[GSBN] dry basis moisture content

[Bob Theis]

Thorough explanation, Derek. 

So if you're really not all wet, what's your oven dry basis moisture content? 

Bob
On Jan 30, 2012, at 7:41 AM, Derek Roff wrote:

> To state the same concept a little differently, we are comparing the weight (actually, the mass) of the water in a sample to the weight/mass of the non-water.  If we want to state the moisture content of a strawbale, we compare the mass of all the moisture (water) in the bale to the mass of everything that remains after the moisture has been removed (the dried straw).  Since the two parts are being measured independently, they can have any ratio.  If a given 40 pound bale contains 22 pounds of water and 18 pounds of dry straw, then the moisture content is stated as 122%.   The simplified formula takes the mass of water divided by mass of dried straw, times 100 (to get percent).  (22/18) x 100 = 122%
> 
> This method of analysis is called the "dry basis", or sometimes "oven dry basis".   In practice, a small sample is weighed, then it is heated in an oven until all the moisture is driven off, and then the sample is weighed again.  The final weighing yields the weight/mass of the dried straw, and by subtracting that from the starting weight/mass, you can calculate the weight/mass of the water that was driven off in the oven.  
> 
> Some industries use the "wet basis" of calculation, which compares the weight/mass of the water to the weight/mass of the whole (damp) sample.  The moisture content of grains, such as rice, wheat, oats, etc, is sometimes stated using the wet basis.  The maximum moisture content possible would be 100%, using the wet basis method of calculation, as Martin mentioned.  But this approach is not used for wood or strawbales used in building.  If you have a bale moisture meter, you may find that it has a way to switch between the two methods, dry basis or wet basis.  SB builders should choose "dry basis".  
> 
> Neither of these calculation methods has anything to do with volume.  But volume does show up in the SB building codes.  If a code specifies that a bale must be at least 8 pounds per cubic foot (pcf), then volume is involved.  The advantage of measuring pcf is that you can get a quick, rough measurement in the field, with a tape measure and a bathroom scale.  The disadvantage is that the approach isn't a direct measure of anything meaningful.  It is supposed to guarantee that the bales are dense enough to be physically strong.  However, it may just be measuring that the bales are overly damp.  If you pour a quart of water into a bale, you won't make it stronger, but you will increase its measured pounds per cubic foot.  For this reason, the codes are likely to specify the minimum pounds per cubic foot within a certain moisture content range.  A bale with a 10% moisture content and a density of 8 pounds per cubic foot is likely to be compressed enough to be sturdy and good for building.  
> 
> Derelict "I'm not all wet" Roff
> 
> Derek Roff
> derek at unm.edu
> 
> 

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