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  1. #1
    Join Date
    Aug 2010
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    Michigan
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    7

    Temperture Readings with IR

    OK, I see very little information out there about the use of Infrared Thermometers and/or how they may or may not be used with regard to things like HVAC.

    So, let me start with asking a question about wet bulb temperature. If I were to take a wet paper towel, or something similar... Place it in the path of some air flow, let it sit and allow evaporation to take place, then use a IR Thermometer to measure the temperature of that paper towel, shouldn't that give me the wet bulb temperature or it's equivalent?


    How about measuring the pipe temperature? if the focus of the IR is small enough and it's aimed correctly, shouldn't a person be able to read the temperature of some tubing/pipe?

    What about Outdoor Ambient temperature? If a IR meter is pointed at some object that isn't directly in the sun (not getting radiant heat from the sun) shouldn't that reading be very close to the Ambient temperature?

    Other then the wet bulb temp situation above, I would not expect these readings to be accurate if moisture and/or air flow is present. What other problems might exist in trying to measure dry bulb type temperatures as described above?

  2. #2
    Join Date
    Feb 2004
    Location
    Midwest
    Posts
    9,932
    IR guns aren't used that much in HVACR work when accurate readings are required for a lot of reasons. One has to do with the the emissivity of the target vs. the ability of the gun to read it accurately. The gun reading can be off by a mile for that reason.
    The other has to do with the size of the target vs. the size of the "spot" of the gun and the distance between them. (The most accurate readings are obtained when the round beam is no larger than the target). That can be a major problem if you can't get close enough to the target, or the target is simply too small.

  3. #3
    Join Date
    Jun 2003
    Location
    Hell Hole Swamp
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    4,180
    IR's measure surface temperature, not air, there are no uses for them in the field where accuracy is required

  4. #4
    Join Date
    Sep 2008
    Location
    Western PA
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    25,545
    Yep. I agree with all of the above. IR thermometers aren't very valuable for the average technician.

    IR thermometers are handy for distracting customers while you work, though.

    Also tend to keep the kids off your back when you're working around the house.

    They have 'niche' type uses, but not everyday service work.

  5. #5
    Join Date
    Jun 2010
    Location
    wa
    Posts
    53
    never use these to measure pipe temp they are very inaccurate.

  6. #6
    Join Date
    Jun 2003
    Location
    Hell Hole Swamp
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    4,180
    How many techs try using them to read supply and return temps?

    Ive seen quite a few

  7. #7
    Join Date
    Jun 2003
    Location
    Hell Hole Swamp
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    And lets not forget home inspectors

    I love writing up those counter reports where they stated the supply air temperature is too high

  8. #8
    Join Date
    Feb 2004
    Location
    Midwest
    Posts
    9,932
    Quote Originally Posted by Swampfox View Post
    How many techs try using them to read supply and return temps?

    Ive seen quite a few
    Measured a flue pipe temp with mine once and it was off almost 100 degrees.
    Learned that after attaching a probe to it (which I did after seeing how fast it burned my hand after I touched it, which I did after seeing that deceptively low IR gun reading).

  9. #9
    Join Date
    Dec 2005
    Location
    Vancouver BC
    Posts
    126
    So many people take the reading from their IR as gospel because they are too lazy to strap on a sensor. I use it to determine if hot or cold air is coming out of a diffuser 15ft up and that is about it. I have seen them used to read refrigerant line temperature and calibrate thermostats both incorrectly. I'd use a pocket thermometer to check superheat before I would ever trust an IR.
    ___________________________________________
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  10. #10
    Join Date
    Feb 2004
    Location
    Midwest
    Posts
    9,932
    I've read a lot of stuff where people stated that they got more accurate readings putting masking or elec. tape or even black paint over the target. Never worked very well for me. Galvanized seems to be the biggest problem for my gun.
    The reading you get is typically the most accurate when the emissivity of the target matches that of the gun. In my case the gun has a fixed emissivity. You can change it to match the target on the more expensive models.

  11. #11
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,545
    Quote Originally Posted by midhvac View Post
    I've read a lot of stuff where people stated that they got more accurate readings putting masking or elec. tape or even black paint over the target. Never worked very well for me. Galvanized seems to be the biggest problem for my gun.
    The reading you get is typically the most accurate when the emissivity of the target matches that of the gun. In my case the gun has a fixed emissivity. You can change it to match the target on the more expensive models.
    A whole h*ll of a lot easier to strap a sensor and some insulation on.

    Another little trick I use for the K-type wire probes is an alligator clip with the wire threaded through it. Holds them in place nicely.

  12. #12
    Join Date
    Aug 2010
    Location
    Swamp land
    Posts
    28
    You can use them when looking for air infiltration. As long as the color and surface is the same you can find them leaks. I’m not saying the reading you’ll be getting will be accurate. A good place to start looking for a leak is the vent fan in the bathrooms. The little swing gate flaps in a lot of cases are not closing or never got put in. Someone try it and give your fee back. They are also good for checking the trim of a door if it is air infiltration. You can also check wall outlets and missing insulation or sagging insulation in walls. Make the reading with the gun close to the target as possible. If it doesn’t work for you then you may need some practice until you get the hang of it. A high dollar IR camera would be the best tool of course.

  13. #13
    Join Date
    Aug 2010
    Location
    Michigan
    Posts
    7
    Quite interesting reading all of these opinions on the matter... The main reason I asked is because I was playing around with a thermistor type thermometer, standard household type thermometers, and various methods of trying to get a IR thermometer to read similarly to that of the other thermometers.

    What I found is that with a bit of practice, I could often seemingly get results with the IR Thermometer that were more or less identical to what I saw with the others. The main exception to this was with my attempts at getting a wet bulb reading.

    Now, mind you for wet bulb, my main method has been to put a bit of wet gauze on the end of my thermistor probe and let that cool down the thermometer as it sat inside of the return air. For one thing I noticed that I could wait as long as 15 minutes before my wet bulb temperature would reach the 'lowest' reading and/or stabilize it's reading. When I tried a wet paper towel in the return air, the reading got to it's lowest level and/or stabilized in as little as two minutes. However what really made me scratch my head is that the IR reading of the wet paper towel reading was typically about 4 degrees cooler then I ever read with the other method.

    Given that in most all other cases I could find a reading that was more or less identical to my other methods, I'm still scratching my head as to why the wet bulb temps I was trying to read were off by as much as they were. Maybe I'm going to have to find a black paper towel...

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