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So I thought that the wildlife in wood would die off at 130 degrees in the kiln. Today I found these two bugs slide under the bark of a junk of wood which was in the solar kiln for 5 weeks and the temperature in the kiln went up to 140 minimally, but the bugs survived
I thought you had to hold 135 degrees, at the core of the board, for 24 hours to sterilize. I understood the kiln had to be at 150 to get that core temp. I don't know what 140 degrees minimally means.
I was thinking that 133 degrees was the temp you needed and it did have to hold over that for several hours to get to the core depending on thickness of lumber. I found that in my solar kiln I moved my temp gauge away from the DH and in the shade area the temp was a lot lower there. I would think that's the true temp.
I build a plenum in the kiln (it's a small kiln, only about a 150 qft) with a pretty good solar fan for air circulation and no direct sun inside, it's all in the dark, so the temperature shouldn't differ all that much throughout. But I started this piece at the end of September and even though my temperature gauge shows that the temperature went up to 140 degrees there is no telling for how long and probably just not long enough to penetrate the bark and wood all the way. This might be one of the short falls of a solar kiln.
From most of the reading I've done on solar kilns the temp will get to the heat you need BUT it CAN'T maintain it long enough to get to the core as needed. Because my heat source in my DH kiln pushes it's limits to get the 135* needed I hold mine for min 24 and prefer 48 hrs to insure the temp creeps to the core ( I push up to 4,000 bd ft). Add a wireless weather station to your kiln that records it's data, mine records every 3 hours. It allows me to scroll back and see the recordings and fluctuations if any. I monitor the furthest point from heat to clarify correct temp readings. I think one of mine was around $125.00 BUT well worth the money!!!
I think you'll find it hits 140* but not long enough to sustain to core.
Although the air was 140 F for a brief time, it is likely that the wood was below 130 F, especially inside the pile and on the side that the air leaves the pile...the exit side.
It is also tricky to accurately measure solar kiln air temperature in most kilns due to direct and indirect exposure of the sensor to sunlight, but not in yours. Also, the collector and the air around it can be hotter than the average air temperature inside the pile.
Heat travels from the surface of the lumber to the core very slowly. Faster when the wood is wet, but then the surface will be at the wet-bulb temperature and not the dry-bulb. With nearly dry wood, the wood is a great heat insultor, so heat travels very, very slowly.
Note that the grub shown in your picture needs moisture to be active, so the wood was still fairly wet. They cannot damage or eat dry wood.
The second picture is one of the anobiid beetles, and they like wetter wood also.
Gene, that is the part that surprised me, both the beetle and the grub where directly under about 1/2 inch of bark and I would thought that the heat would have penetrated that deep, but apearantly not for long enough. It sure seems like the little buggers are tougher that I thought.
Just remember that wet wood is at or very close to the wet-bulb temperature which could easily be 100 to 125 F.
Bark is an excellent insulator.
For example, with 50% relative humidity, the wet bulb is 21 F cooler than the dry bulb, so at 140 F in your case, the wet bulb and wet wood temperature would be 119 F.
Hmm, thanks Gene, when you put it like that it makes sense. I guess for the drying process the solar kiln is great, especially since my work yard is totally off the grid, but for the debugging I'd have to find an other solution, maybe build a second electric kiln nearer to the grid.