503 335 1 moisture content assessment

1 Moisture content Workshop session: Timber properties KDA 503-335: Timber, its origin and characteristics Student Aim This exercise aims to test the moisture content of timber samples three ways: Using a resistance moisture meter Oven drying of: o full sample o the core and case of a sample Background Moisture Content The moisture content (MC) of a piece of timber is defined as the weight of water contained in that timber expressed as a percentage of the oven dry weight Formula 1: MC % = Weight of water in timber × 100 Oven dry weight of timber Under ordinary conditions, all wood contains some water, and the amount of water contained in wood at a particular time is known as its moisture content The moisture content is important as many other parameters such as strength, shrinkage, durability, etc can all be shown to be dependant on the moisture content level The moisture content can be more than 100%, and frequently is Freshly cut plantation grown pine may consist of something like one and a half-part water to one of wood substance The MC in this case would be 150% Freshly cut green timber in hardwood saplings may have a moisture content of about 120%, while the moisture content of mature hardwood such as Southern Blue Gum, may be in the region of 70% Moisture Distribution When timber begins to dry, moisture is evaporated from the surface layers so that the moisture content at the surface soon falls below the fibre saturation point However, the inside of the timber will still have relatively high moisture content If the moisture distribution of a piece of green timber that has been drying for a short period is examined, it will be found that the moisture content varies from its highest value at the centre to its lowest at the surface This variation in MC is called ‘moisture gradient’ A check of the moisture gradient can usually indicate what sort of drying has been given to the piece The gradient is said to be steep and the drying conditions have been severe, when the moisture content decreases rapidly from the centre When the moisture gradient is gradual, the drying conditions have not been severe The importance of maintaining relatively small moisture gradients throughout the seasoning process become more evident when we consider the formation and prevention of drying stresses, particularly in relation to kiln drying Methodology Resistance moisture meter The procedure for using a resistance meter is: Ensure that the batteries in the meter have sufficient charge Most meters are equipped with a battery test phase Choose the location for the measurement It should be at least 400mm from the end of the board and in clear timber Drive the pins into the timber parallel to the grain (unless specified otherwise by meter manufacturer) Drive insulated pins to a depth of one fifth of the timber's thickness This will give an indication of the pieces average moisture content Repeat the process four times Correct the moisture content reading for temperature Correct the temperature-adjusted reading for species o Estimated wood temperature Sample No MC readings % % % % Corrected MC reading % % % % Average MC % Oven dry samples to AS 1080.1: 1997 The procedure for oven drying samples is: Cut two sample at least 500 mm from the end of the board The timber should be sound and relatively free of gum veins, resin pockets and knots The first sample should be at least 40 mm long and weigh not less than 50 grams when dry The second sample should also be 40 mm long Cut the second sample on the band saw as shown in section in Figure Figure 1: Cutting core and case MC samples Scrape or sand off all loose splinters and sawdust and weigh the piece Number the pieces for identification Place the samples in an oven controlled at 103° ± 2°C Remove the samples from the ovens after the expected drying time Weigh the samples immediately Record the weight and the identifying number Return the samples to the oven After to hours, reweigh the samples A sample is considered to be dry if the second weight is within 0.2 % of the first Calculate the moisture content.(Initial weight of the sample [1] - Oven dry weight of the sample) [2] / Oven dry weight of the sample [2] x 100 ([3]/[2] x100) Through all the steps, ensure that dust, debris, or sawdust does not contaminate the scales or samples KDA 503 Timber, its origins and characteristics – Block Worksheet Sample No Initial weight [1] g g g g Oven dry weight [2] g g g g Weight of water lost by drying [3] g g g g % % % % Interim weights Initial weight - Oven dry weight [1] - [2] Moisture content KDA 503 Timber, its origins and characteristics – Block Worksheet Notes Oven drying overview to AS 1080.1: 1997 Oven drying is the standard reference method for determining the moisture content of a test sample It is also the most reliable and accurate method The process of performing an oven dry test is detailed in AS 1080.1: 1997 Timber Methods of test – Moisture Content It involves taking a test sample from a board, weighing it and placing it into an oven at 103±2°C The sample is then periodically reweighed until it reaches a constant mass This is its oven dry weight The difference between the initial weight and oven dry weight is the weight of water that was originally in the sample The moisture content is then calculated as: Formula 2: MC % = Initial weight of the sample - Oven dry weight of the sample × 100 Oven dry weight of timber For using a calculator, this formula is the same as: MC % = Formula 3: Initial weight of the sample × 100 Oven dry weight of timber - For example, if the initial weight of the sample is 154.4 g and the oven dry weight is 95.6g, then using Formula 2: M.C.% = (154.4 - 95.6) / 95.6 x 100 = 58.8 / 95.6 x 100 = 61.5% Using Formula 3: M.C.