Understanding Moisture Readings

7/20/2020 (Permalink)

I was told of a very successful window salesman.  He would use a ‘Heat Loss’ detector to show the prospective buyer how much heat his existing drafty windows were losing.  As he waved the instrument across the glass, the needle would jump alarmingly! So, obviously new windows will make a huge difference, practically pay for themselves, right?  Perhaps.  But you wouldn’t see a difference in heat loss using the same instrument.  He was using a light meter, measuring light – nothing to do with temperature.

Similarly, it helps to understand the instruments we use to measure moisture.  A surface can feel dry to the touch, but still contain and conceal underlying moisture content.  Instruments are necessary to detect this hidden moisture so we can determine what is necessary to dry it, and confirm that the material/space is indeed drying.  

We can use a Forward Looking Infra Red (FLIR) camera to view an area and determine at a glance what is wet.  Typically, wet areas will show up in shades of blue with dry areas showing orange to red.  But while the areas are indeed wet, what is actually being measured is evaporation.  Evaporation lowers temperature and the FLIR camera can detect this temperature difference between the wet and dry areas.  You could be misled if you don’t understand this.  For example, in winter, a poorly insulated – but dry, exterior wall will have areas cooler, thus blue, than the surrounding (orange) areas of adequate insulation.  The camera shows blotches of blue in an orange field even though the wall is dry.

Other instruments measure electrical conductivity.  Water readily conducts electricity.  So an instrument using (invasive) metal pin probes will detect current readily flowing from one probe to the other.  For example, wet drywall will readily conduct electricity and the more saturated the material, the better the current flows from one pin to the other, and the higher the corresponding reading of moisture.  However, if the probes hit an underlying metal corner bead, the current will also flow through the metal and result in a corresponding ‘wet’ reading.

There are non-invasive instruments that measure conductivity with a localized charged field they create.  However, a hidden metal pipe, or metal concrete wire reinforcing will conduct current and register a false reading of ‘wet’.

Lastly, these meters typically have a ‘test’ button to determine battery status.  If the batteries are good, pushing this button will move the needle accordingly.  But pushing this button while probing will also result in a fraudulent reading of wet. 

I once noticed an episode of NCIS where they were using a yellow moisture meter, similar to one we use, to measure radioactivity!  Of course they were merely pushing the ‘Test’ button.