Moisture Meter Technology

Capacitance and Resistance

What’s the difference between pin meters and pinless meters?

Wood moisture meters use two primary methods of determining moisture content: Capacitance and resistance.

Resistance Meters (Pin Meters)
Commonly known as a pin meter, this type used to be the most widely used meter throughout the world. Two or more pins are pushed directly into the wood in two different spots, and a direct current travels from one pin to the other. This process measures the resistance to the current between pins and correlates that resistance to wood moisture content.

Electromagnetic Field Technology (EMF, or Pinless Meters)
Often called a pinless meter, this type measures the moisture content of wood without piercing the wood with pins. An electrical wave is emitted through a sensor pressed against the wood. This creates an electromagnetic field (EMF) the size of the sensor to a depth of .75” to 1.0”, depending on which model is specified. The field behaves differently depending on the amount of moisture present in the wood.

EMF meters are based on the capacitance method, but a properly designed meter will take many more factors into consideration. These meters measure the capacity of wood to store energy (capacitance), the amount of power the wood absorbs from the field (power loss), or the wood’s resistance to the field (impedance). They translate this electrical information to a percentage of moisture content.

Here’s a short list of factors that affect each type of meter:

EMW Meter Pin Meter
Temperature No Yes
Chemicals No Yes
Wood Orientation No Yes
Moisture Gradient No Yes
Wet Pockets No Yes
Wood Species Yes* Yes**
Wood Density Yes* No
Surface Texture Yes No

*Certain EMW moisture meters correct this either digitally or using a lookup table.

**Some pin meters may correct this using a lookup table.

Some factors on this list may be familiar. Since EMW meters read a large 3D-volume average of wood MC, the orientation of the wood won’t affect their readings. Pin meters are affected by wet pockets, even if the surrounding wood is significantly drier, and may give a higher MC reading due to the smaller path of the current traveling through it. Temperature can also affect the resistance of wood measured by pin meters.

From the above list, you may have already developed a healthy skepticism regarding the accuracy of pin meter readings. Although some woodworkers may rarely have to deal with chemicals, the factors affecting pin meters are the most difficult to assess.

Surface texture, density, and species may either be assessed visually or will already be known. Wet pockets, however, are impossible to correct with a pin meter. On the other hand, some EMW meters will read up to 1-2% lower on extremely rough surfaces as you begin to measure the air trapped their instead of the wood itself. On most boards, simply pressing the meter firmly against the wood negates this factor. But on extremely rough wood, you may want to add 1% to your reading.

LEARN MORE TO ENSURE YOUR PROJECT QUALITY:

Additionally, keep in mind this little-known yet significant factor —Many pin moisture meters have a cord connecting the pins that are hammered into the wood (the models with pins and meter connected can’t be driving far enough into the wood to get an accurate reading). On these particular meters, try simply changing the position of the cord and watch the readings change! It would be almost amusing if the consequences of inaccurate readings weren’t so serious.

8 Comments

  1. Phill Anton

    Hey there, I am looking to harvest some downed logs from our land and would like to find a sensor that will give me a rough idea of moisture %. The sawmill that will be cutting and kiln drying the wood for me has a moisture meter but I would like to be able to tell with certainty when the wood I have is under 25% roughly (big discount for under 25%).

    I realize moisture % will vary quite a bit throughout the log but a rough idea would be great. An instrument that can do this with a library of wood species within/around $200 would be great. I am new to the world of rough cut lumber and milling. Any suggestion on which Instrument might be good for my application would be much appreciated. Thanks a lot!

    Reply
    • Jason Spangler

      I recommend the MMC220. Its proven to be what they say it is…fast and accurate. I can change the species settings to anywhere from 7-30%. I’m also very impressed with the level of customer service Wagner Meters staff offers on the phone…You don’t see that a lot these days.

      Reply
  2. Ken Watson

    I am looking for a chart/graph which compares the electrical resistance of wood (in ohms) with its moisture content as a percentage. I plan to use a normal insulation resistance tester with a pair of probes having about 10mm separation between them. Any help/advice would be appreciated.

    Reply
  3. Ken Munson

    I am looking for any application of a hand held moisture meter being used to measure content of logs being delivered to a mill wood yard.

    Any suggestions?
    Thanks,
    Ken Munson

    Reply
    • Jason Spangler

      There are some follow-up questions that would have to be answered first in order to completely answer the original question.

      If you are expecting to get accurate readings for green lumber then there is no meter that we are aware of that can do this. Since most logs delivered to the yard fall under the green category this question may not even be answerable.

      However, if you do get dead or dried logs, here are the questions that would need to be answered first.

      1.What moisture contents (MC) would you be looking for? Anything over the fiber saturation point of about 30%MC is not going to be read accurately by any meter known to us.
      2. What are the minimum and maximum diameters of the logs?
      3. Are the logs debarked?
      4. Would the logs be fully accessible, or stacked?

      Since pinless meters rely on a flat surface to take accurate readings, the round surface of a log prevents this. the depth is also a factor so diameter of the log comes into play. If there is bark on the log a pinless meter is useless until the bark is removed and the logs would have to be fully accessible regardless. We doubt a pinless meter will work very well for this type of application.

      You would have to use a hammer type pin meter. These pin meters are cumbersome but with insulated pins, bark would not be an issue. The curve of a log is also not an issue but the diameter could be as pins can only reach so deep (about 3″ max). To truly check the total moisture content of the log, the core of the board would need to be reached with the moisture meter probes. Accessibility is an issue also. The other issue with a pin meter would be that if any pins were broken in the log, the area would have to be marked so the pins could be removed before any cutting was done to the log. Otherwise, there would be the danger of a saw hitting the broken metal pin and breaking or otherwise causing damage or possible injury.

      The application is workable only under narrowly defined conditions that I am unsure even exist in most mill wood yards.

      Please contact Wagner Meters if you have any questions.
      [email protected]

      Reply
  4. Andrew Kaminker

    Moisture meters are sometimes used to measure the surface wetness of concrete slabs before installing adhered flooring materials. This should not be a substitute for measuring the moisture content or relative humidity within the concrete slab. There are different ASTM tests for doing that. It is critical to measure the intermal moisture content or RH, not just the surface wetness of the slab, or flooring failures may occur.

    Is there any information on:
    a) how moisture meters work when used on concrete slab surfaces(do they electrically measure the surface electrical resistance or resistivity?);
    b) to what depth they are measurng the moisture;
    c) what their accuracy is when used in this manner;
    d) is there any recognized Standard (such as ASTM) covering the use of moisture meters on concrete.

    Reply
  5. Ronald Mukalazi

    would like to no whether coffee moisture meters use the same EMF technology and would moisture in the surrounding/ atmosphere affect the mc reading.

    Reply

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