Kiln Drying Forum
Wood Drying Topics => Moisture Variability => Topic started by: AliciaS on August 26, 2015, 10:13:48 AM
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I use a (Wagner stack probe) moisture meter for performing hot checks. How many readings should I take? Should I throw out the highs and lows?
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Why do you want these readings? Are they used for control (such as to determine when to stop the kiln) or are they used to document the MC for subsequent grading or users? The purpose will determine the actual technique.
IN GENERAL
The first step is determining how the hot readings relate to cool readings. Generally, the hot readings are adjusted and each mill has their own adjustment factors for species, thicknesses, etc.
The second step, is that for a good statistical estimate, about 32 readings are necessary, avoiding unusual pieces of lumber (top of the stack, thin, edge pieces and so on). Obviously, sampling from all the stacks, not just the low ones is important.
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Most mills would do about 100-300 readings, depending on the kiln size and how good of an estimate is needed.
My observation is that these are often done too fast (holding the button down and pulling the meter out to get two or three readings) and are probably not readings of high quality. I like to pull the meter until the reading peaks (so I know the sensor is over a board and not a gap), then push the store button. Personally, I would rather have 50 good readings than 150 bad readings.
Readings need to be taken (as Dr. Wengert says) everywhere, but practically you can reach so high. Often this is only one unit, or slightly higher. You might be able to hire Yao Ming from the Rockets to reach higher. Short of that, most operators know that the upper units will usually be lower in MC than the lower units (or at least no higher) and do not feel a great need to sample there.
Do take readings in both plenums and every unit that can be reached.
Do make sure every operator takes readings the same way and in the same locations.
Do the hot check procedure exactly the same way every time. Record it and follow through at the planer. Record what is measured at the planer and develop the offset for your mill.
Some mills toss readings >30 from the average. If you have a wet-board drop out, this might be justified.
Consider doing a cold check before pulling the lumber from the track.