New weather stick finding

Weather sticks in our test chamber have plenty of outside air (with water vapor) continuously circulated around them but they are protected from liquid water (rain drops) by the design of the air handling system.

The term EMC stands for the Equilibrium Moisture Content, and it is expressed as the percentage of the weight of the water in the wood to the weight of the dry wood.  From the University of Minnesota, Forest Products Management Development Institute we learn:  "Wood located outdoors, but which is protected from rainwater, commonly attains an EMC of 14-18%.  The EMC will be lower during prolonged dry periods.  Under warm humid conditions, as in a green house with a relative humidity of 90%, wood will have an EMC of about 20%.  This is an important point, and so will be repeated.  Again, if wood is in a greenhouse condition with a very high relative humidity the EMC will only be at 20%, sometimes as high as 25%.  Therefore, the moisture within such wood will be held in the cell walls, and none will be contained in the cell lumens.  Once the wood has been dried to a low moisture content, liquid water will not be found in the cell lumens unless the wood is exposed to liquid water".

In other words, as applied to our weather sticks, this suggests that under normal humidity cycling over a period of time for a sheltered location protected from rain, water will be gradually lost from the cell cavity and it will not be replenished by returning to high humidity.  It appears that the bad-weather-deflection depends on the total water in the stick, that is water in the fibrous cell walls plus the water in the inner open space of the cells. This suggests that in the event that the weather stick has been protected so that liquid water (rain) cannot reach it, over time the overall weather stick deflection in the high relative humidity direction may be reduced by as much as 20%. When the stick has the opportunity to absorb liquid water (rain) the larger deflection will be restored.

Deflection loss over a period of a few weeks has been measured in the test chamber.  When liquid water from an ultrasonic humidifier was introduced for half a day, the lost deflection was recovered, since differential expansion with water content between normal cells on top and compression cells on the bottom of the stick depends on the total water content, which in turn depends on water being held by the cellulose fibers that make up the cell walls plus water in the tubular cell cavity.

January 2003 DHR

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