SON":leek9fat said:
Not a good idea to substitute microwave oven as I have heard reports of fire and explosive splitting from these.
Simon
Simon, that's only the case if you don't follow some simple guidelines for oven drying samples in a micro-wave oven. I prefer using a microwave in some ways as you can get results much quicker than by using a conventional oven.
Here are simple rules to follow for anyone that's planning to do the oven-drying test with a micro-wave. Slainte.
Drying wood to oven dry in a microwave oven takes between 20 and 45 minutes. 30 minutes is about average. This is a significant advantage over the 24 hours or so required to do the job in a conventional oven. It does, however, require high accuracy and attention to details. Poor methodology and mistakes in the procedure tend to be punished.
Samples should be 25 mm (1”) long, and the full width and thickness of the board being tested.
The microwave oven must have a carousel (turntable) and, ideally, five or six power settings. A paper towel or towels is also essential.
The following procedure is a good method:
1. Weigh the sample.
2. Place the sample(s) on a paper towel on the outer edge of the rotating tray. The paper towel helps remove the condensed moisture that is drawn out from under wood.
3. If more than one sample is being dried ensure that none of the samples touch each other as touching parts concentrate the energy and could cause smoking or fire.
4. Set the microwave to a low or medium heat. A low setting means the test takes longer but there is much lower risk of spoiling the job due to fire. If the wood starts to smoke it is damaged and you will need to start again with a new sample.
5. If the sample is already partially dried, e.g., about 18% MC or below, cook the wood for between three to five minutes.
6. If the wood is green (about 28% MC) heat it for between 10 and 15 minutes.
7. Weigh the sample(s) again.
8. Dry the wood once more for a minute or less.
9. Re-weigh.
Continue the last two steps until there is no significant weight change, i.e., less than 0.1 of a gramme, assuming your scales are accurate enough. The less accurate your scales, the less accurate will be your results.
The following cautions should observed:
Do not use high power settings. The internal heat built up in the wood needs to dissipate and high settings can cause rapid heat build up, smoke and even fire.
The more wood being tested in one go, the more time will be required to complete the job.
Kiln dried wood samples will perform differently in the oven than green or air dried samples. It’s easier not to mix samples of very different moisture contents during the test, but this can be done if you proceed with care.
Being sure that the wood sample(s) is, or are, truly dry (i.e., at a constant lowest measurable weight after several cycles) requires patience and careful weighing using accurate scales. It’s better, and safer, to use several short cycles in the oven at low settings than it is to try and rush the job by using a higher setting for an extended time. The latter strategy can all too easily result in the wood smoking, charring and possibly a fire.
If you are working with a sample that weighed approximately 100- 125 grammes at the beginning of the process, oven dry means that there is less than 0.1 gramme change in weight between weighings.
These last warnings probably seem obvious, but they’re worth repeating. Removing cooked wood from the oven requires care. It can be hot and will burn. Use an oven glove or gloves. Also be aware that unbeknownst to you, the wood may be charred on the inside. It can smoulder and burn, and could set fire to rubbish in bins, etc.. Disposal needs to be done carefully-- dousing the sample(s) with water before disposal is not a bad idea.
