New solar kiln with drying question

[briand7878]

I just finished a solar kiln from the VT plans. I cut a bunch of 9/4 walnut slabs in June and put them in the kiln last night. I tested the wood using a generic pin moisture tester. The walnut tested 15% everywhere I checked. The slabs were stored stickered in a metal barn.  Is it possible that they would actually be that dry already? How long should it take to finish these slabs from now? I was also thinking about buying a painless Wagner detector. Is it worthwhile for testing 9/4 slabs? 

[Log Jammer]

Watch out with a metal barn, large temperature swings and condensation from ground can wreak havoc.

[WDH]

The pinless wagner is not the best choice for operating a kiln.  You might consider the Delmhorst J-2000X with the slide hammer.  

[Tom the Sawyer]

briand7878,

9/4 walnut, milled 2 months ago, might show 15% on a meter with relatively shallow pins but the core is likely several time higher.  

[mredden]

Try driving two finish nails into a board about one inch. Space them the same distance as the pins on your meter. Attach an alligator clamp lead to the pins and the nails.

Buying a Delmhorst with hammer pins is more accurate but pretty pricey if you are a mere weekend warrior like me.

[briand7878]

$500 is pretty expensive for a meter considering I am a weekend warrior. I do however have so much walnut ready to process I can't see straight. I could probably pay for it in a couple boards but they need to dry first. I will try the trick with the nails first. Has anyone ever done the oven baked pieces for weight comparison? 24 hours in an oven seems a bit excessive to me. I would like the specifics on what to do if anyone else does this. 

Thanks!

[GeneWengert-WoodDoc]

For a moisture section, full width and full thickness and 1" among the grain, cut at least 6" from the uncoated end of the lumbe, we need to make sure we are zero moisture content.  So, you dry the piece at 215 F UNTIL WEIGHT LOSS STOPS.  YOU CAN CHECK THE WEIGHT IN 12 hours and then 13 hours and see if the weight is the same, and continue checking until no more loss.  Twenty four hours is oftentimes too long, but not always.

Regarding the moisture meter cost, it depends how accurate you need the MC.  Inexpensive meters certainly get close.  But if the customer wants it within one or two percent, then a $300 meter is the minimum, especially for under 10% MC.  Also, by using an expensive meter that is made in USA you will have a meter that cannot be challenged. I tested many meters on KD lumber and found that Delmhorst and Lignomat were the closest to the oven test values consistently.

Nail that are without any coating have been used with meters for decades.  Usually the nails are driven to the halfway depth, or half thickness depth, to get the core MC.  The average MC is at 1/4 thickness and you can use that too.  However, these nails measure the wettest moisture along their length, so sometimes the surface moisture due to steaming or fan reversal will give you false readings..  The nails are spaced the same as the needles, so you can touch the needles to the nail heads and get an instant reading.  Take the first reading and not one five seconds or longer from when you touch the heads with the needles.  Make temperature corrections...usually the dry bulb temperature is used, even if the piece is removed from the kiln for a few minutes.  Because the nails carry heat not the lumber, the accuracy with nails left in the wood for a while will read drier than the rest of the piece due to localized drying around the nail.

Questions?

[briand7878]

Thank you for the very detailed explanation. I will give that a shot and see how it works. Also, thank you for your design plans, hopefully this kiln serves me well for a long time.