need to measure wood humidity, is it a matter of resistance?

[Bus Master]

Hi there,

I need to build a wood humidity measurement system using uC. I think it's a matter of wood resistance between two fixed distance points. I expect to measure for a 10k ohms to 40 Mega ohms range. The question is: i need a circuit to convert the resistance petween two probes to an analog voltage from 0 to 5VDC.


I've searched for an ohmmeter or DMM circuit diagram with no results.

Anybody can help?

Thanks in advance.

 

[mdamdam]

Yes,
Professionals uses resistive measurement for humidity in wood. ( also in wall for detecting water leak )
MDamdam

 

[flatulent]

try this

Use an ac current generator to stimulate the electrodes. Then measure the voltage between the electrodes. Amplify the picked up ac voltage and rectify it. Have steps of AC gain. Use a comparator to measure the output voltage being between two limits. The uC then steps the AC gain until the voltage is in the limits and then reads the voltage with an ADC. The ADC reading and gain range will be used to calculate the water content.

As a refinement, have the AC source have several values so that it does not become limited by the power supply voltages.

 

[Analyzer]

Hi,
This reminded me school days.It is a very common experiment in school.Google this please :
"measure humidity of wood"
Analyzer.

 

[jourval]

Re: need to measure wood humidity, is it a matter of resista

Bravo, Flatulent!
Very short, but comprehensive analysis.
The only additional point - if you want to eliminate
influence of local wood density variation you'll need
4 probes (crossed)

 

[Bus Master]

Re: need to measure wood humidity, is it a matter of resista

In fact, I'm looking for a schematic for this part of design. Can anybody help me?

Thanks in advance,

 

[Oslie]

Re: need to measure wood humidity, is it a matter of resista

A classic work in measuring wood moisture is "Electric Moisture Meters for Wood" by William James. It can be found at:

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr06.pdf

This paper gives the theory behind different moisture meter, but no schematics. Fine Woodworking magazine ran an article a number of years ago on building an analog moisure meter, it might be a worth a look.

One of the problems can be that the range of wood resistance vs. moisture content can range from 600 ohms to 700,000 Meg ohms (that is not a typo).

 

[marie65]

take a look at **broken link removed**.
I uploaded there a small file where you have a theoretical background regarding the RH wood measurement. It's not trivial to do it. The resistance (for the same RH) depend on specie, temperature. The relation between RH and resistance is "accurate" only in the range 4-5% to 30% wood humidity. Measurement below 5-6% are very dificult to be done due the high resistance of the wood (over 1 - 2 Gohm). At this resistance the insulation of the cables and of the PCB board have to be taken in consideration.