Kiln temperature confusion

[Stevenjohn21]

I recently built my solar kiln but I'm still tweaking it and trying to get it sealed the best I can before putting lumber inside. Today it was 67 degrees outside and the kiln was reading 109 degrees inside which I think is fairly good considering I still have gaps the size of Texas to fill and my double pane roof isn't sealed at the top so the hot air is escaping and only acting like a single pane. 

My question is, at what temperature do I need to get the kiln at in order to kill bugs? I generally cut 2" slabs so if the middle of the slab has to get to 130 degrees then what temperature does the kiln need to be so I can be sure I have no pests inside? 

[YellowHammer]

The FDA requires 133F core temperature of the wood.

[caveman]

When our solar kiln had a new polycarbonate roof on it and loaded during the summertime, we were not confidant that it would stay hot enough, long enough to ensure the core would get to 133°, although it would get hot.  Our solar kiln will get on average about 50° above the outside temperature when loaded and the wood is close to being dry.  We built a separate "hot box" to stack hardwoods into to sterilize.  

Our solar kiln is based off of Pineywood's design.  Our next solar kiln will hopefully have the provision to run a sterilization cycle using halogen lights for the heat source.

[Stevenjohn21]

The FDA requires 133F core temperature of the wood.

I understand that but how hot does the kiln need to get in order to get the core temperature of the wood at 133 degrees?

[YellowHammer]

The speed at which the core temperature reaches the threshold value is very dependent on several factors including thickness of the board, moisture content the board and also wet bulb temperature.

As a standard rule, I run 24 hours at 150 F to insure a core temp of 133F.  I cannot sterilize reliably in my solar kiln.  

 

[Ianab]

I understand that but how hot does the kiln need to get in order to get the core temperature of the wood at 133 degrees?

Technically about 134 degrees, but it might take a few days  for the heat to soak right through. Problem with the solar kiln is that it only hits peak temp for a few hours a day, so even if the kiln hits 135F, the core of the thicker boards probably wont. There is also a spread of the fatal conditions, like "x" temp for "y" hours. 130 might kill them, but it maybe takes 24 hours. 150F might kill them in 30 mins. (once the heat reaches them). Hence YH's regime of 24 hours @ 150F, after that he KNOWS anything in the wood is now a crispy critter. Maybe they were dead after 12 hours, but he's not going out at midnight to reload the kiln, so check it tomorrow after ~24 hours. 

Think of it like cooking a chicken, hotter the oven, the faster it gets cooked through. Upper limit is that your chicken catches fire. :D

[K-Guy]

The USDA requirement is 133°F core temperature for 45 minutes. I recommend 140°F for 24 hours for 4/4.

[blackhawk]

I embed a temperature probe into the center of a scrap piece of wood that is at least 1/2" thicker than the thickest lumber in the kiln.  I monitor the probe remotely and track the core temperature for sterilization.

[Mooseherder]

How often are probes calibrated?   Is it a one time thing, auto calibration or need to be done scientifically before use?

[YellowHammer]

 

 Here's what to look for, dead, charcoal bugs with X's in their eyes. Classic powder post beetle.  Sterilization is instant gratification and you'll know when it's right.  Dead bugs, some the size of ground black pepper, some much larger.  A mouse or two, some snakes, the occasional scorpion, lots of wasps, carpenter bees, etc.  Kill them, kill them all.  

I don't think I can remember a load of wood, especially live edge slabs, where I didn't inspect and subsequently find dead bugs when sterilized at 150 for 24 hours after reaching temps.  I have found dead bugs from my solar kiln, but I also found live ones.  It's unpredictable in a solar kiln, at least mine. 

I like this picture.  This little guy tried to bore out of the wood and flashed off when he hit the hot air of my kiln.  There was nothing left but a cheese puff looking exoskeleton.  I hate bugs and in the early years probably lost more wood to them than any other reason.



 

[blackhawk]

How often are probes calibrated?   Is it a one time thing, auto calibration or need to be done scientifically before use?

