrier on the inside of the vehicle, are you getting the temp of the glass or the temp of the barrier? Tinted glass. Was trying to find out how hot my glass was getting.

Slight hijack, Brad

 

[deadwood]

You could get one of the cheap cooking thermometers that you can put in a glass of ice water. It should read 32* after a min or so. They usually don't come calibrated and that's how you do it. You basically hold the stem part and turn the thermometer part at the top until it reads 32*. You could then compare that thermometer against any other to see if they match.

 

[gapper2]

Brad if it is a cheap bulb thermometer,  a mixture of chipped ice and water stirred=32 degrees. Then boiling water at sea level (I'd think the Keys are pretty close)=212

If off you can sometimes SLOWLY raise the heat level till the liquid rises and goes out into the upper bulb and let it cool and retest.  Sometimes air is trapped in mixture and this recalibrate sometimes.  Good luck.

 

[BradKW]

are we talking a regular fluid tube thermometer,  or digital,  or the IR type?  well water freezes a 32 and boils at 212 at sea level.  or you could compare to a known good thermometer.  like at a doctors office or an official weather station.  or you could buy an official guarantied scientific thermometer.  boy are they expensive.  highdesertranger

I have one of each actually...and the difference between IR and digital one is always 4 degrees. If you were to count my battery temp remotes, I actually have 5 thermometers that all read differently.

I just googled scientific thermo's ...thinking I don't care enough to drop $80 on it right at the moment. I should just pick the one that reads lowest and go with it for the summer so I feel cooler...

 

[Zil]

It is always better to change instruments until you get the resulting answer you want.

 

[Spaceman Spiff]

So I'm looking at buying a 4th thermometer because the 3 I have don't agree with each other . . .

I have one of each actually...and the difference between IR and digital one is always 4 degrees. If you were to count my battery temp remotes, I actually have 5 thermometers that all read differently . . .

The liquid bulb thermometer is easy to check for accuracy: use deadwood's ice water method.  Use a styrofoam cup or some other insulated container, if possible.  This will get you a first order check on accuracy.  If you want to truly see how accurate it is you need another data point.  Unfortunately, most normal bulb thermometers don't cover the 32ºF to 212ºF range, so you have to get creative.

Good quality digital thermometers would have an adjustment pot in the thermometer circuit, with cheap ones it's a crap shoot.  If you can open the unit up and know what you are doing or are adventurous you can adjust it to read to the (now known) bulb thermometer temp reading.

Same for the IR thermometer.  Open it up and adjust to match the reading on the bulb thermometer.

Or you can just mentally adjust for the temperature differences (once you know the accurate temperature variance). And you have to decide what is 'good enough'.

The battery temp monitor is reading battery temperature, so it likely reads different than room temperature.

 -- Spiff