Wednesday, November 30, 2022


RVelectricity – How infrared temp guns work


By Mike Sokol

Dear Readers, did a bit of backpedaling last week when we inadvertently published a quick tip with misinformation in an RV Daily Tips newsletter about “other” uses for an infrared temperature gun. This was a followup to my JAM session on finding overheated shore power connections where I casually mentioned that one of these infrared temp guns would be ideal for finding overheated shore power plugs, as well as dragging brakes, hot wheel bearings, etc.

But the followup post mentioned that it could be used to measure the temp of your air conditioner, and savvy readers quickly responded that an infrared temp gun WILL NOT measure air temperature, so it’s a lousy instrument for checking air conditioner operation. And they were 100% correct. So here’s a followup to the followup where I demystify how infrared temp guns work, and what they can and cannot do.

It’s getting hot in here…

First of all, here’s what an infrared temp gun looks like. Most of them have a handgrip like a pistol (hence the name “gun”), with a readout screen on the back and a laser pointer on the front. But it’s not quite as simple as “point ‘n shoot.” You first have to understand how infrared temp guns work in general, then you need to know the distance to spot diameter ratio of your particular make and model infrared temp gun. Fun learning ahead, so don’t be scared.

Infrared heat guns don’t read the temperature of the air at all like a regular thermometer would. No, what they’re looking at is something called black body radiation. Basically, as any object is heated above absolute zero (–273.15°C or –459.67°F) it begins to vibrate at a molecular level. And that vibration can be seen as a color.

You may have noticed that as you put a metal poker in a fire it will glow red from the heat. As it gets even hotter the color will change to orange and then yellow. Blacksmiths use this visual color scale all the time to figure out just how hot a piece of metal is in a forge prior to beating it into shape.

But this color scale also occurs in the infrared region. And infrared is simply a color that’s so low in frequency that we can’t see it with our own eyes. But we can easily build an electronic eye that can see that deep red color and correlate it to a temperature above absolute zero. For best accuracy the object should be dull in color to begin with (black body, get it?), so that hints at the first limitation of an infrared temp gun. They won’t do well finding the temperature of anything shiny like a mirror or even the polished aluminum of your Airstream trailer. But for everything else that’s a little dirty or dull in color they can be really accurate.

Next, that little laser beam that’s pointing at the object you’re measuring isn’t really a pinpoint measurement that the laser dot would suggest. Nope, the infrared temp gauge is really like a flashlight beam in reverse. That is, the farther you are away from the object you’re measuring, the bigger the area that it’s going to measure. And if there’s one hot-spot in the middle of a bunch of warm areas, the hot-spot temp won’t be measured at full temp. The meter will indicate the “average” temperature of the measurement spot diameter, and that’s dependent on the D:S ratio (distance to spot ratio), as well as how far you’re away from the object to be measured.

So an infrared meter with a 12:1 D:S ratio will “see” and measure the average temperature in a circle of about 1″ diameter when you’re 1 foot away. Measure something that’s 2 feet away and it’s “seeing” a 2″ diameter circle. And 3 feet away from the object you’re measuring will find the average temperature of a spot diameter around 3″.  A temp gun with an 8:1 D:S ratio will read a 1″ diameter spot at 8″ away. And one with a 16:1 Ratio will read a 1″ diameter spot at 16″ away. So the higher the D:S ratio number, the further you can be away from an object that you’re trying to measure. Get it?

Math alert…

If you try to measure the temperature of a 1″-diameter bolt from 24″ away using an infrared temp gun with a 12:1 D:S ratio, it will average the temp of the bolt that has 0.78 square inch of area (remember, area of a circle is Pi times the radius squared), with the entire 2″-diameter circle having an area of 3.14 square inches. Since the area of your hot bolt is only about 1/4 of the entire spot that’s being measured, its temperature will only contribute about 1/4 of the average reading. So if the bolt was actually 200 degrees F, and the surrounding object was 100 degrees F, then the temp gauge might read only 125 degrees. And that could give you a false sense of confidence.

So what to do? Well, first of all know the D:S ratio of your particular infrared temp gun, and try to keep it within that distance from the object you’re measuring, depending on its size. So a 1″ object should be measured from a distance within 8″, 12″ or 16″ depending on your infrared temp gun’s D:S rating.

