[Stoves] [Fwd: Better than Spit Moisture Meter]

[Thomas Reed]

Dear Stoves/Gasifiers:

We are indeed fortunate that wood is porous and that nutrients and water 
can climb trees.  Unfortunately, when we use wood for energy the water 
is INVISIBLE and we need to know the moisture content.  The best way to 
find it is to weigh the wood, then heat it to constant weight at ~220F 
in an oven and MC (wet basis) is

    MCDB = 100*(1-drywt/wet wt)%

However, that takes time and is tedious, so I was glad to find a 
moisture meter at Harbor Freight (see below) that measures it 
electrically.  I promised to calibrate it this weekend.  I took 5 random 
samples from a bag of moist chips and measured them electrically, and 
then by oven drying them. 

Sample      Wet Weight    Dry Weight    Meter reading      Oven method   
Error (1-M/Ov)*100
1                           4.137         3.529                  14      
                  14.6                 4%
2                           7.713         4.423                  35 
(max)             42.6               18%
3                           0.991         0.801                  13      
                   19.1               42%
4                           8.966         7.689                  12      
                  14.2                 15%
5                           6.020          5.535                 <7      
                     8.1                

I conclude that the meter is qualitatively accurate in the middle of its 
range.  The worst error was on the smallest sample.  The instructions 
say to insert the probe 1/4' into the sample, so this is hard with a 
small sample....  Get bigger chips!

Onward

TOM REED       BEF/BEC


Dear Friends of Biomass/wood:

Biomass has many excellent fuel properties - renewable, carbon neutral, 
low cost - but one problem is that it is by its nature highly absorbing 
for water.  Live biomass typically contains 50% moisture by weight AND, 
you can't look at a piece of wood and judge its water content.   It will 
dry relatively easily to 20% moisture in any climate, but only in Denver 
here do we enjoy 7% "Denver dry" wood.

The founder of the Biomass Energy Foundation, Harry LaFontaine, tells 
that at the beginning of WWII he and Niels Bohr developed wood gasifiers 
when the Nazis took over all liquid fuels.  They set up a supply system 
of bags of wood blocks (usually Beech), match box size, that had to have 
less than 20% moisture by weight and they had uniformed inspectors who 
went to the stores to make test the water content by weighing, then 
heating to 110C and weighing again. 

Halfway through WWII someone developed a simple "spit" test.  If you 
spit on one end of the wood block and could blow bubbles from the other 
end it was <20% MC.  The testing police disappeared overnight. 
------------------------------------------------------------------------
Today we like more quantitative results and don't spit in public.  
Yesterday I was overjoyed to find a two pin Moisture Meter for wood at 
Harbor Freight, about 2"X6"X1", with a nice leather case.  It is made by 
Cen-Tech, uses a 9 V battery (included) and costs ~$14.  
(http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=96472)  
It has a low (0-17%) and high (17-35%) range.  I haven't tested it 
quantitatively yet, but one  of my very dry (EdBurtonCompany) Chunkettes 
measured less than 7%.  I wetted it and it read 16%.  I'll equilibrate 
some bone dry wood and test it quantitatively. 
------------------------------------------------------------------------
Dry wood is not always best.  In some cases wood with 10-15% moisture  
burns more efficiently than bone dry wood because the flame zone is 
localized, rather than allowing the whole log to evolve volatiles faster 
than air can mix and burn.  (Measured by Tom Miles's nephew.)

Onward...

TOM REED               BEF/BEC



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