Hygrometer

[Delwin D Fandrich]

----- Original Message -----
From: "Farrell" <mfarrel2@tampabay.rr.com>
To: <pianotech@ptg.org>
Sent: August 30, 2002 4:34 PM
Subject: Re: Hygrometer


> Hi Greg. I am not trying to directly measure moisture content (MC) of the
board. Intruments for measuring wood MC only go down to 5 or 6% MC. When you
are down at the extreme of most any instrument like this, the inaccuracy
goes way up. Del has mentioned that they do not work well at these low MCs.
---------------------------


The general rule around here--and around here there are folks who measure
wood moisture content for a living--is, "they don't work below 6%." In
reality, though, they begin to get erratic and unreliable some above that.
It's inherent in their nature.

All wood moisture meters work on the principle of measuring the electrical
resistance of the wood in some way. This resistance varies with the species
of wood and with the amount of water bound in the wood. This is typically
done in one of two ways. The most common is to insert two pins into the wood
and measuring it directly by passing a current through it at some known
voltage and comparing its resistance to some known standard. The other,
still less common but growing in popularity, is to expose the wood to an
alternating electromagnetic field and measuring the effect the wood has on
that field. With either type of moisture meter, the closer you get to
oven-dry the more inaccurate they become.

We have a very nice--very expensive--Lignomat moisture meter that spends
most of its miserable life in the bottom drawer of the tool cabinet. We rely
on checking the temperature and humidity of our conditioning room every day
and comparing those two readings to a chart which tells us the equilibrium
moisture content of wood at that temperature and humidity. We then slightly
adjust the temperature of the room accordingly.

Del