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Author Topic: lumber temperature / wet bulb temp / fiber saturation  (Read 2521 times)

Offline Craig Jensen

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lumber temperature / wet bulb temp / fiber saturation
« on: September 24, 2014, 02:52:56 PM »
I have what may be a stupid question, but I'm gonna ask it anyway.  ;D
I know that before fiber saturation point, I can pretty much assume that my lumber temperature in a charge is about the same as my wet bulb temperature.

Is there a simple means with normal kiln control to tell when the lumber temperature starts to rise past the wet bulb temperature?  It would seem to me that we could then assume we were past fiber saturation point, and then once we were able to determine our typical drying rate in a specific species and schedule, we would be able to more accurately predict when we hit our moisture target.  I am thinking this would result in fewer interruptions in the schedule to check moisture, and improve quality.
Suggestions?  Am I missing something obvious?
Craig Jensen

Offline MichaelM

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Re: lumber temperature / wet bulb temp / fiber saturation
« Reply #1 on: September 25, 2014, 03:31:12 PM »

People have tried that in the past and, yes, it can be done.  The Lignomat control system can tell you about the shell and core but a capacitance system could also tell when the wood reaches 30%.  These don't use lumber temperature, however, except maybe to correct the meter.

In your question, you ask "with normal kiln control."  I suppose one could calibrate the TDAL for that purpose (for a given schedule and product).  Often people just use it for end point determination, but it can (and is) correlated to MC in some control systems.  Again, this is not lumber temperature, it is the air temperature.

I would say the most effective way (of using wood temperature) is to place a thermocouple or other sensor at the center of the piece.  This involves drilling, putting in the sensor, sealing the hole, and placing the board(s) in a representative location.  The surface of the board will lose the free water first and the surface temperature rises first, then the mid layers, then the core.  The temperature rises at the core when the free water leaves the core.

To make the measurement easier, cameras can be used to detect the surface moisture content.  This might be done with visible, NIR, or IR, light but the surface MC.  Surface MC, however, can vary a lot with conditions and with the previous wood handling.

Not sure if that's the whole answer, but it's what I have.  And, no question is a stupid question except the one that does not get asked.

Mike M.



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