I'm looking at building a wood moisture meter, there seems to have been some discussion on this in the past and a few attempts with different methods. This post seems the most promising:
I've attached a couple of breadboard photos, I dont have a 2M resistor so for now just used x2 1M resistors in series.
5V through resistor to A1
A1 then to probe
A1 also through a 0.1uf capacitor to Gnd
Gnd to second probe
Serial.println(analogRead(A1));
gives me a reading of 1020, if I touch the pins together reading goes to 0
If I put the probes into wood that is about 10% moisture reading the pin reading decreases slightly, across my finger it drops to approx 550.
Am I on the right tracks and is it just a cause of calibrating or am I well off the mark so far?
That might work, but I imagine that it would be very difficult to standardize and calibrate.
Conductivity does depend on moisture, but it is a bulk property of material. The value you get will depend on the distance between the probes, and the area of wood that is in very close contact with the probe metal.
You would have to standardize the probes and then verify that different wood samples with the same known moisture content give very similar readings.
Once that is done, to calibrate the sensor, you then need readings from wood with different known moisture contents, accurately determined by other means.
It is much, much easier to determine moisture content by weighing, then drying and weighing again.
The fractional water content of the wet wood is equivalent to "moisture level", but has no units.
Define "moisture level" in scientific terms, so that, if needed, a conversion factor can be determined. If it is percentage, multiply the fractional water content by 100%.
I hadn't ever really related it to being weight before, I just used a moisture meter with probes and could see it was 20% and after drying 6%.
100kg piece of wood original weight
After drying it weighs 80kg. You will never get it to 0% so though we know it's lost 20% of its weight not sure how that gives us what it's current moisture content is now.
Of course drying will never remove all the water as some of the water is "locked in" to the wood fibers. Same with drying food and any other biological material.
A conductivity meter will respond to both free and "locked in" water, so there is a fundamental difference between the two techniques. They both give useful information, and you can work out a useful, quantitative relationship between them.
If you have a commercial moisture content meter, why try to make your own?
Just saw your edit. I'd be making one mainly to be able to gauge when a slab will be dry enough to come out the kiln. Once it's out the kiln I'd still use the moisture sensor to confirm it is actually dry enough.
I think you're doing good, but water content in a material is usually measured in weight, as wood is tough and might damage your probes, so calculating water content by weighing your material is recommended.
The proper way to judge by weight is to have a standard time to heat and weigh the sample. Perhaps in one hour cycles to start with. I remember working in an asphalt roofing mill where the UL standards guy would do this type cycle to determine the moisture content of the paper felt that was the basis of the roofing material.
For anyone who is interested, this publication introduces wood drying methods, defines "moisture content" and discusses the difference between free and bound water.
The moisture content formula is presented above as the ratio of water weight to "oven-dry" wood weight, rather than the fractional water content that I mentioned in posts #4 and #6, but the two formulas can easily be interconverted.
*Moisture Content (MC) is the amount of water in lumber measured as a percentage of the lumber’s oven-dry weight. This value is calculated as the weight of the water divided by the oven-dry weight, times 100 to convert to a percentage.
The water content in a sample piece of wood should affect the capacitance of the sample.
Machinery's handbook will have some of the properties of wood but maybe not by wood species. It didn't 50 years ago, just hard wood, soft wood and grades of types. References like the old Audel's Guides may have tables with dry wood densities and strengths. There are standards to measure against and wood will change with humidity over time.
Dry one sample and soak, even boil or steam another until it bends easily (wood forming technique) and weigh those to get your own data. Get the capacitance of each and you won't need to cut into your pieces to judge. Wood is an insulator, the water has the capacity... gine wood that is not rotted or fungal infected.
My wooden doors swell with humidity....
Another test may be opacity to radio signal or changing magnetic field that a linear Hall sensor could measure.
With a small piece, temperature change after microwaving could tell but be aware that wood contains methanol, oils and other volatiles besides water.
