Verifying moisture meters' accuracy

A discussion of ways to verify the accuracy of moisture meters. Includes info on calibration. June 21, 2000

Q.
Is there a way to verify the calibration of a pin-type moisture meter?

A.
Yes. The meter is calibrated to read a certain percentage of moisture content (MC) when you put a known resistor across the needles. Each manufacturer has a slightly different resistance value. A full set of resistance values as a function of MC and species is in "Electric Moisture Meters for Wood" by Wm. James., U.S. Forest Products Lab Report FPL-GTR-6, 1988.
Gene Wengert, forum moderator


The manufacturer might supply a calibration module. It's a resister with a high value. You connect the resistance to the probe. The hard part is knowing where to turn trimpots to bring the meter into calibration, but the module should come with instructions.

In addition to tuning in my Delmhorst RC-1E with the 12/22 percent calibrator stick, I occasionally check my meters on actual wood samples that I know the exact moisture content of.

In other words, once in a while when I have completed a charge of red oak or soft maple, I'll oven-test wafers off of the samples (using two wafers - not sliced just off the edges either) to find out the exact sample moisture content.

Then I get to see just how my meter reacts to measuring the center segment of the sample.

I try and keep a piece or two laying around the office (which has 35 percent relative humidity) to use as a reference, if I doubt my meters for whatever reason.


From the original questioner:
After reading "Electric Moisture Meters for Wood," I have one question.

It mentions in one of the notes for Table 1 that the resistance is measured in megohms. Some species would have a resistance of 200,000 megohms or higher. Is this correct, or are the resistances 200,000 ohms?


That is correct, 200 billion ohms. That high resistance is why the pin meters do not work well under 7 percent MC -- the resistance is too high to be measured with inexpensive equipment.
Gene

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