Of course if you’re standing near a hot spring at Yellowstone Park and you’re 10 feet away, then the 10″ diameter measurement spot is going to be all the same temp, and you’ll get an accurate reading that will wow the crowd. So have fun with it, but at an appropriate (and safe) distance.

Want to learn even more about how infrared temperature guns work? Then read this article from Fluke, which is written in very understandable language. See, I told you not to be afraid.

Let’s play safe out there….



Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at For more info on Mike’s qualifications as an electrical expert, click here.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.


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2 years ago

I own an infrared temp. Gun and use it frequently to check our ac temperature, water temperature, among other things. It cannot read ambient temperatures, however, it will read the temperature of the vent where the air is flowing, which is relatively close to the ambient air temp.
I work for a D.O.T., where when I was on the highway construction site, managing asphalt paving, we use these temp guns, continuously, and they are calibrated to be extremely accurate, and the accuracy with the range, is dependent on your environment and surroundings. I have accurate measurements at about 10-15 ft all around, and they are a great tool to have. Especially in the middle of summer, here in the NW, pavement temperatures correlate with ambient temps., and I inform people when they are walking their dogs on asphalt, and how it burns their paws. I am a believer in them and use it for numerous things.
Oregon RVer.

Tommy Molnar
2 years ago

You’re right, Mike. The Fluke article makes it understandable how these things work. So I quickly went out to my shop and retrieved my cheapo Harbor Freight infrared thermometer. No mention of a D:S ratio. Of course, I don’t expect anything from Harbor Freight to be great, but it WOULD be interesting what they at least “claim” that ratio to be. No dice.

But what I DID get from your article is, get as close as possible to the object to be measured and maaaybe it will be close to accurate (at least with this cheesy unit). No more standing in the house and hitting the fence with the laser – ha.

3 years ago

I can & do, in fact, measure the air coming out of my air conditioner, at least, in a relative sense. It’s really pretty simple. I hold the laser temp gun close to the exit air vents & aim the red laser dot in between the vent slots. The laser reflects off of one of the exit vent walls inside the air conditioner where the cold air is travelling past. I wait until the air conditioner has been running for at least 15 minutes, so the spot I’m measuring is close to the same temp as the air passing by it. I typically get a reading of around 35-45 degrees. If I try it too soon, the inside walls will not be as cool as the air passing by & it will measure 10-20 degrees warmer. But having done this dozens of times over the years, I now know what temp number to expect to see & how long to wait before taking the measurement. From that relative measurement, I can tell if the air conditioner is working properly. I’ve also learned that extremely warm days, 95 deg+, will produce slightly warmer temps, because an air conditioner can only cool the air a fixed number of degrees below the temp of the air coming into the intake. Once the intake air gets cooler & cooler, then the exiting air gets cooler & cooler.

Gene Bjerke
3 years ago

So it sounds like you will get the most accurate reading by being as close as possible to what you are reading.

3 years ago
Reply to  Gene Bjerke

That would be the simplest explanation…lol.

Also, these guns can be refocussed for a tighter beam similar to optical focussing. The reason they aren’t crazy tight OEM is because then you could completely miss smaller anomalies. I sweep suspect areas and then drive down the messurement cone (approach) to the hottest/coldest reading.

Lawrence Fuchs
3 years ago

Thanks for that article. Never even noticed the label on the side of my temperature gun. But now I see it is a 12:1. It’s a very handy tool to have. When I was coming back down from Pike’s Peak there was a booth in the middle of the road where you had to stop and they would use one of these guns to measure your brakes. If you were over a certain temperature you would have to pull over and let them cool down.

Jeff Arthur
3 years ago

You can find frozen pipes. Thanks for the ratio explanation

Mike Sokol
3 years ago
Reply to  Jeff Arthur

Yes, it would work well for that as long as you’re close enough, probably 12” or so.

3 years ago

Thanks for clearing that up Mike! Now I shall go back and read the book on mine. Not a MANLY thing to do, but oh well! LOL!

Mike Sokol
3 years ago
Reply to  Ran

I promise not to tell anybody that you read the manual. 😁

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