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Author Topic: RH issues?  (Read 1903 times)

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Offline Don P

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Re: RH issues?
« Reply #20 on: October 14, 2018, 08:35:30 AM »
If it's belt drive a couple of step pulleys might do the trick.

Offline GeneWengert-WoodDoc

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Re: RH issues?
« Reply #21 on: October 14, 2018, 08:18:38 PM »
A variety of MCs as the wood dries is not unusual, as I stated already.  The risk is that we might over-dry the drier pieces (creating more warp, poorer machining, and poorer gluing) while waiting for the wettest to dry.  So, the best procedure is to put humidity in the kiln that prevents over drying but still allows the wetter pieces to dry.  For example, when drying wood that we want to have at 7.0% MC at the end, we might put in a humidity that prevents drying under 6.0% MC, which is 6.0% EMC.  Some might use 5.5% EMC.  In other words, never let the kiln conditions or the wood MC of the driest pieces go under 6.0% EMC.

If we do not do this, but instead use lower humidities or EMCs, then there is a process called equalization.  The kiln humidity is increased at the end of drying (using special rules on when to start and when to end).  However, cupping and poor machining caused by over-drying are not fully repaired in equalization.  Equalization can take 24 to 72 hours in order to get uniform MC, and the correct final MC.  Equalization is described in all drying texts.

In the first process described in the first paragraph above, the kiln is essentially equalizing for a long time, which does slow drying perhaps by a day, but this extra time is offset because no standard equalization is needed...24 to 72 hours is not needed.  Plus the lumber quality is higher.

Regarding air flow, when we get under 30% MC, air flow no longer controls drying speed...it is RH and temperature.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline YellowHammer

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Re: RH issues?
« Reply #22 on: October 15, 2018, 08:14:54 AM »

The graph that says it all, Drying Rate vs Air Velocity.  You can see how at say 40% MC, higher airflow, such as 600 fpm will dry wood about twice as fast as 200 fpm.  So assuming the wood can tolerate it, you can save a lot of time with with higher velocities.  

Conversely, for oak and other wood that requires a low drying rate, and isn't prone to sticker stain, a lower air velocity will inherently dry slower and provide a safety margin.  

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Offline SoftWood

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Re: RH issues?
« Reply #23 on: November 29, 2018, 10:05:07 PM »
May seem like a ridiculous idea once I hear from one of you guys but is there any reason not to pull my lumber out piece by piece as it reaches say %6 MC & then put it back in, all at the same time to heat back up & equalize to %8? 

Offline GeneWengert-WoodDoc

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Re: RH issues?
« Reply #24 on: November 30, 2018, 09:19:12 PM »
What we do in most kilns is to keep the kiln at 6% EMC. This prevents any lumber from drying under 6% MC. Continue with this EMC until the wettest gets to 8% MC.  Ok?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline YellowHammer

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Re: RH issues?
« Reply #25 on: November 30, 2018, 11:08:53 PM »
The key is to set the EMC conditions at a minimum acceptable level so the wood canít get below that.  All the wood in the system will converge to but not go under that level, it wonít just keep drying out more and more. Itís an EMCSN (EMC Safety Net) and will keep from overdying the wood even if all the boards start at different levels.  
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Offline SoftWood

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Re: RH issues?
« Reply #26 on: December 05, 2018, 06:39:38 PM »
Thank you for the continual help Gene & YellowHammer! 
Itís making sense what youíre saying & im aiming for that now but I am getting defeated.
Iím getting close, RH wise, to start checking lumber to see where Iím at & did so today & im pretty far off from what the ecm charts say I should be at. 
I have a hygrometer/temp gauge in the kiln that records the min & max.
My temp is constant between 102-104 & RH is down to 41-44 last night/today, it dropped a few percent each of the last couple days. I havnt bothered checking the wood since the first week because I figured I am so far off still, but when I probed the wood today, 3/4 of it is showing %6, as low as my meter goes. But by the charts I should only be in the mid 7ís.
Had planned to try & keep it around %35 RH in there until all lumber showed the same MC. 


Offline GeneWengert-WoodDoc

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Re: RH issues?
« Reply #27 on: December 06, 2018, 05:56:48 AM »
How are you measuring MC...Equipment brand name?  If needle type, are they insulated and how deep?  Species or temperature corrections? 
Do you measure humidity with a wet-bulb and have good air flow on the bulb?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline SoftWood

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Re: RH issues?
« Reply #28 on: December 06, 2018, 01:01:50 PM »
Itís a Lignomat mini E/D meter with 3/16 pins, I donít think they are insulated.  I used a dry bulb, species is western red cedar. 

Offline YellowHammer

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Re: RH issues?
« Reply #29 on: December 06, 2018, 02:05:30 PM »
One of the most useful skills you can learn is how to oven dry wood to measure true moisture content. It sounds a little intimidating, but is very easy, and with an accurate food or better scale, will tell you exactly what your wood is at, if your meter is reading incorrectly, and will give you a gold standard approach to measurements.  Ironically, once you have a sample specimen in the kiln, it takes aout the same effort to put it on a scale and calculate the result as it does to hammer pins in.  Well... not quite as fast, ;D  but certainly more accurate. 

 
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Offline GeneWengert-WoodDoc

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Re: RH issues?
« Reply #30 on: December 06, 2018, 03:38:44 PM »
With this meter, you are getting mainly the surface MC.  The core is likely wetter, so the average MC of the piece is likely higher than 6% MC.

Even so, at 45% RH at room temperature, we would expect to see around 8% MC.  The temperature correction, using the temperature where the needles are driven, is about 1% MC for every 20 F hotter than room temperature.  Add when colder and subtract when hotter.  So, in your case, either the meter is wrong or your humidity is wrong.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline SoftWood

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Re: RH issues?
« Reply #31 on: December 08, 2018, 03:27:53 PM »
The gf isnít convinced that itíll be fine to oven test the wood, but Iím working on her.

I did get the moisture meter somewhat sorted. Got some longer pins, 7/16Ē, & now its showing a higher MC. 
May be a late question now but where is the proper place to probe the lumber?  Obviously getting a different number when I prove the end vs the face.  Itís only 4/4 stuff but I hate sacrificing a board by probing

Offline btulloh

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Re: RH issues?
« Reply #32 on: December 08, 2018, 03:34:17 PM »
Pick up an older microwave that somebody is ditching or off craigslist.  In this case the gf is on firm ground.  I wouldn't do it in the house either, even with a dedicated mw.  You can, but .... interesting fumes come out of the sample.  It is really instructive and useful to try the oven dry method even if you don't use it all the time.  There are multiple threads on here about using the microwave oven.
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Online Ianab

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Re: RH issues?
« Reply #33 on: December 08, 2018, 03:57:25 PM »
Yup, the microwave ends up smelling of whatever wood you happened to be cooking. As well a boiling off the water vapour there seems to be some resins and other chemicals come out as well (same as any high temp kiln) Likely depends on the wood, but any species of cedar is going to leave something behind.

And then if you use too much power, and set fire to the wood, you have a whole other smokey mess to clean up.  :D
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)


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