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Old 08-04-2009, 03:27 AM   #1
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?

 
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Old 08-04-2009, 12:35 PM   #2
bakins
 
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You have to calibrate them all - even the thermoresister. Do you have a good quality "lab" thermometer to use as a baseline?

 
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Old 08-04-2009, 01:05 PM   #3
HairyDogBrewing
 
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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.

 
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Old 08-04-2009, 01:08 PM   #4
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Quote:
Originally Posted by psyber View Post
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?
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.
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Old 08-04-2009, 01:25 PM   #5
psyber
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Jun 2009
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Quote:
Originally Posted by HairyDogBrewing View Post
There are several thermometer threads, everyone has the same problem.
::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.

 
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Old 08-04-2009, 02:05 PM   #6
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Quote:
Originally Posted by psyber View Post
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.
Stick it in one of Derrin's probe ends Welcome to Brewer's Hardware

That's what I went with for the DS18B20. Couldn't be happier.

 
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Old 08-04-2009, 04:53 PM   #7
evanmars
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Quote:
Originally Posted by Sawdustguy View Post
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.
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.
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Old 08-04-2009, 09:35 PM   #8
jpc
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Quote:
Originally Posted by freddyb View Post
Stick it in one of Derrin's probe ends
Yeah, but I'd feel obliged to buy him dinner first...

 
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Old 08-05-2009, 03:28 AM   #9
psyber
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Jun 2009
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Quote:
Originally Posted by freddyb View Post
Stick it in one of Derrin's probe ends Welcome to Brewer's Hardware

That's what I went with for the DS18B20. Couldn't be happier.
Wow, he even has compression fittings that double as weldless thermowells! I know what I am ordering this weekend.

 
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Old 08-05-2009, 03:41 AM   #10
psyber
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Jun 2009
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Quote:
Originally Posted by evanmars View Post
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.
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.

 
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