Question

There is variation in timber test results due to species and wood temperature and from moisture content? The Internet site of Timber Queensland Limited "technical data sheet for correction factors for resistance moisture meters" lists correction factors for these effects. What are these?

Answer 1

METHODS OF MEASURING THE MOISTURE CONTENT OF TIMBER AS PER TECHNICAL DATA SHEET ISSUED BY TIMBER QUEENSLAND 2014:

Resistance meters measure the highest moisture across the exposed ends of the pins whereas capacitance meters measure an average through the piece. Oven dry testing measures the average moisture content of the sample placed in the oven but by cutting the sample up into applicable smaller pieces, case and core moisture contents can also be determined.

The three common methods of measurement, including their application, benefits, limitations and accuracy are outlined as follows:

OVEN DRY METHOD

When is it used?

Oven dry testing is often carried out where variations in moisture content in the final product can have a significant effect on the performance of the product.

In case of disputes Australian Standards generally refer to this method as it provides measurements that are more accurate and reliable.

Manufacturers of board products often undertake oven dry testing as a check in the manufacture of their products.

Organizations such as TQ also have the appropriate testing equipment and contract out these services.

Testing equipment and facilities

The equipment required is an accurate balance or set of scales and a laboratory oven that is able to maintain a temperature of 103°C ± 2°C.

Sampling from a pack

The samples need to be representative of the timber in the pack being tested and capture the variation present. This may therefore include some outside boards as well as some from within the pack.

If cupping is present or there is variation in the cover width by more than 1 mm, samples should be provided which include 2 boards that are cupped, 2 with wider cover widths and 2 with narrower cover widths. (Packing pieces are not to be provided as samples).

If boards are not cupped and there is little variation in cover width throughout the pack, 5 boards should be chosen. (Packing pieces are not to be provided as samples).

The samples from which test pieces will be cut should be taken not less than 400 mm from the end of a board and should be approximately 300 mm long.

Testing Procedure

The samples should be individually wrapped in “glad wrap” or similar to reduce moisture content changes during transport.

The samples should be stored in a cool place and delivered to the testing facility within 24 hours.

From the 300 mm long pack samples, test pieces are cut with a length between 15 mm and 30mm so that the required mass is achieved to suit the accuracy of the mass measuring equipment. If the equipment measures to 0.1g then a test sample of at least 50gm is required. The sample may be less that 50g if the equipment measures to 0.01g.

The initial masses of the test pieces (and usually the cover widths) are recorded. The test pieces are placed in the oven for at least 24 hours and then reassessed at four hour intervals until there is minimal change in mass. For longer samples in denser species, times of 48 hours or so may be required. The mass after drying in the oven (i.e. oven dry weight) is recorded.

The moisture content is then calculated for each test piece by applying the following equation.

Moisture content (%) = ((Initial mass – oven dry mass)/ oven dry mass) x 100 %

This method provides the average moisture content for the test pieces. Case and core measurements can be obtained by cutting the appropriate sections out of larger test pieces.

Benefits and limitations

The main advantage of this method is its accuracy.

The method is time consuming, not portable and more expensive.

The most common error results from insufficient drying, which underestimates the moisture content. If sample masses are small then measuring errors can significantly affect the moisture content calculation.

Microwave ovens can produce good results and speed up testing, however there are no formal procedures and there is the risk of evaporating volatile compounds in addition to the water which affects accuracy.

RESISTANCE METER

Principal of operation

The electrical resistance of timber reduces as the moisture in timber increases. These meters measure the flow of electricity between two pins where the timber acts as an electrical resistor between the pins. The scale on the moisture meter is graduated to read moisture content. Wood temperature affects the readings and for this reason wood temperature above or below 20°C, requires correction to the reading. Temperature correction, if not already taken care of by the meter, is applied before species correction. Species correction is necessary as two different timber species at the same moisture content may not have the same electrical resistance.

Meters are generally set up relative to one species and that is Douglas Fir (Oregon) and species corrections are then applied for other species. There comes a point where the moisture in timber is so low that the resistance is difficult to measure accurately or on the other hand sufficiently high that the resistance does not change greatly and is prone to greater errors. These meters generally provide reliable results between 6% and 25 % moisture content.

Types of meters

A wide variety of meters are available. All have two pins that are used to penetrate the timber but the pins may vary in length from approximately 6 mm up to 50 mm. The longer pins are often insulated up to the pointed ends to prevent surface moisture effects from interfering with core measurements. Those with longer pins are also usually of the ‘sliding hammer’ type, which provides a means of driving the pins into the timber. The sophistication of the meters varies greatly in terms of features such as inbuilt temperature correction, preprogrammed species calibration and depth indication. Many of the meters now come with a calibration box.

