My humidity control cabinet - created 06-06-2008

Dellinger, Gene - 06/06/2008.11:47:39

After reading everything I could find in the archives, I designed and built an humidity control cabinet in my shop. I finished it, and put it into operation early last week. My cabinet is 32" wide x 24" deep x 46" high. I have three slatted shelves of 3/4" stock, with 1-1/4" between the slats, to allow for air/heat flow throughout the interior, and it is heated by one 100 Watt bulb, mounted in the center of the bottom, and is controlled by a common dimmer switch.

My shop is in a valley just west of the Tuscarora Mountain, in south central Pennsylvania, and it appears that we are shooting for a high humidity record, this Spring. I've dumped over 16" from my rain gage, since April 1, and the current humidity in my shop is 65%. That's with an AC running.

After putting the cabinet into operation, four days passed, with the bulb at full power, with only a 2% drop in humidity, from 55% to 53%. I drilled a single 1/2" hole through the front of the cabinet, right at the very top. Within eight-hours, after drilling the vent-hole, the humidity inside the cabinet had leveled out at 45%, at 87-degrees F, and stayed there even when I reduced the power to the bulb, by roughly half, until my shop's humidity sky-rocketed from 55% - 65% during the past three-days of rain. Last evening, the cabinet humidity had climbed to 49%, so I ran the bulb back up to full power, and this morning it was back down to 45%, and has held at that, so far. I have three uncarved mandolin neck assemblies, and three sets of mandolin tops, awaiting glue-up, inside the cabinet, and the humidity fluctuation was interesting, when these were introduced, last weekend. Within a couple of hours, the humidity had climbed from 45% to 48%, but by the next morning, it had dropped back to 45%, where it still is, today.

I really appreciated all the material available on the subject, in the archives, and thought that perhaps someone might be interested in seeing my results.

Thanks,

Gene

Basically, it's merely a tight rectangular box of approximately 20 cu. ft. interior, with three equally spaced, slatted shelves. (Keep the bottom shelf several inches above the bulb. You don't want to set the thing on fire, and protect the bulb with wire mesh, or other sort of non-flammable cage, to protect it against breaking.) It has a tight fitting door 22" x 36", and there is a simple ceramic fixture, with a 100W bulb mounted in the center of the floor of the cabinet, which is wired through a common dial type rheostat (dimmer switch), then to a 110V circuit. I bought a thermometer/hygrometer from the local True Value, calibrated the hygrometer, marked the 42-47% range with a permanent marker, and mounted the unit on the center of the cabinet's rear wall, just under the top shelf. As I mentioned, I drilled one 1/2" hole in the center of the cabinet's face, about an inch below the top.

I mounted my cabinet, in one corner of the shop, with the top about ten-inches from my shop's ceiling, so the back and one side are insulated by the insulation in the shop walls. I'll probably insulate the top, bottom, front, door, and remaining side, with foam insulation, so the bulb won't have to work so hard maintaining interior equilibrium. I'm also planning to lay out a simple gage on the dimmer switch dial, so I have a little better control of its positioning.

I played around with the control throughout the day, yesterday, and finally left it set at about half power, when I went to the house, last evening. The humidity, outdoors, was above 90%, all day yesterday, and this morning the fog is so thick that I can barely see my board fence, 75' away. The shop's humidity is at 61%, and the interior of the cabinet is standing at an even 43%. Seems to be working beautifully.

FWIW - I think the key is to have the overall cabinet and door very tight, but I do believe it is very necessary to have the small vent near the top, to allow initial moisture (from wood being introduced; moisture entering every time you open the door) to leave the interior, otherwise you will merely have heated moisture trapped inside.

Hope this helps.

Gene

Gene, I know something about your area; I grew up in western PA (Butler) and lived near Philly for a long time. The seasonality of Pennsylvania makes it a beautiful place to live and a great farming country.

About your cabinet, I think it's a great idea, but 'as is' I wonder if it will take up too much of your time and attention unless it's part of the place where you spend most of your time every day. It might be better to add some automation. Have you thought about adding some sort of humidistat? What will happen next winter, when the shop dries out?

Maybe you can add a household humidistat, and calibrate it by using the temp-hygrometer that you've installed.

It's been going for nearly a month now, and even with opening and closing the door several times a day, the RH inside the cabinet hasn't gone above 46% or below 42%, and stays pretty much dead on 43% all the time. The RH in my shop hasn't gone below 55% during that time. I've only tweaked the dimmer switch a couple of times, and then only when I've introduced several bulky items such as mandolin neck block assemblies. Sometimes methinks we tend to over-think our projects.

Thanks,

Gene

Bob -

Sorry - missed your last line. Yeah, Winter may bring on it's own requirements, and I may have to go with an humidistat. When I was burning wood, I kept a big kettle of water simmering all the time, and my ambient humidity in the shop stayed right around 45%. Not sure I'm going to go with the hassle of wood again this year (costs dang near as much as fuel oil now). May just use the electric heat, and that will likely require some automation inside the cabinet.

Thanks,

Gene

I haven't come down with W.A.S. yet, but if I did, your cabinet sounds like it would be a good thing to have. To be on the safe side, I'd probably put a smoke detector inside.

You might want to think about adding some hygroscopic material to the box. It doesn't need to be fancy--a pile of short off-cuts would work fine, the shorter the better to maximise end grain. The idea is to help absorb moisture when it gets in from a damp shop, and help hold moisture when it's trying to get out into a dry shop. This slows down the transfer of moisture into and out of your important pieces of wood, letting them adjust slowly and without stress to your desired equilibrium moisture content.

When I worked in historic conservation we used silica gel beads for this, since they are VERY hygroscopic, but you don't need that quality and you sure don't need that cost.

Rob

Thanks Rob, after squirreling away dozens of silica gel packets that I've acquired over the years, I finally have a use for them. By the time I get around to making a cabinet I'll probably have a few dozen more.