How dry is too dry? - Page 3 - SailNet Community
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post #21 of 24 Old 10-11-2008
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Originally Posted by Stillraining View Post
isn't there is a huge storage facility for aircraft in the desert where the humidity stays around 10 %? also in furniture building you want your wood well below that level even so I fail to see how this would hurt a thing..Ill gander a guess that the wood used in boat construction is below 7% moisture content..IMHO
Uh ... ambient relative humidity is different from MC of interior woodwork. Which is measured as percentage by weight. All that cellulose to deal with, you know. Hardwoods are air-dried from 25-35% MC (full saturation) to around 15-18%; often (tho not always) they are then kiln-dried to 6-8% MC. That drives off all the free and some of the bound water. Here in the desert, it stays there more or less year round; in damper climates, wood will creep back up toward 15%. In difficult climates, like Michigan or West Texas, MC of interior woodwork will fluctuate between 6 and 12% on a six month sine wave, driven by but lagging a few months behind ambient humidity. That's a real bastard to cope with. That's the sort of long-period, large amplitude swing I'm warning Faster to avoid.

Certain woods, like mahoganies (true and Phillipine), don't much care. They have low absolute movement and low T/R ratios (tangential to radial movement). Other woods like maple or beech are high in both and are likely to distort if subjected to large changes in ambient humidity. Wood-veneered panel products can be exceptionally damage prone, as the veneer is thin and highly stressed during the manufacturing process; it is then bonded with inelastic glue (typically urea resin) to a core that may have very different movement properties. The result of multiple large moisture cycles can be severe surface checking or even failure of the glue bond.

Upshot: stability of the environment is more important than damp vs dry climate. Actually, the very finest woodworking belongs to Japan, in part because it is cool and damp there year round. Highly conducive to fine assemblies using air-dried woods.

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post #22 of 24 Old 10-11-2008
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Thanks for the lesson..
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post #23 of 24 Old 10-13-2008
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Originally Posted by anthony11 View Post
I recently saw a boat for sale that included two units kinda like this:

It was the driest boat I've ever seen, no musty smell, the bilge was spotless and looked like it'd never seen a drop of water in 23 years. Now that I've seen the apparent results, I wonder why everyone with shore power doesn't use one.
I used to build a product for the greenhouse industry with a keyboard on it. Greenhouses are extemely humid, and our machines did the watering! We sealed up our keyboard really well. To test them we put them in bucket of water with a brick holding htem down for a week. They'd come out completely dry. Put them in the greenhouse and aftera month they'd have a couple of ounces of water in the them and everything was badly corroded! Up on the machine itself was an electrical enclosure that was ws nothing more than a fiberglass tote turned upside down, no gaskets at all around the bottom edge. No water, no corrosion EVER, even after years in the same enviroment that killed a half dozen attached keyboards. We finally figured it out.

At night the greenhouse gets cool and real damp. The sealed keyboard breathes in through the strands of wiring in the cable attaching it to the other electrical enclosure. It condenses in the cool keyboard enclosure, because there is no heat generated by only switches. When it warms up during the day the condensed water doesn't get pushed out. Over a month you collect a lot of water. the other enclosure has power supplies in it. Thermocouples inside and out recorded that it is always about 10 degrees warmer than ambient, night and day. The higher temperature keeps the moisture out at all times, and guarantees it won't condense.

The little unit you pictured is nothing more than a 90 watt heater, that will keep the boat warmer than the surrounding air even on a hot summer day, and especially at night. 90 watts is way less than a humidifier draws, and it will work just about as well, as you found out.
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post #24 of 24 Old 10-20-2008 Thread Starter
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An update to this story.... we've been using this dehumidifier for a month or so now... yesterday we managed to get out for a bit after our mainsheet modifications and noticed that one of our sliding doors is suddenly about to fall out of it's upper track... normally the door is too tall to fall out on its own, of course. It seems to have shrunk that 1/8" or so.

I think we'll set it for its lowest setting for a while rather than constant duty.


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