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

[Rob Tom]

On Tue, 17 Oct 2006 16:15:35 -0400, Mark Bigland-Pritchard  
<mark at lowenergydesign.com> wrote:

> It seems to me that various things are required of a moisture sensor.   
> Which option you choose depends on how you rate these parameters:
> (1) accuracy of the sensor itself, across the full range of likely  
> humidities and temperatures
> (2) cost of the total system
> (3) robustness
> (4) size of sensor + associated protective layers (the bigger it is the  
> more it will affect wall hygrothermal conditions - especially if you  
> have a permanent tube going most of the way through the wall - and so  
> the less reliable the reading will be)
> (5) [where appropriate] ability to give a reading that can be  
> interpreted by a data logger for continuous monitoring purposes
> (6) anything I've missed?
>
> Now, the wood block resistive method is good on (3) and (if the cheap  
> meters that Rob mentioned before are reliable) very good on (2).  It  
> could probably be made good on (4), which makes me wonder why all the  
> assemblies I've seen, possessed or seen piccies of involve a plastic  
> tube going most of the way through the wall.  But it's useless on (5),  
> and not very good on (1).

Mark;

It appears that we agree on points 2 & 3.

On point 4, in particular, the length of the tube, I think that you are  
probably looking at some very, very old pictures. Even Rob Jolly's sensors  
which were used in his 1997-98 monitoring report were only about as long  
as your baby finger and there's no reason they need to be that long.

Obviously it's a bad idea to have a tube that runs through the entire  
thickness of the wall since you would be creating a possible conduit for  
heat and moisture.

If for some reason you wanted to have sensors at three locations in the  
wall profile (ie inner and outer skins and mid-bale) then you would place  
three separate small sensor assemblies at each location.

A single 6-conductor cable could be used to run out so that there is only  
a single penetration for the three sensors at the interior plaster to seal.

On point 5, I have to ask why a SB homeowner would want or need a  
data-logger.
All they are concerned about is whether or not their walls are healthy.

Watching the hourly fluctuations 24/7/365 would likely be as appealing to  
them as watching paint dry, not that there's anything wrong with that of  
course, if watching paint dry is up one's alley.

>  And the variability of wood  samples means that either each sensor has  
> to be individually recalibrated or else there is going to be a  
> significant loss of accuracy.

For a single building, I doubt that a person would use more than one wood  
species for the sensors. For the most part, pine has been the wood of  
choice.

> RS components in Britain are selling high-quality Honeywell ceramic  
> humidity sensors for under GBP20 each.

A stick of pre-ripped wood stock long enough to yield an entire houseful  
of sensors would cost $5 at most even if you were to buy it from a hobby  
shop. Making a similaarly sized stick of stock on one's home table saw  
 from scrap lumber might cost a nickel for the electricity to run the saw  
motor.

If a GBP is still around $1.50 CDN or USD, then we're talking about a  
significant chunk of change for a houseful of sensors. That would tend to  
be a deterrent for a good portion of the SBH-building population.

> I agree that r.h. readings are useless without temperature readings.  So  
> are m.c. readings.

On this I disagree.

%MC let's you know how much moisture is present in the straw in proportion  
to the dry weight of the straw and that % is independent of temperature.  
(ie The mass of water does not vary with temperature). It is the amount of  
moisture in the straw that one is concerned about.

Humidity only tells you the moisture content of the air at a given point  
in time and that can change several times over the course of a single day.

High humidty levels are only a problem when they are accompanied by warm  
enough temperatures to support microbial activity and it is the latter  
that is the bottom line that will be of concern to homeowners .

> I wouldn't, even in the simplest monitoring exercise, consider measuring  
> bale moisture >without also  measuring bale temperature.

Obviously, but why would a homeowner need to ?

If the straw is known to be at 30% moisture content as a result of a  
readings taken in June and December, then you know that the walls are in  
trouble.

If the straw is known to be at say, bewteen 10 and 15% (or lower) at any  
time through the year, then there's nothing to be concerned about.

A few precentage points either way isn't going to matter to the health of  
the straw.


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