Replies

1. 
   bill_1010 | Oct 05, 2002 09:51pm | #1

   that time factor has many varibles. rough wood, infinished wood, oil finish, varnish, lacquer, etc. And then you figure in the number and thickness of the coatings.  I think that would be a hard number to come up with.

1. 
   LD | Oct 05, 2002 10:44pm | #2

   The chart was based on unfinished wood.  I would be shocked if there wasn't some info someplace in a wood tech book, I just don't have one available.  Also,  even though a finish will retard moisture migration, I don't believe most finishes will prevent the migration, only slow it down.  If moisture migration could be prevented by finishes, there would not be a concern with cross grain joints.

2. 
   StanleyNiemiec | Oct 06, 2002 07:27am | #3

   Besides time, there are other variables that must be considered as well -- 

   The steepness of the gradient (amount of difference) between wood MC and ambient relative humidity (EMC conditions).  If there is a 5% difference between wood MC and EMC the rate of MC change will be greater than if there is only a 1% difference.

   Type of wood is another variable.  Different woods have different diffusion coefficients.  A comparision between red oak and white oak is a good example.

   Air movement is also a factor.  If the air is static, the rate of change will be less than if there is air velocity/movement.

   Finishes, as indicated by others makes a difference.  The Moisture Excluding Effectiveness (MEE) of different finish types and the number of coats vis a vis vaporous moisture readsorption has been studied and published by the FPL. 

   And finally there is the issue of hysterisis.  Wood readsorbs moisture at a slower rate than it desorbs moisture, because energy must be applied to break the inter-cellulosic H+/OH- bonding sites to provide a place for the water to bond.   The higher the temperature, the more rapidly this will occur.

   Your question is theoretical and very general, what specifically do you need to know?

1. 
   LD | Oct 06, 2002 06:09pm | #4

   I believe my question was couched in general terms.  I understand that the specifics of a particular wood, it's species, it's surface treatment, finishes, length, and any other variables that enter into the picture would affect the results. 

   However, the referred to chart gave a best fit curve together with what is either outliers, or a standard deviation relationship to determine outliers.

   My question is, is there a similar curve for time?  In other words, if you change the RH environment of a generic piece of wood by 5% how long does it take for that piece of wood to reach moisture equilibrium?  If the environment is changed by 25% RH how long does it take?  Etc.  That info should be available someplace.  I was just wondering where.

   Specifically, I have read that wood should be acclimated in your shop for 2 weeks before using.  I'm questioning that time element.  I keep my shop between 50 and 60% RH.  I also keep some wood outside in a protected shed.   If I notice that the external RH has been at 50% for the last three days, is it necessary for me to acclimate the wood I bring into the shop for 2 weeks before using, or would 1 week do?  An RH, time chart similar to the RH moisture chart I referred to earlier would be beneficial in determining that time element.

   Thanks for your response.

   Dave

1. 
   StanleyNiemiec | Oct 07, 2002 03:57am | #5

   Dave:

   Within the USDA publication Dry Kiln Operators' Manual (DKOM) there is an extensive table of temperature/relative humidity conditions and the resultant wood EMC values.  If you can somehow find a copy of that book, it would provide you with all the details you seek.

   However it cannot give you the time requirement  --  as I stated previously, wood species (with differing diffusion coefficients) and things like gradient and air movement will all make a difference. 

   And as I think further about the problem, wood thickness is another factor.  It is reasonable to state that the time required to equilibrate 8/4 stock will be >4 times the time required for 4/4 material.

   If this is a critical situation for you (eg a business where you are moving large volumes of material and have limited inside space available for storage), then I suggest you obtain a moisture meter.

    

1. 
   LD | Oct 07, 2002 05:07am | #6

   Thanks for the info.  No it's not that big of a problem,   I was asking more out of curiosity and hoping to learn more about wood technology.

   Thanks again, Dave

1. 
   LeeGrinding1 | Oct 08, 2002 05:34pm | #8

   Here's a more complete chart. Space restrictions prevented the publication of the entire thing.

   EMC Chart

   For practical purposes a couple of weeks of acclimation is enough as long as the air is allowed around the lumber and the gradient isn't too high. In real life an MC difference of a point or two doesn't cause much effect.

   I will stress that if you're briniging air dried lumber into the shop for the first time weeks of acclimation would be in order. Most areas of this country will only get down to 10% or 12% EMC and even that requires timing. Once the lumber is brought into the shop it will lose several more percentage points depending on the time of year.

   Oh, it was issue #151.

   Lee Grindinger

   Furniture Carver

2. 
   jonsherryl | Oct 08, 2002 03:33pm | #7

   Dave, I think Stanley has given you the relationships. You can't compare RH without considering temperature, because RH is a measure of the atmosphere's capacity to hold additional moisture, which is based on both its temperature and the amount of moisture it is currently carrying. I'm no mathematician, but I think you'd need a three dimensional chart to express this relationship, since you are simultaneously altering two variables relative to a third (time.) 

3. 
   LD | Oct 08, 2002 05:53pm | #9

   Thanks to you all for your responses.  I appreciate them.

   Dave