% = [(154.4 / 95.6) – 1] x 100 = (1.615 - 1) x 100 = 61.5% Oven dry testing can also be used to determine the moisture profile of the sample Smaller samples are cut from the edges of the initial sample to an established pattern These smaller samples are then weighed and dried and the moisture content calculated The simplest pattern of these detailed in AS 1080.1: 1997 See Figure Cutting samples to establish a moisture gradient The two outside or case pieces can be weighed together to give a heavier sample, or separately to compare the moisture contents of each face of the board While generally very accurate, the disadvantages of oven dry sampling are that it is relatively time consuming, samples take up to a day (or longer) to dry in the oven, and sampling is destructive The accuracy of oven dry testing is dependent on the control accuracy of the oven, the accuracy of the equipment used to weigh the samples, and the mass of the sample The smaller the mass of the sample or the less accurate the equipment, the more uncertain the result Table shows that an error of one percent in measuring the oven dry weigh results in a difference of at least one percent in the calculated moisture content Table 1: Error from inaccurate weighing of samples Initial Weight Oven dry weight Calculated moisture content 33.3 grams 29.7 grams 12.8% 30.0 grams 11.7% 30.3 grams 10.7% Overview of electrical Resistance moisture meters Electrical resistance moisture meters are an indirect method of measuring moisture content and their use is also detailed in AS 1080.1: 1997 Timber Methods of test – Moisture Content This method is only used below FSP Oven dry wood is a very good electrical insulator and has a very high resistance However, water has low resistance So, the high resistance of timber falls as the amount of water in the timber increases KDA 503 Timber, its origins and characteristics – Block Worksheet Measurements are taken by placing two or more electrodes into the wood and reading the resistance between them If the electrodes are insulated except at the tip, the reading is for the path of least resistance in the wood between the tips of the electrodes If the electrodes are not insulated, the reading is for the path of least resistance in the wood anywhere it makes contact with the electrodes Generally, for uninsulated electrodes (eg nails) the resistance will be measured at the wettest point between the electrodes In either case, the resistance is then converted to a moisture content and displayed on a dial or digital display The accuracy of resistance moisture meters is highest at lower moisture contents and decreases as the actual moisture contents approaches fibre saturation point, generally about 30% moisture content They are inaccurate above that figure, though a correctly configured meter may still be useful in assessing drying trends At lower moisture levels, resistance meters are sufficiently accurate and easy to use so that quick readings can be taken at many locations Correction of resistance moisture meter readings The relationship between moisture content and resistance changes with the species of the timber and with its temperature Meters are generally calibrated for Douglas Fir at 20 °C Readings have to be corrected first for temperature and then species Corrections factors are listed in AS 1080.1: 1997 Timber Methods of test – Moisture Content Considerations and limitations There are several major considerations when using resistance moisture meters Moisture distribution Resistance moisture meters will always register the moisture content at the wettest point between the bare parts of the electrodes If uninsulated nail electrodes are driven into the centre of a piece of timber, they will read the moisture content of the centre of the board during fine weather but only the surface reading if the timber has been recently wet Thickness of specimen The length and penetration of electrodes must be taken into account For example, blade type electrodes are usually only 10-15 mm long and so are only useful in determining the core moisture content on thin stock Longer nail or pin type electrodes will penetrate further into thick stock Estimating wood temperature As discussed above, resistance moisture meters are normally calibrated for testing timber at 20 °C so the reading has to be corrected for the temperature of the wood Wood is a good insulator and its temperature changes slowly unless temperature variations are high It tends to even out from day to night So, the wood temperature should not be assumed to be that of the air or even the temperature of the kiln It should be estimated from the average likely temperature Contact There needs to be firm contact between electrodes and the wood Poor electrical contacts increase the resistance and may result in lower readings Calibration Electrical equipment in a production environment should be checked to an established procedure regularly to ensure it is functioning properly The accuracy of meter readings should be confirmed by occasional oven dry testing Preservatives, glues and additives Glues and preservatives such as CCA affect the resistance of timber significantly Preservative treatment appears to affect readings in sometimes unpredictable ways Resistance readings taken on treated material should be regarded as having limited reliability KDA 503 Timber, its origins and characteristics – Block Worksheet ... 95.6 x 10 0 = 58.8 / 95.6 x 10 0 = 61. 5% Using Formula 3: M.C.% = [ (15 4.4 / 95.6) – 1] x 10 0 = (1. 615 - 1) x 10 0 = 61. 5% Oven dry testing can also be used to determine the moisture profile of the... calculated moisture content Table 1: Error from inaccurate weighing of samples Initial Weight Oven dry weight Calculated moisture content 33.3 grams 29.7 grams 12 .8% 30.0 grams 11 .7% 30.3 grams 10 .7%... Resistance moisture meters Electrical resistance moisture meters are an indirect method of measuring moisture content and their use is also detailed in AS 10 80 .1: 19 97 Timber Methods of test – Moisture