Dry Box - The details

[Chuck Tweedy]

Hey Everyone,
I've not built a thing for a coupla' months now - stopped by 2 things: Lacrosse season, and high humidity.
SO, I finally did something about the one obstacle I could actually do something about ;

I built a dry-box.

It's just a simple box about 20" x 24" x 8" inside, big enough to fit a completed body. Made from scrap MDF & plywood - with a light fixture in the back.
The one "deluxe" feature is that I fitted the lid with a humidity gauge I got from McMaster for $20. It correlates well with my shop sensor so in it went.

I just turned it on this morning and I have a couple questions, they are simple:

I have a 40W oven bulb in there now - is that generally all I need??

I left 3/16" between the door and frame for either 1) an air gap, or 2) weatherstripping.
I really don't know if it should be sealed or vented. Right now it is vented. Which??

Overview - box closed

Humidity at 70% now - same as the room

A view inside box - You can see the back of the humidity gauge is exposed to the inside air

[Chuck Tweedy]

Okay people, I'm not feelin' the love here.
I've had this thing on all day and the RH in there is still only down to 65%.
I added the weatherstripping around 2pm and closed it up.
Still the same.

60W maybe??

[Brad Heinzen]

I don't know if what I did was right or wrong, but I assumed that the point of the dry box was to create a little area that was a bit warmer than the rest of the shop so that the relative humidity would be correspondingly lower. In my case, I turned an upper cabinet into a dry box by adding a light bulb to the area below the bottom shelf, drilling all the shelves for air circulation, and popping a little line voltage computer cooling fan in there to circulate the air. Mine isn't terribly air tight - just as tight as a regular cabinet door would be. The end result was that the measured humidity inside the cabinet is generally about ten points lower than it is in the rest of the shop. I drilled a little gauge into the door so that I could read it easily. Also, I don't store materials in the lower section where the light is, just in case the light might cause color changes or some such distress. I use mine mostly for seasoning materials like fingerboards. The next few guitars always go into the dry box while they wait their turn in the shop.

[Brad Heinzen]

Hey Chuck, the 40W didn't do it for me either. I think I ended up going with a 60W clear bulb.

[Chuck Tweedy]

"...the point of the dry box was to create a little area that was a bit warmer than the rest of the shop so that the relative humidity would be correspondingly lower."

Exactly. I'm not going to be able to make an airtight chamber and fill it with nitrogen from a cylinder to absolutely control the humidity, so for a practical dry-box, the air in the box is the same are as in the room.

However, if there is major circulation, then you have to continually pump heat into the new air to bring it up to temp. I therefore designed mine to "seal" (not perfectly, but mostly) so that the inside air just stays hot with a minimum of energy input.
That's the theory at least.
40W ain't doin' schist right now.

(schist is a mineral - look it up)

EDIT: I see your post Brad - I'll put in the 60W right now.

[Chuck Tweedy]

Here is a good site to do quick calculations: http://einstein.atmos.colostate.edu/~mc ... idity.html

I used it like this:
Set the room conditions = 23C and 67%RH
That calculates the dew point = 17.49 C
Reset the page.
Enter the dew point and...
my desired RH (I'm hoping for 50% - so I entered that)
It calculates that I need to bring the temp up to 28.9 C to achieve the desired RH.

That is definitley do-able. Only about 90F, not a big deal.

The dew-point of the shop air is the key:
If the concentration of water in the air stays the same (same air in box as out), the dew point is constant.

[Chuck Tweedy]

I think I may add a little muffin-fan in the box to just move the hot air around.
I should also finish the box to stop (slow) moisture tranfer into the box. That hot air is going to be moisture-hungry. I'm guesing there is going to be an osmotic effect.

[Chuck Tweedy]

Sorry, I'm turning this into my own little blog here, but...
I've got a fan, and my other (higher quality) RH & Temperture unit in the box now.
The temperature is 32C and the RH is 61% (by both sensors now)

That means that the box (the wood it is made of) is still out-gassing water.
I need to wait longer AND I need to leave the door open so that the moisture can get out.
Next update tomorrow!

