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Thread: Non-radio electromagnetic radiation question and some deep thinking.

  1. #1

    Default Non-radio electromagnetic radiation question and some deep thinking.

    Prepare yourselves for what might be a totally dumb question! I am posting this non-radio question because it is electromagnetically involved, and although it has nothing to do with radio communications, I think there are enough smart people here to give me the DiNozzo treatment and show me what I'm missing here.

    Setting the stage: Its a perfectly calm night with clear skies and -9F outside. I have a brand new infrared (laser sighted) thermometer with a temp range of -56F to 718F having an alleged accuracy of 2.7F. In and around the house, the device performed absolutely amazing. Everything it showed was on par with the digital wall thermometer and my highly accurate distillation thermometer. So far so good.

    I shouldn't have, but I went outside and started pointing it at things... My outer wall reads -17F and both my trucks (measured on the fender) and snow plow blade read -46F. Now, I know there is a difference between air temps at certain heights above ground and differences in surface temperatures for various materials because they lose their energy at different rates due to their different thermoconductivity and specific heat capacity. But I always assumed all the surfaces would eventually, given enough time, reach the temperature of the surrounding air. Aside from the trend that surfaces with greater thermoconductivity seemed to be colder, I initially assumed my new toy was broken.

    Then, the science nerd in me attempted to create a logical explanation for the great differences in temperature readings having no expectation to discover one ~ but, after a few captain cokes, I actually did think of one. Unfortunately, I have no clue if the theory holds water because my calculator broke an hour ago. Step one was to consider how the planet drops in temperature, specifically on nights like this. The answer is a simple one ~ the infrared radiation leaves the surface of the earth and has no clouds to reflect back to earth on giving us these exceptionally cold nights (I know, -9 isn't cold, but we do see -40F a few times a year here in northern MN and -30 is way too common). Step 2 never came as I was hung up on step one. Is my truck any different than the surface of the earth in that respect? Temperature is just a means to measure energy contained in molecular vibration, and like a car out of gas, these little molecular structures want to slow down if nothing stops them, right???

    So, my super dumb question of the night is: Is it possible that the black-body (or whatever you physicists call it) radiation is causing a loss of molecular energy from the material (truck) faster than the non-moving and slightly warmer air can transfer it back to the material (truck)? Is my truck body really that cold? Or, is the fact these materials are more thermally conductive having that serious of an affect on the IR thermometers accuracy? My glass thermometer on the deck (right next to the air temp sensor, but sitting on a bucket lid, read -16F (remember, the other one says -9). That is on par with the wall temp the IR gave me but my glass thermometer does not go low enough to put it on the truck at the expected temp to confirm that one.

    Someone please break this down for me Is it possible to have an air temp of -9 and have a vehicle fender be at -46?????????

  2. #2

    Join Date
    Nov 2016
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    In a word, emissitivity

    Simply put, infrared thermometers must be calibrated for the material they are measuring.
    -Jeff NE1U

  3. #3

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    Thank you!

    I missed emissivity day in school! New plan... I am going to buy various rolls of black tape and see which one is closest to E=0.95 (what this IR is calibrated to) so I can just toss a piece of tape on whatever it is I'm commonly measuring, otherwise, this $25 was a total waste. I guess I will have to get a new thermocouple for my DVM and go around with dabs of thermal paste if I ever want accurate results.

  4. #4

    Join Date
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    At work, we had one of those laser range finders. One of the guys bought it with the idea that they could measure the surface height of water in wells and bore-holes, instead of dropping a tape measure down.
    It worked fine on most solid objects, but failed miserably on water

  5. #5

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    Good to know! I used a fishing line, bobber and sinker to to check our well by feeling when the bobber took on the weight of the sinker.

    At least this emissivity thing can be readily corrected for. Luckily, it seems to be accurate pointing it at water, which is one of the main reasons I bought it. I should have bought one that can be programmed. Unfortunately, I dont think I will be using it on my glass distillation apparatus or on my hotplate testing salt melting points without some prior testing. Maybe a spot of soot or something other than tape will give me a decent target to check hot surfaces. Lesson learned lol!
    Last edited by brandon lind; Sat 11th Jan 2020 at 18:18.

  6. #6

    Join Date
    Oct 2009
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    Quote Originally Posted by brandon lind View Post
    Good to know! I used a fishing line, bobber and sinker to to check our well by feeling when the bobber took on the weight of the sinker. !
    What we now use is a tape measure with contact on the end. When it hits the water surface, a box at the other end of the measure goes "Beeeeeeep!"

    Then again, I could always try and sell you one of our environmental monitoring stations that would monitor the water level, give you the measurements online and send an alarm if the level gets too low or too high ;D

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