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testing a thermometer

post #1 of 23
Thread Starter 
I got a new electronic thermometer and I think it is off. What is a good way to test it to see how far off it is.
post #2 of 23
boiling water...........212

crushed ice full cup........then with water added..........stir with probe.........32
post #3 of 23
Thread Starter 
ok this might sound like a dumb question but does that mean water temp will not get any higher than 212 degrees
post #4 of 23
yes 212 boils no more
post #5 of 23
Actually, some of it depends on where you live. You're in Georgia and you probably don't have much altitude to deal with.

I live in Denver so I have a Mile of altitude to deal with. For us, water boils +/- 198 degrees so it's an important factor in cooking times, temps and how to calibrate a thermomoter.

212 degrees is sea level for boiling water.
post #6 of 23
Thread Starter 
ok. I have to say I ask a question on here and within 2 minutes I got a answer. I am a member of alot of fourms but I must say I always get a answer from here in lightning speed. Thanks all. Scott
post #7 of 23
all you are trying to do...........is to see how far off the temp is........at those test temps...........and most meat you don't take that high.......but if you do......you know about how many degree's YOUR reading will be, and adjust in your mind on what real temps are......gives you a better idea..........
post #8 of 23
Kinda off topic, but a fun way to teach kids a little science is to boil water in a paper cup over an open flame. Water in it's liquid state will (with a few exceptions) always stay below 212 degrees, at which point it will turn to steam. Paper doesn't burn until 451 degrees. So as long as there is water in the cup, the paper can't get hot enough to burn. PDT_Armataz_01_25.gif
post #9 of 23
i learned that trick in boy scouts........hehehe
post #10 of 23
Because of the difference in altitude a number of the country's Health Department are encouraging the use of the ice and water method of checking/calibrating thermometers. Reguardless of where you live (high altitude or low) water always freezes at 32*.
post #11 of 23
What WD said, clear and concise.
post #12 of 23
I just got a brand new et 73. Still in box. Do you recommend I try the boiling water and the ice water test before using it?
post #13 of 23
I would. On mine one probe was 2º off. Not enough of a difference to warrant replacement by the manufacturer though, but you can mark that probe and know how much it's off.
post #14 of 23
I agree both probes on my ET-73 read 6 deg low
post #15 of 23
What's your elevation in Ohio? It might be a function of altitude since both probes are the same. A 6 degree difference seems like it might be about right.

I'm in Denver at 5200 feet and water boils at 198. So, roughly, it's a 1 degree change for every 371 foot rise in elevation. At least using Kentucky windage, wink.gif

If I'm right and that's the cause, I would say your altitude is right around 2,200 feet?

P.S. so, it's not that your probes are off, it's just that water is boiling at 206 degrees rather than 212 degrees. If that's the case, you're good to go.
post #16 of 23
I will check but I am pretty sure it is the unit. My et-72 and several electronic thermometers all read 212 on the hot water test. I am going to try the ice water method Who knows maybe the other 4 units are wrong. thanks for the input I need to check my elevation.

Just goggled it I am 922ft above sea level I guess I am flat lander
post #17 of 23
>Reguardless of where you live (high altitude or low) water always freezes at 32*.<

That is true only for pure water and tap water is seldom pure. Here in OK, my water is relatively soft, so the induced error is small. When I lived in Wyoming, the water had bunches of dissolved solids in it, so it did not freeze at exactly 32 degrees F.

Ice water, made with distilled water and boiling distilled water are very good methods, if one knows how high above sea level he lives for the second. Measuring both points will give you a better idea of the overall scale accuracy, too, because you can plot this on a graph and get an idea of whether or not the error is linear.
post #18 of 23
I find myself wondering that also. If a probe is reading 212 as 210, then will 182 in fact be 180, or will it be something else.

Cmon Geek with Fire, you seem like the type to test this out with a known accurate thermometer and read the differences.

I'm sure it doesn't matter. But, inquiring (and lazy) minds want to know.
post #19 of 23
your right.......it WILL be a sliding scale........butt 2 degree's i wouldnt sweat it.........like some people pull at 190 and others pull at 200, while still others pull at 205 or above........
post #20 of 23
OK, you sucked me in.

The problem here is that most of the probes used in these digital thermos are thermistors. Thermistors return to a digital processor a resistance. The computer converts that resistance to temperature. The problem is, the resistance to temperature ratio is not linear, it's curved. Meaning, if the resistance changes by a factor of 2, it doesn't mean that the temperature moves by a factor of 2.

This means that a probe can be accurate on one end of the spectrum and be off on the other side. So I recomend testing on both sides. It's also important to note that when you are taking a measurement, thermistors are slow to respond, sometimes up to 60 seconds. So make sure you give them time to idle to ensure that the proper temp is reached. Also, it takes longer for a probe to read a temperature change from high to low.

When I use thermistors in my program, I disregard the formula to calculate the temperature curve. I create a cross reference table from 40 degrees to 400 degrees. I measure the resistance every 5 degrees and store that value in a database. This is a calibration process I perform and store for every thermistor probe I use.

Geek out.....
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