Hygrometer

[Farrell]

I feel like I should know this, but I don't. Using the chart for EMC based on sets of RH and temp., why does the species of wood apparently not affect the EMC value? Isn't the EMC a percentage of the weight of the wood? So shouldn't wood density affect EMC? Or is EMC a percentage of wood volume? Or should I just go back to making my hot box and follow the friggin' charts?

I think maybe I'll fire up the grill, go get some meat, bread and a bottle of wine. Talk with y'all tomorrow or whenever!

Terry Farrell
  
----- Original Message ----- 
From: "Delwin D Fandrich" <pianobuilders@olynet.com>
To: <pianotech@ptg.org>
Sent: Saturday, August 31, 2002 12:06 PM
Subject: Re: Hygrometer


> 
> ----- 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
> 
> 
>