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Thread: How to check your digital thermometer for accuracy

  1. #1
    Moderator scooter's Avatar
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    How to check your digital thermometer for accuracy

    Let me first say that I am striving to learn how to be a pitmaster that doesn't need a thermometer to tell me when the meat is done. To know when the meat is done by sight, smell and mostly by feel is my goal. I'm a ways off from that point yet so to help me I rely on digital thermometers to give me thermal information that I can use to help understand, to a point, when the meat is done.

    Most digital thermometers are fairly accurate. Some are hyper accurate and it usually depends on how much you spent on your unit to how accurate (and fast) it is. In my mind, the most important first step a pitmaster can take with a new digital thermometer is to know how accurate it actually is and there's a simple test any pitmaster can do to determine the accuracy of your thermometer. If you can boil water, you can do this test.

    First take a pot and boil some water, when you get to a rolling boil stick the tip of your thermometer into the middle of the boiling water and wait for the display to stop changing. Depending on your thermometer, it should stabilize after 5-20 seconds unless you have a Thermapen which will stabilize in 3-5 seconds.
    It should read 212 degrees or within 1 or 2 degrees either direction. If it reads 212 +/- 2 degrees your unit is pretty darn accurate, however, there's a way to determine precisely how accurate your thermometer is. Most pitmasters are satisfied at this point but if you really want to know how accurate your thermometer is, pour out the pot of water, fill it with filtered water, start heating it and read on.

    There are two factors which will impact the accuracy of the boiling water test. Atmospheric pressure and altitude.
    Quote Originally Posted by From about.com Chemistry
    The boiling point of water depends on the atmospheric pressure, which changes according to elevation. The boiling point of water is 100°C or 212° F at 1 atmosphere of pressure (sea level), but water boils at a lower temperature as you gain altitude (e.g., on a mountain) and boils at a higher temperature if you increase atmospheric pressure (lived below sea level).

    The boiling point of water also depends on the purity of the water. Water which contains impurities (such as salted water) boils at a higher temperature than pure water. This phenomenon is called boiling point elevation, which is one of the colligative properties of matter.
    So, if the temperature of the boiling point of water changes with altitude and atmospheric pressure, how can you know what temperature the water in your pot will actually boil given that atmospheric pressure changes almost constantly? You will need 3 websites (only two if you already know the elevation of your house) to gather the information you need.

    First stop is to find the elevation of your house. Google Earth will tell you the elevation of any point on the earth. You'll need to download Google Earth if you don't already have it on your PC. It's a fantastic piece of software and I use it all the time. Open Google Earth and type in your address and hit enter
    1) Place your mouse pointer right over your house
    2) The elevation will read at the bottom. In this example it reads 186 feet of elevation


    Now that you have the altitude you need to know the atmospheric pressure in your city. There are many ways to get this but I like Intellicast - Local and National Weather Forecast, Radar, Maps and Severe Report. Just type in your city and state and read the pressure displayed. In this example it reads 29.58" inches of pressure.


    So now you have the two pieces of information necessary to determine the accurate temperature that water will boil in your house at the time of the test. Now you need to calculate what that temperature is. Scientists can probably do the math in their heads but I prefer to use a free boiling point calculator on the Thermoworks website. Go to ThermoWorks Boiling Point Calculator and click begin


    Now enter in the elevation and pressure information you collected and click enter


    The correct temperature that water will boil given your elevation and current atmospheric pressure will be calculated and displayed. In this example it is 211.12F. If your thermometer reads 211.12F in your pot of boiling water then you know your thermometer is spot on accurate. Some digital thermometers will allow you to calibrate them. That means you can adjust them to read 211.12F right there during the test. If yours doesn't allow calibration you will simply have to do the math in your head. If your digital thermometer read 213.12F during the above test, you know you'll need to subtract 2 degrees to get the accurate temp of your briskets and butts.


    There's also an ice slurry test you can use. We know that as surely as water boils at 212F, it will freeze at 32F. Take a coffee mug and fill it to the top with crushed ice. Put just enough water in the cup to make a slurry. Dip your thermometer probe into the ice slurry, it should read 32F. I admit I don't know the effects of altitude and atmospheric pressure on the ice slurry test. I just through it out there for you to discuss amongst yourselves...
    Last edited by scooter; 03-17-2012 at 02:40 PM.
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  2. #2
    Senior Member RickB's Avatar
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    And anyone who actually goes through all this needs to talk to thier therapist!
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  3. #3
    Moderator TentHunter's Avatar
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    Good info Scooter!


    I follow similar steps, so I must need a therapist (my wife & daughter already say I do, and now so does Rick ).

    You can also go to Wikipedia.org and search for your town/city (or the one nearest to you). They usually list the elevation on the right-hand side. Subtract 1° for every 500' of altitude.

    Example: Where I live is about 1000' above sea level (974' to be exact) so water will boil at approx. 210°

    Of course this doesn't account for atmospheric pressure changes from weather, but gets you pretty close.
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  4. #4
    Moderator scooter's Avatar
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    Yeah Tent, I guess we both need therapists although I've had other people tell me that from time to time!
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  5. #5
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    Wow way to involved. I check the accuracy with my hot tub water. It is always at 104 and so is my thermometer. Test done. I have a thermoworks tw8060 with the pro bbq kit. I have a buddy that has been a chef for 30 years at Ruth Chris. He will tell you that yes he can visually thell you exactly how done the meat is but he still checks them all. It sounds like a concept but I would rather be safe than sorry. I do not know how many people actually follow the 40-140 rule in 4 hours but I do everytime. I do not want to get sick.
    Last edited by rsp076; 03-18-2012 at 12:33 PM.
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  6. #6
    Senior Member MossyMO's Avatar
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    So, do competition smokers recalibrate for each competition location because of change in elevation?

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