That would be the Nuclear Regulator Commission if memory serves me correctly. I used to be nuke certified with one of the consulting firms I previously worked for, and performed nuke surveys on everything from prison roofs for DOC in Virginia, to colleges, hospitals, etc…
The reason I like the infrared scan better is because you get to view 100 percent of the roof surface, and someone who knows what they are doing can disregard areas that glow and give false readings. Take the photograph below for example:
The roof surface has cooled down, and the underlying wet roof board insulation is glowing white (the squared off area in center of photo), but the pile of gravel also retains heat and is glowing too. Now, I knew the gravel was stockpiled and would retain heat longer than the rest of the gravel-surfaced BUR, so I disregarded those areas. I was, however, able to detect the wet insulation beneath the gravel, so I marked it and we indeed found water damaged insulation.
Now, take a look at the photo below:
The heat anomaly was an unusual shape and probably not wet. I still marked the area out of curiosity, but I was later proven right in that the area was not wet. Here is a photo of the same area in the daylight:
Now, when you do a nuclear backscatter scan of a roof, you mark a grid on the roof and take readings in specific locations. I can’t remember if the grid is usually 5 or 10 feet, but I believe it is every 5 feet. So you are taking reading in specific spots that may or may not be representative of the surrounding roof area. Alright, here is where my memory gets fuzzy, but when you push the bottom on the Troxler gauge you send out neutrons (heck it could be protons, or something along those lines) and they bounce back up toward the gauge off of hydrogen molecules. So, the more water (H2O) you have in a roof, the more neutrons (or whatever) bounce back up to the gauge where a reading is taken. The count of the gauge is representative of the neutrons or whatever that bounced back up (back scatter) and were counted. Now here is the problem, there is also hydrogen in asphalt, so in places where you have heavy pours of asphalt you are going to get a high reading. You have to plot that number on your histography, and unlike the infrared you can’t just ignore it. Also, if you have a 3’ x 3’ area of water collected beneath the roof membrane, and it falls in between your grid, you will not get a high reading and will be ignorant that you have water in that location.
Alright, I know I’ve probably given you bad information with regard to how exactly the Troxler gauge works, but the principle that I discribed is generally correct. If you look at the Troxler site, they say the top four inches are where they get the readings, and it is from gamma rays. I know the gauges I used didn’t rely on gamma rays, so things may have changed. Still, I prefer infrared over nuclear back scatter for finding moisture in a roof system. I do like the nuke gauges, however, for testing soil compaction.
FWIW, the reason you didn’t know I WAS nuke certified, is because you never asked and I never talked about it until now. If you have anymore questions about nuke and infrared, I’d be more than happy to answer them to the best of my knowledge.