Q. When cutting prongs to determine results of stress relief or conditioning, should the results be interpreted immediately upon cutting or should the samples sit for an amount of time?
A. The standard prong test is only valid if there is no moisture gradient, so it is best if the prongs sit for a few hours so that any gradients will dissipate. If you are in a hurry, you can put one or two prongs in a microwave oven for 15 seconds at high power. This will move the moisture around. Then let the prongs sit for several minutes. The results you see will be the final results.
Incidentally, the length of the prongs should be around 6 inches. The width of the prongs should be 1 inch. The thickness of the prong’s legs should be about 1/4 of the lumber’s thickness, but no thicker than 3/8 inch.
Note that the leg itself does contain some wood that will be planed off in subsequent processing. As this outer wood has the most stress, a little bit of stress on the prongs (that is, they move slightly inward, but do not touch) is often OK. Stated another way, the planer itself does remove some drying stresses from the lumber.
The prong test measure across the grain stress, but is not a good predictor of lengthwise stress; a different sample is needed for lengthwise stress measurement.
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