Limitations, accuracy and precautions

when using resistance moisture meters When using meters a common sense approach is necessary and each reading should be evaluated and if not as expected, then the reasons for this should be investigated. The meters generally provide a reasonable estimate of the moisture content to ± 2% in the measuring range from 8% to 25% and as stated above readings should be recorded to the nearest 0.5%. There are a number of factors that require consideration when using these meters:-

Measurement necessitates damaging the surface of the timber

The method is conducive to only taking a relatively small number of sample readings.

Readings near the board surface can be significantly different from the core.

Low battery can cause low readings in high moisture content material.

Uncertainty over the species can make species corrections difficult.

Species such as Brush Box have very high species correction factors and are prone to greater error

Use for extended periods in high humidity environments can raise meter readings.

Meters only read the wettest part that the exposed surfaces of the pins are in contact with.

Surface moisture can provide artificially high readings not reflecting wood moisture content.

Salt water or any preservative treatment salts can affect meter readings and will usually raise them.

Electrical wiring in walls can affect the readings.

If meter readings are not in line with what is expected, then this may necessitate oven dry testing to more accurately estimate the moisture content.

CAPACITANCE METER

Principal of operation

These meters measure an electrical property called the ‘dielectric constant’ and in so doing an electric field produced by the meter, and the presence of the timber on which the meter is positioned, form a ‘capacitor’ type of arrangement. The electric field can penetrate deep into the timber but meter readings are biased toward moisture in the surface layers. Both the moisture content and the density of the timber affect this electrical property. The effective range of capacitance meters is from approximately 0% to 30% moisture content. The more sophisticated meters can be adjusted for timbers of different densities. Less expensive meters do not have density compensation and for these meters corrections to meter readings must be applied based on the density of the species being tested. Such meters are usually preset to be more suited to softwoods and lower density hardwoods and this can cause limitations with higher density species (i.e. large correction factors are necessary).

Types of meters

Meters are imported from overseas and range from those with few features to those with a wider range. Features may include settings for timber density (or specific gravity) and timber thickness as well as the ability to store readings and apply some statistics to the results. It is necessary to ensure that the meter is going to meet your specific needs and if being used with higher density hardwoods, timber density (or specific gravity) adjustment must be seriously considered.

Using capacitance meters

The appropriate meter settings for density and board thickness etc. should be applied and the meter checked for calibration.

The density (or specific gravity) is often calculated differently for different reasons (i.e. green density, density at 12% moisture content or basic density). Specific gravity is the density of a material divided by the density of water (approximately 1000kg/ m3). It is necessary to obtain from the meter supplier the relevant figures applicable to the meter being used. Table 3 provides densities at 12 % moisture content.

Measurements are then taken in clear timber away from knots etc.

Some meters require measurements to be taken with the meter in a particular orientation on the board (check with the manufacturer’s manual).

The plate of the meter must be in firm contact with the board before a reading is taken.

Readings should be recorded to the nearest 0.5%. If no density (specific gravity) settings are available, then these meter readings, need correcting.

Limitations, accuracy and precautions when using capacitance moisture meters

Similar to resistance meters, common sense must prevail when using these meters with readings evaluated and investigated if not as expected. Providing the density is accurately assessed then these meters also provide a reasonable estimate of the average moisture content in a board up to approximately 25% moisture content. Again there are a number of aspects that need to be considered when using these meters:-

Readings can be taken very quickly both within a board and in a number of boards.

The meters do not damage the surface of the timber that is being measured.

Within species density variations can be quite high, particularly between mature and young growth material. This can result in significant variation in meter readings.

Estimating the correct density adjustment can be difficult, particularly if the meter is being used on a wide range of different timbers.

Density (specific gravity) information for Australian species relating to specific meters is not well documented.

Difficulties with setting density (specific gravity) adjustment often reduces field measurement accuracy.

If no timber thickness adjustment is provided then thicker pieces at the same moisture content are likely to read high.

Any gap between the meter and the board (e.g. a cupped surface) will cause lower readings.

Framing raises meter readings where exposed timbers cross (e.g. softwood floor over hardwood joists).

The presence of salts (either from salt water or preservation treatment) will cause readings to be higher.

Readings are also considered to be less reliable with Brush Box.

Again, if meter readings are not in line with what is expected, then this may necessitate oven dry testing to more accurately estimate the moisture content.

MEASURING THE MOISTURE CONTENT OF PLYWOOD AND PARTICLEBOARD

Meters do not provide an accurate and reliable measure of moisture content in these materials. To determine the moisture content of these material the oven dry method should be used.