Originally Posted By: Caoimh?n P. Connell
This post was automatically imported from our archived forum.
As I mentioned in your other post, the 20% rule isn?t really a very safe bet; simply because fungi may proliferate even where the apparent moisture content of surrounding wood is less than 20%.
Regarding the accuracy of the meters ? the actual accuracy of the meter, in the context of fungal growth, is largely a moot point, since, oddly, the moisture measured by the meters is not the same concept of moisture that is used by the fungi. In microbiological terms, water availability (i.e., the water available to the organism growing on a particular substrate) is usually discussed as water activity, (aW
). Water activity is expressed as the ratio of the vapor pressure of the water in the air in equilibrium with the substance divided by the vapor pressure at the same temperature of pure water. Thus, aW
values range from 1.0 (for free standing water) to 0.95 for freshly baked bread, 0.85 for salamis and summer sausages to 0.7 for hard candy and dried cereals and dried fruits.
Water content of a substance is not the same as water availability. Two substances with the same water (moisture) content may have very different aW
values. Kiln-drying wood removes water which is tightly bound to molecular sites such as hydroxyl groups of polysaccharides, amino groups of proteins, and other polar sites, thus lowering water availability. Air dried wood does not release that tightly bound water since there is an equilibrium which exists between the water vapor in the air and the movement of water molecules in the substrate called the ?water potential? (a complex phenomenon dealing with the entropy of the water molecules that is beyond what any one here wants to hear about).
In general, pin moisture meters, by contrast, don?t measure moisture at all ? rather, they measure electrical conductivity. For example, I took my Tramex and placed the pins in 100% PURE water (ultra distilled water), and the moisture meter diligently read less than 10% moisture. Then, I slowly added table salt to the water, literally a grain at a time at first. As the salt content of the water increased, the moisture readings began to increase. Eventually, the meter reached a reading of 37% and did not increase any further regardless of the amount of salt in the water.
So, as always, it?s important to use instruments only within the limits of the purpose of the instrument and not attempt to extrapolate information beyond the capabilities of any instrument.
Just my thoughts!
Caoimh?n P. Connell
Forensic Industrial Hygienist
(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)