[Strawbale] Moisture sensor redux (was Re: cement stucco problems?)

[Rob Tom]

On Sun, 15 Oct 2006 03:11:58 -0400, Mark Bigland-Pritchard  
<mark at lowenergydesign.com> wrote:

>>> Have a look at the (CA winery) document - it's on the EBN site.

>  have a look at the photo of the sensors that John used - I suspect that  
> only ceramic capacitative sensors could be made that small.  Unless you  
> know otherwise.

Ah, okay. *That* winery document.
It turns out that it was already on my HD and I hadn't looked at it yet  
until now and then, only just a quick glance.

I know that people's eyes tend to glaze over and their mouths yawn  
whenever the subject of moisture monitoring gizmos comes up so I'll try to  
keep it brief.

I'm guessing that Mark's "smallness of capacitative sensors" query has to  
do with a comment I made recently to HW on the SB-r-us list; something to  
the effect:

"Why bother futzing trying to re-invent the wheel with moisture sensors  
when the CMHC-style wood block sensors do the job just fine ?"


But first to answer Mark's question on "size":

The wood blocks themselves (in the CMHC-style sensors) are actually very  
small -- a wood disk the size of a 25 cent piece in the case of the "first  
generation" type for which Habib Gonzalez provided fabrication notes -- or  
a wood plug that can be smaller than the first segment on one's baby  
finger, in the case of the "second generation" type for which Rob Jolly  
provided fabrication notes.  [Both of the preceding sets of notes with  
images are stored in the FILES section at SB-r-us]

If the wood blocks were any smaller, I suspect that most baleheads would  
have difficulty getting their fingers to behave during DIY home  
fabrication of the sensors.

Even at their comparatively "bulky" size (when compared to a capacitor) it  
requires a drastic change in headspace from the everyday (unless one is a  
watchmaker or such-like) to develop the fine touch necessary to drill the  
tiny wood blocks, insert the two even tinier bolts/nuts/washers and  
connect the tiny small gauge telephone wires.

Any bulk the sensors may have is due more to the protective sheath around  
the sensor itself, the sheath being made from readily-available, stock  
plumbing parts that in many cases, people would already have, as a result  
of home plumbing jobs that they've done .

Even so, the entire sensor assembly including protective sheath can be as  
small as the cap on a fountain pen (actaully, a bit shorter in length) ,  
which, in relation to the size of the bales into which it will be  
inserted, is negligible IMO.

What is more relevant (again IMO only) is that (from my cursory  
understanding of them) capacitative sensors can only yield humidity  
readings whereas the CMHC-style wood block sensors give direct readings of  
moisture content.

Humidity readings are by nature (again IMO) useless without the  
corresponding temperatures at which the humidity readings were taken.

Further, humidity  readings can only give one a picture of the  
instantaneous and transient conditions unless one has some means of  
continuously logging a series of temperature and humidity readings over a  
long period of time and that usually means additional gizmology which in  
turn means a source of power to run the gizmology which in turn means that  
if for some reason the power is interrupted, data is lost and you have  
nothing.

[ Interestingly, almost as if on cue, the grid-supplied electricity to my  
home just blacked out as I wrote and re-wrote the above.]

And humidity *is* relative.  ie 90% relative humidity at minus 10 degrees  
C represents very little actual water in terms of actual %moisture content  
and would pose no threat whatsoever to bales.    OTOH, 90% relative  
humidity at 30 degrees C represents a great deal more actual water in  
terms of %moisture content and could be a threat to the health of the  
bales in one's wall.

I say "could be a threat" because it would depend upon the duration of  
those high humidity/temperature conditions and if there was any sort of a  
regular pattern of those conditions over the long term. Small spikes over  
the short term are to be expected with plastered walls exposed to wetting  
and not really anything to get worked-up about.

OTOH, with the CMHC-style wood block sensors, the reading that is yielded  
gives you an instantaneous reading of the longer-term health of the bales,  
which is, what I suspect, most SB homeowners are interested in when  
monitoring their walls.


PS: The "redux" descriptor in the subject heading refers to the moisture  
sensor development discussions that were carried out on the CREST SB list  
long ago and I didn't fully realise how "long" ago that was until  
yesterday when I went for a walk in the woods with Don (Fugler) (and Don  
chased a poor widdle porcupine up a tree and then got us lost).

He (DOn that is, not the porcupine) reminded me that the last time we saw  
each other was over 10 years ago and it was a number of years before that  
when I met with him the time prior, to discuss the moisture sensor  
evaluation study.

===* ===
Rob Tom
Kanata, Ontario, Canada
<archilogic at chaffyahoo dot ca>
winnow the chaff from my edress in your reply