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Lesson 2 of 7
Measuring moisture and sampling a charge
You cannot control what you cannot measure. This lesson covers the reference method, the meters, and how to sample a kiln charge.
The oven-dry method is the reference
The method follows the definition of MC. As summarised from the Wood Handbook (ASTM D4442 Method A, North America):
- Cut a full-cross-section specimen about 25 mm long, at least 500 mm from the board end, free of knots, bark and pitch pockets.
- Weigh it immediately.
- Dry at 101 to 105 degrees C until the mass stops changing between weighings (the sources give 3-hour or 4-hour intervals; typically 12 to 48 hours for 25 mm).
- Compute MC from the wet and oven-dry masses.
Worked example. A specimen weighs 120 g when cut and 96 g oven-dry. Water is 24 g, so MC = 24 / 96 x 100 = 25%.
The method is accurate but slow and destructive, and it can slightly overestimate MC in woods with volatile extractives. Use it to calibrate other tools and settle disputes, not to steer production.
Electrical meters
| Meter | Range quoted | Limitations |
|---|---|---|
| Resistance (pin) | about 6 to 30% (sources differ) | unreliable above about 30%; species and temperature corrections; pin depth sets the layer read |
| Capacitance (surface plate) | about 4 to 30% (sources differ) | affected by density; reads mainly the outer layer |
Above about 30% MC, resistance has too much variability and changes too little for reliable readings (Boone and Wengert, North America). That is why charges are often brought to about 30% by weighing samples, with probes controlling the rest (FPL manual).
Rules from the sources:
- Pin depth. One-fifth to one-quarter of the thickness gives an average; shallow pins read the surface.
- Temperature. A resistance meter needs about 0.5 points of MC correction per 10 degrees F (about 5.6 degrees C) away from the calibration temperature, added when the wood is colder.
Worked example. A pin meter reads 12.0% on a board 30 degrees F colder than the calibration temperature. Correction = 0.5 x (30 / 10) = 1.5 points. Corrected MC = 13.5%.
Surface readings can mislead
Swedish Wood (Europe) warns that right after drying to a 16% average, surfaces are often below 10% while the centre may be above 20%. A surface reading at the edge of a pile can overstate how dry the charge is. Lesson 6 returns to gradients.
Kiln samples (FPL manual, North America)
- Samples at least 30 inches long, with 1-inch moisture sections cut from each end and oven-dried.
- At least four samples for charges up to 20,000 board feet; usually 10 to 12 for 100,000 board feet or more.
- Place samples near the top and bottom of the stack, spaced 10 to 16 ft along the kiln. Slow samples reveal cold or low-circulation zones; fast ones reveal hot zones.
- Under the recommended schedules, conditions change when the average of the wettest 50% of samples reaches the scheduled value; equalising starts on the driest sample.
Sampling a lot
The Wood Handbook (1999) says to test at least 10% of each item, for example 10 of about 60 joists chosen at random. The portal could not verify the EN 14298 sampling plan (Europe), so use the standard itself for contract work.
Key points
- Oven-drying at 101 to 105 degrees C is the reference; MC = (wet - dry) / dry x 100.
- Electrical meters are unreliable above about 30% MC; resistance meters need about 0.5 points per 10 degrees F, added when the wood is cold.
- Surfaces can be far drier than the core, so one surface reading is not the charge average.
- FPL guidance: at least four samples for small charges, 10 to 12 for large ones, placed in the slowest and fastest zones.
- Lot sampling: at least 10% of an item in the Wood Handbook example; read EN 14298 for contracts.