False Positives in Thermography
Full Disclosure: The use of thermography and thermal imaging is not perfect or 100% accurate.
As a Level III thermographer, I’ve taken countless (residential) images in just about every conceivable condition imaginable. In many cases, we encounter conditions which can only be troubleshot from the exterior side of the building envelope where the condition occurs, in other words, troubleshooting without destructive testing (removing wall finishes).
It is sufficient to say, that when we test the plumbing, we also take moisture measurements with a moisture meter and infrared camera periodically simultaneously. If an anomaly is noted we monitor it throughout the course of the inspection. This way, we can visually see if the affected area increases in the infrared image or not. Additionally, when using the moisture metering device, we’ll check to see of the moisture levels also increase.
In many cases where moisture is suspect, however, without actual water dripping from the ceiling (or wall) we can only presume that moisture is current and active.
For this simple example, we’ll use a state of the art moisture meter, state of the art infrared camera, a small piece of drywall, a steel bar and a sponge to illustrate my point.
To begin, well dampen the drywall with the sponge on the front side and the rear.
Measurements of Drywall Without an Infrared Camera
This image shows the left side as dry and the right side as wet (from sponge). Upon applying the moisture meter over the dry area one can notice that the reading is low.
This subject image This image shows a dry side on the left and a wet side on the right, however, this time the reading is elevated when the moisture meter is placed over the moist drywall area.
The third subject imagThis image shows the drywall with a steel plate placed on the top. The moisture meter is then placed on the top of the steel and measured. The measurement shows an elevated measurement almost equal to that of a wet surface, or in this example, wet wall board.
Measurements of the Interior Wall with the Use of an Infrared Camera
This image displays both a wet and dry area on the wall board. With infrared, the pallet chosen displays cold as purple or dark and dryer areas in lighter tones such as yellow.
This image displays In this image there is wall board with a moist area and a dry area (on the top of the drywall is a steel plate). Note that the steel plate is the same temperature as is the dry wall board.
This image displays In this image, the wall board is viewed from the backside (opposite as all of the other images previously). As visible, the darker discoloration is where the moisture was directly applied on the wall board.
With the Testing of wall board primarily (other materials offer different challenges), as you can see in the subject image 1.3 that metals within wall board materials such as metal screen material which is very common when repairing drywall patches and holes, metal corner bead and metal bullnose corner bead can read high, as a matter of fact, just as high as if the material measured was actually wet. Thus the term false positive can be applied.
To rule out a false positive conclusively, one must physically view everything on the opposite side of the materials measured.
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