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microwaving prong tests???4/24
I've met with you on several occasions and you've mentioned that, to eliminate the moisture scale in the prong stress test, to microwave them 15-30 seconds on high heat and let them set for a few minutes. A year later you tell me no more than 15 seconds. My boss, spoke with an unnamed source and says that I should go longer.
The issue is that I've conditioned a charge of 4/4 Red Oak for 10 hrs @ 168 dry bulb and 11 degree depression. initially cutting the prong test, the sample appears to be stress free and the rip test confirms this. When I microwave the prong test (30 seconds or 15 seconds) the sample shows (severe) case hardening.
Its a 600w microwave. I've conditioned the charge 13 hrs now checking for stress at 7 am, 730 am, 9 am and again at 12 pm with the stress only getting worse. The temperatures are right on, verified with an electronic windmate and standard wetbulb and drybulb thermometers.
management would like to know if the microwave is the problem or if something else might be the issue. Your thoughts appreciated.
Haessly Hardwood Lumber Sales
The key for stress relief is rapid addition of moisture to the wood at high temperatures.
The recommendation is to use 15 seconds for a prong (single one) in the microwave for steam conditioning, and 30 seconds if you use water spray instead of steam spray.
The intent of the microwave is to move the moisture within the piece and not to dry it further. Then we wait a few minutes to allow moisture movement before we read the sample. This has worked for years, so I do not know what you are doing wrong to get such wierd results. Are you cutting the prong from a kiln sample, cutting off 6” before taking the prong?
Your settings of 168 and 157 are OK, if these are the actual values. Normally, we set the WB and then let the DB rise on its own so we 11 depression actual.
Stress relief occurs when we rapidly add moisture to the dry shell, so we do need saturated low pressure steam. With a high pressure boiler, we use a desuperheater so get saturation after we lower the pressure and just before the steam enters a kiln.