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Old 10-29-2012, 08:31 PM   #1
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Default Thermometer question

How can we be sure our thermometer is accurate at 68 deg C. I ask because my former one (calibrated against a national standard thermometer at 70 deg C) has packed up, I bought a new one, but my friend who worked in a lab has retired, and I am too mean to pay six times the cost of the new thermometer to get it calibrated.

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Old 10-30-2012, 01:15 AM   #2
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If the endpoints are good, there’s a pretty good chance the middle is too. I use a cheap $8 lab type liquid thermometer. It reads a little low (-.5C) in an icebath and right on at boiling, corrected for barometric pressure and elevation. So I figure I can be within half a degree between 0-100C.

Occasionally I get all my thermometers together for a party and see how closely they agree, especially at mash temperature.

If you suspect your thermometer is wacky at mash temps and you just have the one you could make a quick and dirty temperature reference. Boil 700mL of water and add 300mL of liquid from a well stirred ice bath. It ought to be within a degree if you’re fairly quick.

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Old 10-30-2012, 12:17 PM   #3
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thank you for that tip - I will use 14L boiling and 6L ice water to minimise cooling effect and add ice water to boiling water and stir quick and well - will report back...

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Old 11-01-2012, 06:33 PM   #4
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If you're interested in half degrees (which you should be!) you should get some distilled water and freeze it.

Crush it up into a glass and let the ice melt itself- record the temperature of the ice melt.

Then, boil your distilled water and measure the boiling temperature.

Go online and google your elevation and humidity and find out what temperature distilled water should boil in your house. Then you'll have all the data you need!

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Old 11-01-2012, 08:23 PM   #5
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That’s one way to make an ice bath. Just be sure to stir it well. The maximum density of water is at 4C so you could have serious error with a static solution.

I’m not sure what you mean about a half degree. A proper ice bath is 0.0C, better than most thermometer accuracy and resolution.

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Old 11-01-2012, 09:01 PM   #6
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Quote:
Originally Posted by Wynne-R View Post
That’s one way to make an ice bath. Just be sure to stir it well. The maximum density of water is at 4C so you could have serious error with a static solution.

I’m not sure what you mean about a half degree. A proper ice bath is 0.0C, better than most thermometer accuracy and resolution.
The idea was half a degree at mash temps. While most thermometers aren't perfectly linear, if you get the two endpoints correct that's about as much calibration as a normal person can stomach. To really dial in the precision, you need to calculate where water boils at your altitude and humidity.
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Old 11-01-2012, 09:18 PM   #7
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Remember to add some pebbles or a pinch of chalk or something when you're checking your thermometer at a boil, else you could be way off. BTDT.

Also, if you're using a digital thermometer, it's hard to speculate on how linear it reads between 0F and 212F. Liquid-filled lab thermometers should be pretty linear though. And remember to only immerse them to the indicator line.

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Old 11-01-2012, 09:26 PM   #8
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We have had some issues with the Blichman therms. We have calibrated at freezing and it will be off at boiling, and vice versa. We didnt realize till after accidently mashing super low and getting a dry-ass beer. Now we are using 2 new ones and triple checking everything.

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Old 11-01-2012, 10:41 PM   #9
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Ok daksin, I see what you’re saying. SpeedYellow your points are well taken too. Digitals can be wacky in the middle, especially if the cable or the works have ever been wet.

I’d like to throw in a caution about the difference between resolution and accuracy. Even the much vaunted Thermapen is rated ± .7°F (.4°C). So if you carefully set your strike temperature to 163.0°F that means it’s somewhere between 162.3 and 163.7. With the equipment we are using ± 1°F is about the best we can hope for.

I hope we didn’t lose Bob with all this silly talk about Fahrenheit.

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Old 11-01-2012, 11:48 PM   #10
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Quote:
Originally Posted by Wynne-R View Post
Ok daksin, I see what you’re saying. SpeedYellow your points are well taken too. Digitals can be wacky in the middle, especially if the cable or the works have ever been wet.

I’d like to throw in a caution about the difference between resolution and accuracy. Even the much vaunted Thermapen is rated ± .7°F (.4°C). So if you carefully set your strike temperature to 163.0°F that means it’s somewhere between 162.3 and 163.7. With the equipment we are using ± 1°F is about the best we can hope for.

I hope we didn’t lose Bob with all this silly talk about Fahrenheit.
I'm a molecular biologist- you're right in that we don't want to get into precision vs accuracy. I think, personally, for homebrewing, most thermometers will be off as far as accuracy goes, but as long as your thermometer is precise, you can tune your recipes to your equipment and still make the beer you intend.
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