Infrared Thermometers.. Can Cause Confusion

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There are three reasons why I don’t like infrared thermometers for many HVAC tasks.

#1 – The Laser is Misleading
The laser dot is just a point of reference, not an exact point where it is reading. Often the thermometer will read lower, higher or over a MUCH wider area. Unless you are right up on what you are measuring you can’t be sure the result you are getting is correct.

#2 – They Only Read Surfaces
An infrared reads surface temp only, not air temp. This is necessarily a problem, but “shooting a vent” is not the same as measuring the air temperature coming out of it.

#3 – They Can be VERY Inaccurate
Basic infrared thermometers are only accurate on a surface that has high “emissivity” of near 1.0. These are usually darker, less reflective, generally non-metallic surfaces. Metals have a low emissivity (much less than 1 generally) which means that if you are reading a pipe an infrared could read much lower than the correct temperature.

Infrared thermometers can be useful to do comparisons where reading the correct temperature is less important than comparing one spot to another, such as looking for hot spots in a panel, or checking a zone to see if a damper is open.

So long as you use the right tool for the job you should be fine, but in general….

I don’t like techs using infrared thermometers for most tasks.

— Bryan

P.S. – While I don’t generally like infrared, I REALLY like thermal imaging. Check out these nice products from Trutech tools 

3 comments

  1. Avatar Robert Gore says:

    THANK YOU! I’ve told two different bosses that swear by IR guns exactly what you’ve stated about emmisivity. They looked at me like I was crazy until I told them shoot the supply air diffuser and I’ll use my Cooper air probe. Eye opener for them!

  2. Avatar Bill Spohn says:

    Good solid info.

    Regarding point #1, better IR thermometers actually paint points on the diameter or a circular ring with the laser pointer, to show you the target area, not just a center dot.
    Even the inexpensive testo 805i smart probe IR thermometer paints a target ring.
    http://www.trutechtools.com/Testo805i

  3. Avatar emjayw71 says:

    Most IR thermometers have the emissitivity preset to 0.95.

    It is possible to buy IR thermometers that allow the user to input the expected emisitivity factor for the surface whose temperature is being measured.

    If you do your research, you can buy such IR thermometers with this feature for under $20!

    Indeed, I own 2 of these style of IR thermometers. That said, it required then to access a table of emissitivity factors for given surfaces and to then take the time to set the correct emisitivity factor before taking measurements. This will significantly improve accuracy.

    Given that most techs will not bother to lookup emissitivity tables and program their IR gun prior to use each and every time, I look at IR guns as a means of monitoring systems roughly.

    For example, I am performing a service and I want to test that the system is working in both heating and cooling. By pointing my IR gun at supply air registers, I can get a quick feel for if the temperature is falling or rising as well as which supply registers are at least working as well as a reasonable comparison of supply air temperatures to the return air temperature.

    As long as users understand and work within the limitations of using an IR thermometer (ie.. accuracy), they are still a useful and timesaving tool.

    The problem is when techs do no understand the inherent limitations of these tools and wish to rely on their readings as accurate representations of temperature.

    Understanding your tools and how they work is part of being a good tech.

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