I checked my probe before using it by placing it in a calibrated oven where I work.  Then when I installed it in my kiln in open air, I checked it against the Nyle dry bulb probe and they matched within 1 to 2°F.  If both probes match, it is very unlikely that they are both defective but still reading the same temperatures.  You can always take the wick of the wet bulb probe also and now you have 2 probes that you can compare it to.  The comparison to the Nyle probes is a quick check that is easy to do at the start of every load.

[YellowHammer]

The gold standard way to check temperature probes is to put them in both ice water and boiling water.  That checks both ends of the polynomial curve fit equation they use to convert volts or ohms to scaled digital units.  This is also how calibrated standards are calibrated.  Depending on your altitude you may also correct for air pressure or distance above sea level.  

[Mooseherder]

Those are the same methods we used in the meat business.  My experience with the pocket thermometers was they didn't hold calibration if you threw it in a drawer or it got handled improperly and had to be calibrated before use.   The ice water method was quick and easy enough and we kept people away from boiling water. :D

[kelLOGg]

I do like Blackhawk but with a thermocouple inserted into a sample board (usually thicker to be on the safe side) and seal the opening with soft putty to keep moving air from influencing the reading.

[River Mtn Man]

It seems like many people have solar kilns, but recognize they don't serve the full function of drying and sterilization needs. Are solar kilns used as a supplement to dehumidification or vacuum kilns? Or are they a romantic idea for natural drying, but still require additional steps?

What is the best place to start with kiln drying on a tight budget? Solar kiln with a hot box or supplemental heat? 

For those with both solar and dehumidification/vacuum kilns, what role do the solar kilns play in your operation?

My plan has been to build a solar kiln for both budgetary and romantic/natural reasons, but I am curious if anyone looks back and wishes they directed their initial energy towards a nyle or idry?

[caveman]

We have a solar kiln that was loosely modeled after Planman's and Pineywood's designs.  We used materials available for free or cheap.  We initially ran two dehumidifiers along with the solar heat, but the woods that we dry seem to appreciate the nighttime to relax and we get a better finished product without losing too many days in the kiln.  

If we ever get around to building another, we'll likely be able to supplementally heat in the solar kiln.  If we changed out the polyethylene panels on the one we have now, we could probably achieve sterilizing temperatures most of the year, but it does such a good job, we are reluctant to mess with it.  If we do build another, we'll likely configure a hinged insulated panel between the plenum and the top of the stack to hold the supplementary heat in. 

[scsmith42]

It seems like many people have solar kilns, but recognize they don't serve the full function of drying and sterilization needs. Are solar kilns used as a supplement to dehumidification or vacuum kilns? Or are they a romantic idea for natural drying, but still require additional steps?

What is the best place to start with kiln drying on a tight budget? Solar kiln with a hot box or supplemental heat?

For those with both solar and dehumidification/vacuum kilns, what role do the solar kilns play in your operation?

My plan has been to build a solar kiln for both budgetary and romantic/natural reasons, but I am curious if anyone looks back and wishes they directed their initial energy towards a nyle or idry?

I have both solar and dehumidification

The benefit of solar is that you can mix and match different species, thickness, and moisture content material in them, and not have to worry much about damaging it. Typically mine get hot enough in the summertime to sterilize also

There are times that I'll unload solar and place it in the nyle for sterilization. I also have one solar killed with augmented heat for sterilizing.

Typically, solar is a great way for a beginner to get started and kill him drying. It's inexpensive to get into and it's much more forgiving.

[YellowHammer]

A solar kiln is great.  Easy, slow, forgiving, cheap, and will last a long time.  On the other hand, even an Easy Bake Oven has limitations. 

Learn with a solar, if you need more, then get a more. 

[Kit B]

I've had my solar kiln with solar powered fans for a couple of years now and really like it.  I struggled with reaching/sustaining sterilization temperatures until I happened upon an old post by SCSmith42 that showed a set up using a propane torpedo heater set up outside the kiln.