Are there any accuracy studies done for EMC wafer devices? I am on the fence on just how worthy these are.

(This question was received from a mill worker and is posted here for the benefit of others in the industry.)

I am not aware of any studies regarding the accuracy of EMC wafers to measure kiln EMC (equilibrium moisture content), nor did a quick look at literature find any studies.  These wafers consist of a cellulose fiber pad held between electrodes.  A voltage between the electrodes is applied and the current through the cellulose fiber pad is measured.  The cellulose fiber pad is hygroscopic, meaning it picks up (or loses) moisture from the air, so the current varies depending on the moisture content of the air (that is, EMC).

As pointed out in the book Quality Drying of Hardwood Lumber (get the pdf at http://www.fpl.fs.fed.us/documnts/fplgtr/impgtr02.pdf)
 "Like the wet-bulb wick, the cellulose wafer or pad becomes dirty and must be changed at specified intervals for the EMC measurements
to be accurate. "
Additionally, others have pointed out that given their exposure in the harsh environment of the kiln, wafers and their electrical connections are susceptible to the corrosive effects that can take place leading to inaccurate reading of kiln conditions.