Air movement in sealed containers?

Suppose I had something like an aquarium that was 1’ X 1’ X 6 ft long with a sealed lid. Stored indoors at relatively stable temps. Would I have some air movement going on most of the time? How much doesn’t matter as long as it moves. If not a small fan can be installed easy enough. Will be used for final drying of wood using desiccants.

Another related question, rice is listed as a desiccant. Would it simply reach an equilibrium with the wood or would it’s higher hydroscopic nature cause it to absorb a higher percentage of water?

HoneyBadgerDC,

A completely sealed box will have the air molecules bouncing all around depending on the temperature. Water being a small molecule will bounce around enthusiastically (:slight_smile: ) compared to say Nitrogen in the air. So no need to add a fan.

Everything will reach equilibrium “eventually” - so that’s a redundant question.

The mechanism of drying in this case involves the following steps:

  1. Migration of water molecules from inside the wood to the surface of the wood.
  2. Evaporation of water from the surface of the wood to the air inside the box.
  3. Migration of the evaporated water to the surface of the desiccant.
  4. Adsorption/Absorption of the water by the desiccant.

A higher temperature favors all the above steps and step 1 above is the slowest step. Here are the two things I would think about:

  1. How fast will it dry ? - The slowest step is the rate determining step. So a fan will not help with that. Heating it up will help but then you may get warps.

  2. How much dessicant do I need ? Typically raw wood has 50 to 70% moisture by weight, so you need dessicant that can absorb that much water.

I have a freezer that still works but the fan doesn’t. The back wall where the coils are grows a thick coat of ice. Food touching any of the walls freezes solid. Food not touching the walls stays only cold, and doesn’t freeze. It suprises me how the temperature inside the whole thing doesn’t equalize to below freezing.

Great answer, my wood will be below 12% when it goes into the box. 6% is the final goal that i would like to maintain. Beyond 6% the properties of the wood start to degrade. I have found typical room temperatures and about 50% relative humidity will get me to about 6% but in my area higher relative humidity is usually present and most of my wood usually stays at around 8% not bad but won’t give me peak performance either.

Unless the enclosure is in an environment that has no temperature gradients in any direction, there will always be some natural convection going on inside the enclosure.

Rice is not a good desiccant - it’s not even the best desiccant you have in your pantry. Use the real stuff (silica gel).

Have you done this kind of wood drying before? I’m aware that kiln-drying is commonly done for lumber, and this Wikipedia article lists 9 methods for drying wood, but none of them use desiccants.

This is not really wood drying as much as it is maintaining somewhat precision moisture levels. I air dry all my wood and then right before I make a bow out of it I like to dial in the final moisture levels. I am always looking for better ways of storing and finishing the wood.

If it was a larger container a fan would be necessary. That’s pretty small and there must be some air circulation in there. Desiccants can hold much more moisture than the air they are in, there is some equilibrium point but it’s not like the wood and desiccants will end up with the same moisture content. I’m not sure about rice, silica gel is probably much better. There are color changing versions that let you know when they are saturated. Keep the desiccant in a cloth bag so you can easily swap in fresh stuff if it reaches it’s limit.

Why not fill the box halfway with rice, put in your wood piece and then cover it up with rice ? It probably will work faster.

I wouldn’t do that with dessicant since it can get the wood fibers drier than 6%

nm

It is not about speed, it is about storage and maintenance. Too dry is just as bad as not dry enough so would have to find the balance.

50 % RH air will not result in 6 % MC wood in typical room temperatures, or even at 130 F. It takes roughly 30 % RH at room temps to reach 6 % MC. 50 % RH and room temps gets you very close to 9 % MC wood, or 50 % “wetter” wood than the 6 %.

Is it possible that indoors inside the closet where they are stored may be below the outdoor 50% humidity. Our humidity here is typically in the mid 60’s and my bows that are kept indoors usually settle in at about 8%.

This whole rice this has gotten rather out of hand. As a first approximation rice is going to be about as good a desiccant as wood. Curiously this might be a good thing for your purpose as it may reach a more stable equilibrium. The useful thing about proper desiccants is that the water chemically binds to them - and you you don’t just reach a simple equilibrium, the desiccant scours the water from the air.

You won’t have air circulation unless you have a thermal gradient. You will have diffusion, as the individual molecules are still whizzing about, they will tend to try to equalise the concentrations of different gas components over time. So if you are scavenging in one area, water will always be diffusing towards the scavenger. The effective speed of diffusion is very slow compared to a fan or convention, but it is possible it will be adequate - depends upon how long you are waiting. (You can work it out from first principles, but you probably won’t know the parameters needed.)

Easiest thing in the world to check. Put a humidity meter and a thermometer right where you store your bows. Let the meters settle and take readings. Those two numbers tell you with pretty good accuracy (and way more accurately than direct MC readings off the bows) what the EMC of wood inside the closet is. My guess, based on taking such readings, is that the closet contains air no drier than other indoor areas in the house.

Where do you get the 8 % number? If from direct moisture reading of the wood, it is imperative to check this against the ambient RH / temp, and the EMC those determine.

I have a good quality moisture meter. The behavior of the wood does seem to suggest that the reading I am getting are pretty accurate. But I have been needing a humidity gauge for a long time anyway.

For storage, can’t you make it airtight? Caulking or some other sort of goop (sounds like a bit large for ground glass). Use what other people have said to figure out the RH you need, dessicate the heck out of your aquarium to get that RH, then remove dessicant, add wood, and seal?

Does getting the wood too dry damage the wood, or is it just the moisture content at time of use that’s important?

At the time of use is what is most important. So a month or so before a shoot I would want to start tuning in the moisture content.

Adding some numbers is probably needed to gain an understanding of what you need to do.

So lets say you have 1kg of wood, and it is currently at 12% and goal is 6%. So you have 60g of water to remove. As noted, if your air has 30% RH at room temperature, your wood will reach an equilibrium of 6%. So, given enough time, you can simply maintain the air in your aquarium at 30% RH, and the wood will reach the desired water content. Maintaining this RH will minimally require you to scavenge 60g of water from the air whilst the wood dries. More if the air is over 30% when you start.

This is probably no big deal. Get a hydrometer, a bag of desiccant, and put both in your aquarium along with your wood. Looking at silica gel desiccant, they adsorb 10% of their weight when the RH is 20%, RH=50% and absorption capacity is 23% of its weight. Call it 15% of its weight at 30% RH. So you would need 400g of fresh desiccant minimum to get down to and maintain 30% RH if there is 60g of water to scavenge.
At this point you would hope things would reach equilibrium.