Ambient temp in kiln vs temp inside wood

[low_48]

I'm using a converted upright freezer box to sterilize turning blocks. Some are up to 6" thick and harvested over 10 years ago. I made up my own kiln schedule of 1 day at 100 deg, then two days at 125, and then bring the air in the box up to 150deg, I hold that temperature for a week. This bothered my sound and temperature engineer friend from our days at Caterpillar. He thought I was wasting money. So he brought over 3 thermocouples and temperature readers. We drilled a hole in 1 1/2" thick stock and another drilling in a 6" block. I stopped the experiment when the center of the wood was at 145 and held it there for almost 2 days. What surprised me was that the wood never matched the air temperature inside the freezer box of 153. The wood never matched through the entire test and he tested the equipment in his home oven to make sure it wasn't bad hardware. So is there always a mismatch in the temperatures between the wood and the air in the kiln, or do I need to move more or less air? My heat source is a 110v hot plate with a large aluminum finned heat sink on it and a 5" fan blowing through the fins.

[doc henderson]

It should eventually come to equilibrium, but with fairly fast temp increases, you are not just increasing the wood temp but also the air and water in the container. with an increase in temp relative to the water in the container you are essentially lowering the EMC, and that takes time reach equalibrium.  as the water evaporates and is motivated to move across the wood, it also uses energy and lowers the temp for the phase change.  the dryer the wood, the more insulating properties it would have as well.  so slow to get heat and temp increase to the core. That temp should sterilize it, and then let it equalize at the EMC/ RH where it will live.  raising the temp and maintaining the same water in the kiln, is the equivalent of increasing the wet bulb depression/dropping the RH, and EMC.

[beenthere]

Doc said it, and you just need to wait longer. Temperature in the wood will get closer and closer to the air in the box, the longer you wait.. but will "almost never" get there.

Maybe if you quit calling it a freezer box and call it a hot box instead, that may help.  ;D

[doc henderson]

you could add water, so it stops trying to reduce MC, and just heats.

[YellowHammer]

There are two things at work here that happen when wood is heated and trying to reach EMC.  The guys have very clearly discussed the first, the cooling of the wood due to the enthalpy phase change of the water in the wood.  So this indicates that the wood has still not reached full EMC with the chamber, or that the chamber is still dropping in EMC and the WB and DB would be different.

At that point, the delay in wood to hit the tail of the asymptotic curve to match dry bulb temperatures is due to the reduced thermal conductivity of dry wood, from case to core.  The way to increase that conductivity is to increase the WB, or increase the moisture in the kiln environment.  Otherwise there is a significant time delay to reach equilibrium temperatures, or in contrast, when the WB meets the DB, then the enthalpy stops to become a factor and the temperature conductivity is at it's max and the wood will match temperatures faster.

The FDA requires 133F core temperature, and this difficulty and extended time constant involved with reaching equilibrium temperatures between the wood and the chamber air is a major reason I overheat my kilns to 150F to sterilize. 

In addition, in order to properly measure the wood temperature with a thermocouple, the TC junction must be physically touching the wood fibers, and they must be at the same moisture content as the fibers surrounding them.  Seems easy, but it is not, and that's one reason the kiln manufacturers have so much trouble making direct temperature measurement kiln probes.  So if the holes drilled into the wood core and are open and the TC junction is merely taped or kind of touching the wood, then you won't be measuring the actual representative wood core, you will be measuring the instantaneous wood surface plus enthalpy effect.  That means low thermal conductivity of the wood fibers in the hole (exposed to the air) plus the surface cooling effect of enthalpy will cause lower reading.  The correct way would be to drill the hole and install a wooden plug from the same material that physically crushes the junction against the fibers in both the plug and the hole at the same depth as the wood core.

And yes, turn the freezer off.   :D :D :D

     

[low_48]

Much appreciated gentlemen! Hot box it is. I'm going out tomorrow with a Sharpie to make it official! Maybe I'll even draw some wiggly lines above the name to drive home the point.