temperature conundrum

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psyber

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Currently, I am prototyping my PID software for what will (hopefully) become part of my electric rig controls and found something odd.

I have three temperature reading devices and all three are registering a different temp:
A digital aquarium thermometer: 76.3
A digital cooking thermometer: 78.8,
and a thermoresister (DS18B20) using Dallas 1-wire and an Arduino board: 75.5.

Supposedly the thermoresister is accurate w/in +-.5 degrees.

I realize that it is silly to expect them all to be in agreement, but this does beg the question of which is correct. Also makes me reconsider using this as my temp probe. Then again, some basic testing suggests that the DS is hypersensitive (which should be a good thing).

Any thoughts?
 
You have to calibrate them all - even the thermoresister. Do you have a good quality "lab" thermometer to use as a baseline?
 
Try them in ice water (should be 32F) and boiling water (212F at sea level, less at altitude).
Consumer thermometers are usually off by several degrees.
I use a Thermapen which is certified as traceable to the NIST standard.
Beware of the Traceable brand thermometers.
There are several thermometer threads, everyone has the same problem.
 
psyber said:
Currently, I am prototyping my PID software for what will (hopefully) become part of my electric rig controls and found something odd.

I have three temperature reading devices and all three are registering a different temp:
A digital aquarium thermometer: 76.3
A digital cooking thermometer: 78.8,
and a thermoresister (DS18B20) using Dallas 1-wire and an Arduino board: 75.5.

Supposedly the thermoresister is accurate w/in +-.5 degrees.

I realize that it is silly to expect them all to be in agreement, but this does beg the question of which is correct. Also makes me reconsider using this as my temp probe. Then again, some basic testing suggests that the DS is hypersensitive (which should be a good thing).

Any thoughts?
Click to expand...

Make up an "Ice Bath" (crushed ice and a little water) in a large glass. The Ice Bath will be at exactly 32* F. Try each of the thermometers on the Ice Bath and you will be able to tell which one is the most accurate. I spent a summer in a calibration lab during college and this is how we checked thermometers and thermocouples.
 
HairyDogBrewing said:
There are several thermometer threads, everyone has the same problem.
Click to expand...

::facepalm::: Why didn't I think to look before posting.

The ice bath test is a good idea, as soon as I figure out how to waterproof the resister I will give it a shot.

Thanks a bunch.
 
Sawdustguy said:
Make up an "Ice Bath" (crushed ice and a little water) in a large glass. The Ice Bath will be at exactly 32* F. Try each of the thermometers on the Ice Bath and you will be able to tell which one is the most accurate. I spent a summer in a calibration lab during college and this is how we checked thermometers and thermocouples.
Click to expand...

The devices won't necessarily have a perfectly linear response. The most accurate at 32*F may be more inaccurate at 212*F. Since the temps we are working with are closer to 212 than 0, I'd check them at boiling after finding out what temperature water boils at at your elevation.
 
evanmars said:
The devices won't necessarily have a perfectly linear response. The most accurate at 32*F may be more inaccurate at 212*F. Since the temps we are working with are closer to 212 than 0, I'd check them at boiling after finding out what temperature water boils at at your elevation.
Click to expand...

Good point.

I could be way off here, but if I have a pot of water boiling how do I know it is at the minimum boiling temp? That is, if boiling is anything > 212 how can I tell if I am at 212 or 240? Then again, the same can be said for ice cubes in water.
 
Boiling water will be 212 F (@ std press & temp blah, blah, blah) and it will not exceed that temperature. Water will freeze at 32 F. A mixture of slushy ice and water will be very near 32 F as the mix is right at the phase change state. Think of the texture of a slurpee or whatever they call those frozen slush drinks. Ice cubes in water don't work nearly as well for for this.
 

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