While the overall article is good I wanted to point out two things. In the first picture with the "inspector" using an IR gun, while the scale of distance can not easily be determined, if you are more than a few inches from the surface being measured, you probably did not get a good reading. IR guns are relatively accurate at inches but not at 10 to 20 feet. I would not accept the temperature taken in this picture.
Back in the days of mercury thermometers, slings were generally fairly accurate. If you have a thermometer that uses the red spirit instead of mercury, they are relative accurate up to about 85F. Over this they are almost always wrong. Two ways to determine this, on a hot day, read the dry bulb temperature before the sling and then after the sling. Dry bulb should not change but I bet it did. Second method. Check WB and DB the normal way. Then redo the test by putting the sling in front of a fan. again, you will get a different number. In hot weather stick to an electronic meter.