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

[Mark Bigland-Pritchard]

Responses in text below
Mark


Rob Tom wrote:

> 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.
>
OK, I exaggerate.  I went back and looked at the Rob Jolly pics, and see 
that the sensor housing is about 120mm long.  About a quarter of the 
width of the wall.  (Or about a third if you have bales on edge.)  The 
Rob Tom pics suggest a total length of about 70mm (but don't indicate 
how to insert it accurately).  So better than I said, but (i) it is 
still going to change the heat and moisture flows through the wall more 
than I feel happy with (especially if your plastic housing is 
aluminium-lined), and (ii) you end up unable to pinpoint exactly where 
in the bale you are measuring anyway.  (The latter usually won't matter, 
but it might sometimes.)

[The housing I have used for ceramic sensors is not a lot smaller than 
that, but that's only because I swapped from timber capacitative sensors 
to ceramic capacitative sensors partway through the process and had a 
load of plastic tubes and bits of wood to recycle into something useful.]

> 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.

Clearly John Barlow prefers a datalogger.  What's your answer, John?  I 
can think of a few for myself (dataloggers have better memories than 
Marks, possibility of automatic calibration adjustment, can take 
readings when you're away), though my next project will use direct 
measurement on a multimeter.

> 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.
>
OK, so maybe a single calibration will do.  But how many people are 
geared up to do even that?  (or are you telling me that you trust the 
adjustment figures for different species that the moisture meter 
manufacturers may or may not include with their product?  I don't, 
because the species of wood isn't the only factor which will affect it)

>> 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.

As I say, I don't know the price in North America.  It might be a lot 
less, it might not be.  [I do know that a wall-moisture Protimeter cost 
me nearly GBP200 in Britain a few years ago, and you're claiming the 
equivalent (if it is the equivalent) can be had for a fifth of the price 
over here, Rob.]  But either way, as a proportion of the total building 
cost, it's peanuts and it shouldn't deter anybody.

>
>> 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).

Actually the mass of water (or specifically the mass of water bound to 
the bale) does vary with temperature.  Most changes in moisture levels 
are very slow.  So it is reasonable to assume that - except for the most 
rapid changes in moisture content, say those from a sudden leak - the 
straw in the bales is in all places in equilibrium with the air around 
it (inside the bale).  So as the temperature goes up, the r.h. of the 
air goes down, so moisture evaporates from the straw into the air to 
restore the equilibrium.  (This is, for example, why someone from rural 
Saskatchewan complained to me the other day that towels aren't dried 
properly in Ontario - he's in for a shocking experience if he ever makes 
it to Britain or to coastal BC...)
The sorption isotherms that have been determined so far suggest that 
there is very little change in the r.h.-m.c. relationship as temperature 
changes.  There's no reliable data for sorption isotherm variation with 
temperature for straw that I know of (and it's clearly not an easy 
procedure - I've read of one failed attempt to do so using state of the 
art equipment at the Danish Technical University).  But I have on file a 
1986 collection of sorption isotherm measurements (also from the DTU) 
which includes graphs for "wood" (doesn't specify what sort, but the way 
it's laid out suggests the same sample was used each time) at 20, 40, 60 
and 80 degC.  At about 80% equilibrium rh, the moisture content 
measurements are given as follows:
  20 degC:  rh=79.1%,  mc=14.1%
  40 degC:  rh=79.1%,  mc=13.3%
  60 degC:  rh=79.2%,  mc=12.6%
  80 degC:  rh=78.9%,  mc=10.6%
So some variation, but not very much - and very little over the range 
which is of interest to us (say 0 to 40 degC)
So, to summarise, we can assume that (i) there is a direct relationship 
between r.h. and m.c. in straw, (ii) the constants in the equation 
describing this relationship will change a little bit with temperature, 
but not very much.

But I've so far failed to mention the big error in your argument, Rob.  
You are measuring moisture content in timber.  You want to measure 
moisture content in straw.  If you compare sorption isotherms for straw 
and timber you will find that they are not the same.  So to get to the 
figure you want, you need to use the relevant timber sorption isotherm 
(which you won't necessarily know accurately enough for the sample you 
have) to convert the meter reading to an equilibrium r.h. value, then 
use a sorption isotherm for straw to convert the r.h. value to a straw 
moisture content value.  This strikes me as just a little cumbersome.  
Better to just read off the equilibrium r.h. in the first place.

>
>> 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.

If it's somewhere between the extremes you quote, Rob - say 17% or 20% - 
then a few percentage points _do_ make a big difference.  And so can 
temperature.

Of course it would be entirely possible for said homeowner to think in 
terms of equilibrium r.h. instead of straw moisture content - risk 
starts somewhere between 75% and 80% for local temperature of 20 to 30 
degC, somewhere between 80% and 90% at 10 degC, etc.

I suppose the big question then is along the lines of:  I have had 80% 
rh in my walls every night for the last week, at temperatures of 15 
degC.  What do I _do_  to reduce the risk of mould growth?

In the end, though, Rob, you're right to suggest that many homeowners in 
many locations will neither need nor want a great deal of precision.  If 
you're in a place where the "background" wall moisture content - as 
dictated by diffusion/adsorption/desorption processes - is low, then a 
high value will be all the more noticeable and it should be possible to 
troubleshoot before it gets serious.  If, however, you live in Cardiff 
or Bristol or Manchester, and diffusion/adsorption/desorption processes 
set a background wall moisture content within a few percentage points of 
the danger level at some times of the year, you need to be a bit more 
careful and notice all the relatively small building flaws before they 
have a chance to cause problems.  Or, likewise, if you have 17 people 
living in your house, all with a liking for cooking gourmet soups with 
the pan lid off and a tendency to shower 5 times a day.  Or, for that 
matter, if your building is a winery rather than a house.