[Clay Schaeffer]

Hi Chuck,
Leaving the door open so the moisture can get out makes no sense to me. As you mentioned in a previous post the air inside the box is essentially the same as the air outside the box, it is only the _relative_ humidity that is changing because of the temperature difference. Unless the wood is extremely wet and the seal around the box very tight the rise in temperature alone should lower the relative humidity gneiss-ly.
I've been using an oil filled space heater in a closet in Maryland's high humidity and it will lower the RH from 80% to about 40% in a very short time. I adjust the thermostat to adjust the RH. I have a drying cabinet with light bulbs, but have been storing tools in it lately. The heater/closet is easier to regulate.

[Chuck Tweedy]

"Leaving the door open so the moisture can get out makes no sense ... Unless the wood is extremely wet and the seal around the box very tight"

Exactly! The wood lived outside before yesterday, and it has been misty here in So Cal. The wood is - in fact - still drying out. At least it was yesterday.
After all night with the 40W bulb, and the door ajar I'm down to just over 51%RH

[Jeff Highland]

Probably the most effective method would be to have the box well sealed, but in the first week or so with fresh wood, open the door and exchange air once a day.

[Chuck Tweedy]

Hey Jeff, that's what I've been doing. Amazing how the humidity plummets every time I open the box - well, it's actually stabilized now but that's what it was doing before.

This morning at work I found a couple CPU fans with heat-sink fins attached to them.
I think I'm going to give up on the light-bulb and make a dedicated heater using a cartridge heater in a block directly attached to the heat-sink.
Even with the fan, heating in the box is not very uniform.

[Clay Schaeffer]

Does anyone have any data on how quickly wood changes it's moisture content when cut to the thicknesses used for lutherie? It has been suggested that makers only remove wood from the hot box while it is being worked on, and quickly return it to the box after glue ups. I remember reading that some makers would warm the wood over a heat source just prior to gluing bracing. Rather than running light bulbs and heaters 24/7 might it be possible to just warm things up for an hour or two before glue ups and let things cool down after the glue has set?

[Barry Daniels]

Got any silica gel?

[Alan Owler]

As a possible alternative to a light bulb Lee Valley sells these things,
http://www.leevalley.com/en/wood/page.a ... at=51&ap=2

Al

[Chuck Tweedy]

Clay, that is a good question. From what I'm observing it takes a while for things to stabilize when I put something new in the box. About 12 hours. I what worries me about just heating the parts for a shorter time is you don't really know what state it is in. If it is in the dry-box for an extended period of time, then it is at equilibrium with the box and I know what that RH is.
Currently I'm using only a 40W oven bulb, not a big deal to have that on 24/7 for a few weeks while I need it.

Desiccant is a great idea for that "clear the moisture" phase of the box stabilizing. I have some that needs baking to get back to a "thirsty" state, and I'm gonna' throw it in there to see where the needle goes.

[Randy Roberts]

Chuck,
Working out of the garage in the midwest is the definition of frustration when it comes to humidity control. Here's what has worked well for me the past 4-5 years.

They make ZipLock bags in very large sizes for storing clothing, blankets, etc., and I use them to store all the parts for each guitar all together. I have silica gel dessicant coming out of my ears at work as each antigen test kit for blood tests has one in it. I store the guitar parts in the bag and throw some dessicant in with the parts. Can't hurt to ask your Vet to save them for you, they just get thrown away otherwise (doughnuts always make a good trade). Put a humidity monitor in the bag and your all set.

I've found quite a few advantages to doing it this way. If the humidity in the bag gets lower than I want, I just take a few dessicant packets out and leave the bag open for a few minutes. If too high a humidity I toss a few more dessicant packs in. It's pretty easy to keep a constant target humidity this way. Once the bag is zipped shut the humidity stays constant for months at a time.

It keeps all the parts at the same humidity, so when I'm going to glue up something I'm ready to go.

I lose a lot fewer parts because everything is kept all together.

I hang each project's bag on the wall and it keeps everything out of danger from me dropping things on it. Far far fewer dents nicks etc. since I started storing stuff this way. when parts get to a size that they could bang into each other (neck and body) I just wrap one or more piece in a towel. (with necks, the leg cut off an old pair of jeans works well)

Definitely less clutter in the shop since I went to this system.

[Chuck Tweedy]

Now you tell me!!
That is a very good solution Randy. I may have to abandon the box or work this into my long term storage.
I do like the way the parts come out of the box all toasty, like it is an Easy-Bake oven.

[Steve Senseney]

I was unaware of the LARGE zip lock bags. Where are you buying them?

[Bob Menzel]

The ZipLock & dessicant bag seems a whole lot safer than a combustible box with a heat source inside.