Wednesday, February 26, 2014

Log Log

Yes it's time for another installment of my Log Log. If you look over to the links column to the left of this post, you'll see links to three previous thrilling episodes in which I find ingenious ways to show how much water dries out of my firewood logs.

My friend Rob has a log moisture content meter, it has two prongs you stick into the wood then a readout shows you the percentage of moisture.   You can get them for as little as £15 on ebay.  One amusing advert I saw said the meter gave a reliable and accurate reading of approximate water content.  So a sample  reading might be exactly about 20%  - or so :-)

Anyway, I thought it might be fun to make one of these gadgets.  The only problem is, I didn't know how they work.  My first idea was to see if measuring the electrical resistance did the trick, so I bashed a couple of panel pins into each of a couple of logs (one drier than the other) and stuck my multimeter across them.

 Here was the first problem.  Although the resistance was much lower in the wetter log, I couldn't get a stable reading, the number of Kilohms kept steadily climbing one log and steadily falling on the other although I confess I should perhaps have given it ten minutes to see if it stabilised.  Maybe there's some galvanic action going on.

Hmm, more to this than meets the eye. Time to consult uncle Google, and eventually I tracked down what I suspect to be the definitive work on these matters published United States Department of Agriculture Forest Service

Electric Moisture Meters for Wood by William L. James

Now I don't suggest you read this unless you are a sucker for technical matters and tables of data, but a cursory reading of it told me two things

1. Conductance or electrical resistance (the one being the reciprocal of the other) of wood wetter than 30% cannot accurately be measured as "conductance is only a weak and erratic function of moisture content greater than fiber saturation." or put more fully: " The direct-current conductance (resistance) of wood varies greatly with moisture content below fiber saturation. As the moisture content decreases from fiber saturation (about 30 pct moisture, based on the dry weight of the wood) to the ovendry condition, the conductance decreases by a factor of over 10 million . In this range of moisture content, a roughly linear relationship exists between the logarithm of conductance and the logarithm of moisture content. At moisture content levels beyond fiber saturation, the electrical conductance correlates very poorly with moisture content."   ( although why you should believe a man who can't spell fibre properly I'm not sure.)  Anyway, fresh logs cut from a tree (often gathered canal side by boaters) can be anything up to  140% saturated, which is to say  the water content weighs 1.4 times the dry weight of the wood.  Hence probably the erratic nature of my multimeter readings.  In a nutshell what this tells us is that moisture meters only start to be useful when the wood is getting towards dry enough to burn.  Useful in itself, but I like to know how far away from that my wood is, so I can estimate when it will be ready.

2. Results vary strongly with temperature, wood species, direction of grain etc etc.
So even when the meters are in their comfortable range, it still all depends on other stuff.  Now I understand where the accurately approximate claim comes from!

So I propose to stick with my tried and tested method of weighing a newly cut log, and writing the weight on the end, then weighing it to check every so often. Sometimes the simple cheap way is the best.

Seeing me writing this, Kath showed me a link to  really great post on wood drying by Mike on Nb Rose of Arden in which he finds out how to dry wood in a plastic bag.  Brilliant!  Much more useful information than my futile electrical experimentation.


Maffi said...
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Maffi said...

I know this might seem radical but I have a trivet on my stove upon which I place the next log to go on the fire by the time it goes in the fire the moisture content is too low to measure so I don't bother! :)

Halfie said...

Isn't that a bit of a fire risk, Maffi?