Re: moisture measurement

[Strawnet@aol.com]

While I'm feeling flush (throwing around my few cents and perhaps scarcer
sense) I think I'll share a few thoughts on moisture testing that have been
racing around in my head and finally reached escape velocity.

Back to a question brought up a little while back, I think by wwb Tom (what
his name sounds like under water - too much :-) ), we (Matts and I) bought
one of those cheapo soil moisture meters and it absolutely doesn't work for
straw.  We tried several - one he had, one I had and the one we bought -
since we wanted to be sure it wasn't just that our old ones weren't working.
 Couldn't even get a reading on a bale that was far from dry, though not
"sopping - heading for slime - wet".  

I'll send Ross Burkhardt (ARE YOU OUT THERE ROSS?) a private e-mail message
to see if he has done any work toward figuring out the circuitry of these
moisture contraptions.  What I do know is that the better straw and hay
moisture meters cost a lot of money because they probably have a somewhat
limited market, they need to be fairly accurate (I sort of disagree with
whoever said they thought we didn't need very accurate readings for moisture
- depending upon what you are intending to do with the information.  If you
are using it to tell whether you are in the dangerous zone for the potential
moisture damage that might occur in the walls of your house - well - how good
would you want that info to be?  That might be at your personal discretion.
  If you are trying to determine the actual density of bales to use in a
loadbearing structure, and you need to know how much of the weight of the
bale is water and how much is compressed straw, then I think most of us would
want accurate info).

I wonder if there really was a way to make an accurate meter, cheaper than
the ones available out there commercially, wouldn't there be one we could
already buy?  On the other hand, there are some really smart people on this
list and maybe we could invent one online, start a new business and make
enough money to pay someone else to do the testing.   

I realize we could make one of a kind meters that will no doubt work, but to
do serious research (ok I don't mean :-( SERIOUS research) we need some good
standards and we need to know that Rob's (wwb's) 28% and Kim's (the
untarnished) 28% and DEsert Dave's (certainly tarnished) 28% and Toby's  well
you get the idea ... are all the same 28%.  Otherwise we know a lot less than
we think we know (we already know that don't we?).

One other thing, which I think someone posted a while back, is a discovery
Carr Everbach at Swarthmore College in Pennsylvania told us about when we
were in Kingston, NM at the Natural Building Colloquium/Straw Bale Gathering.
 Basically, it uses the innards of a microwave oven (properly sheiled of
course) and a temperature probe.  You drill a small hole throught the plaster
(he says this thing will work on plastered walls - with or without stucco
netting or expanded metal lath) so you can measure the temperature in the
straw - which you do first.  Then you apply a certain amount of microwave
energy to the wall, using this device.  Then you measure the temperature rise
in the straw.  The microwaves only heat water.  The temperature rise is
directly proportional to the amount of moisture present in the straw.  Carr
said they had already done some preliminary testing and it works and he
thinks it can be very accurately calibrated to quickly give moisture info in
any straw bale wall, finished or not, with only the need for a 1/2 inch or so
diameter hole for the temperature probe.  Pretty cool/hot idea eh?  We should
get Carr on this list.  He is the professor at Swarthmore whose class built a
straw bale building on campus last year.  There is a web page on this
subject.  Search for Swarthmore and  Strawbale on the www.

Finally, some thoughts WRT to the mineral content of water, the relationship
of humidity (moisture in the air spaces in the wall actually, in this case,
as opposed to the moisture content of the straw itself- though they are very
much related) and moisture content as a percentage of weight of the straw -
in terms of how they relate to the readings from resistance type moisture
meters:  

First, they actually are reading the ability of somewhat wet straw to conduct
electricity.  They will not read moisture content of air.  And when they are
calibrated, one presumes that this is done by testing actual straw that has
been wet and is at various levels of moisture content.  The question of
whether different circumstances within the wall (mineral content, etc.) will
affect the accuracy of the readings is a valid one, but one which may be
impossible to answer except in the most general way or only with very
specific information about a particular situation.  Water condensed out of
the air should be fairly pure water until it is contaminated by what is on or
in what it condenses on.  This could be an area of legitimate research,
although it is more esoteric than anything that really interests me at the
moment (or in any other conceivable moment in the future given my current
state of mind).

 Typically, these meters have a metal tip, an insulated (electrically that
is) section of the shaft, and then the rest of the shaft is metal.  They read
the amount of electricity that is conducted through the straw between the two
sections of metal on the shaft.  There are two other factors that affect the
reading.  They are the density of the straw (how much contact, both surface
area and pressure, is made between the straw and the shaft of the meter) and
the temperature.  Matts has both meters at the moment and thus he also has
the literature that came with them, but they do mention temperature
correction factors as I recall.  Matts and I also discovered that if you take
a handful of damp straw and stick the probe into it, you can "adjust" the
reading by how tightly you squeeze the straw around the shaft.  These are all
confounding factors to getting "highly accurate and consistent data".
And that's with the best commercially available meters.  Well, that's more
than enough for now and my relative humility is at a tolerable level now, so
I can quit.  

DEsert David Eisenberg (under his own steam still)