dh kiln load stuck at 12% MC

Dan_Shade · February 18, 2013, 09:56:59 AM

The load is air dried white oak, some cherry and walnut. All species are at 12% after 4 weeks.

How do I get it to drop?

Jay C. White Cloud · February 18, 2013, 10:06:34 AM

Thank's for posting this Dan, I work in green wood most of the time, so I'm just following along, but 12% sound like maybe it has reached some form of equilibrium. Could that be it? Can't that be low enough?

Regards,

Jay

GeneWengert-WoodDoc · February 18, 2013, 10:07:56 AM

It can only stay at 12% MC if your kiln is at 12% EMC or about 65% RH. Otherwise the wood must dry. So, lower the RH in your kiln to under 30% RH.

For any readers that have a solar kiln, you will stay at 12% MC if you run the fans all the time. You should run only when the sun has raised the temperature in the kiln. Vents should be almost fully closed at low MCs to get maximum heating.

How are you measuring the MC? It might be that your measurement is wrong. If you are using a meter, give us the exact procedure you are using and any calibration technique you are using. Also, give us the brand, model, and other details. Do you make a species and temperature correction? Can you check a piece of wood in your house (maybe a cabinet) that is at 6% MC and get the correct answer? Or do you know someone else that has a different meter? If using oven dry, again, give us the procedures.

Dan_Shade · February 18, 2013, 10:53:09 AM

Meter: delmhorst J-2000
Using external probe with insulated pins
Temp is set at 120°F
Species is set at 32 for white oak
The meters self calibration reads "12.0"

Kiln: nyle/woodmizer DH-4000
Set at 120°F, 40% humidity

OneWithWood · February 18, 2013, 12:40:28 PM

Dan, drop the humidity to 20%. You should be able to get to 6% in a few days, then flood the floor and raise the humidity to 100% for a day or two until the lumber comes back up to 8%.
How big is the charge?

GeneWengert-WoodDoc · February 18, 2013, 12:55:15 PM

When you turn on the meter and take a reading without the needles in a piece of wood, what do you get?

Even with air dried, there should be some variation from piece to piece. Do you see this? Can you get a low MC reading when checking a dry piece of wood, like a cabinet...no need to change species for this test or even temperature?

Do you see a MC gradient as you drive the needles from the surface to the core?

I can think of no reason why at 40% RH (8% EMC) and 120 F that the wood would not dry to 8% MC, especially at the surface. So, maybe your RH is incorrect. Radio Shack has a digital humidity measurement device for $30; not the most accurate but good enough for lumber drying.

OneWithWood · February 18, 2013, 12:58:27 PM

Good point about the readings being all the same, Doc. I have never experienced any two test boards reading exactly the same. I also weigh each sample and do the math to verify the accuracy of the meter. It is never the same but should be close.

Dan_Shade · February 18, 2013, 01:11:30 PM

I went back out this morning, and left the probe in the kiln for a while. Surface MC was around 5%, core around 8.4% on several boards.

I think the probe was too cold yesterday ?!?!