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Wind Sensor for Wood Drying Kiln

Posted: Mon Dec 28, 2020 12:18 am
by JohnLPoole
Background of Wood Drying

I'm designing a wood drying kiln. A wood drying kiln is essentially a pile of boards that are stickered so that air can flow through the pile and the moisture in the wood can evaporate. One way to dry wood is simply to sticker it and let it naturally dry with whatever prevailing winds there are; the rule of thumb for the length of time it takes is about 1 year for every inch.

In a kiln, the environment is completely enclosed and the air is recirulated by fans. Dry air enters the pile from the front as as the air travels across the faces of the stickered boards, it picks up moisture through evaporation. At the back of the pile, the air, having picked up moisture from the wood, is cooler and its humidity raised. Behind the pile is a dehumidifier and heater so the air is dried and heated and then directed over the pile to again enter the pile as hot dry air. Kiln drying can accomplish in a matter of weeks what would take a year or more. The magic is to not dry the wood too quickly as the wood will crack and bend if the moisture is taken out too rapidly. Practice in the industry is to have an airflow of 100-800 cubic feet per minute over the wood; 300 CFM seems to be a highly desired rate. One reason for the high air flow is to discourage mold growth. Airflow is a variable as part of the drying process, so it needs to be monitored and adjusted.

I have a photo of a stack of wood stickered showing air flow, but there seems to be a setting preventing images. In Preview Mode, I encountered "You can’t post image, email or url links. "

My Issues
I would like to explore having sensors placed at various locations within the kiln, most likely at points on the front or back face of the stack. A design consideration is directing the air flow from the fan or fans so that there is a uniform flow or air entering the stack. Baffles (large Styrofoam boards or plywood pieces) can be placed about to break up the air flow and try to cause a more uniform flow at any given location on the face of the stack. One uses an anemometer taking readings at the entry face of the stack to set up the baffles.

The kiln I'm designing is to be collapsible so I can set it up in a garage, dry a pile of woods, e.g. 10' x 4' high x 3' wide, and then disassemble the kiln afterwards. I'm planning to use polyisocyanurate, see Polyisocyanurate on Wikipedia, which is a rigid foam, aka urethane foam, for the walls and roof and I'm trying to keep the kiln small. Unlike large industry kilns where you can walk in, taking reading in a fully enclosed situation with this small kiln will be problematic. So, sensors hooked up to an Arduino looks very attractive.

I've played with temperature sensors using the 1-wire protocol and quickly ran into a roadblock when I learned about distances and signal degradation, interference and/or attenuation. Various solutions included have a star hub multiplexer and it just became very complicated if I wanted to have sensors more than 20' away from the computer. What I am wondering here is if I were to have several Wind Sensor Rev. P, would there be multiplexing issues with the protocol? Humidity is another factor, I saw a posting in the forum that discussed encasing almost everything. High humidity is something you can have in a wood drying cycle and with the heated element in the Wind Sensor Rev. P, will condensation occur that might cause shorts? I've gone to a considerable effort here to explain the application where I might use the Wind Sensor Rev. P units because it is a highly humid, hot (I plan to have up to 120 degrees Fahrenheit) and possibly corrosive... what comes out of the wood is not necessarily pure water.

If I can get past the multiplexing issue (I'd want 10-30' feet of wire between a sensor and the multiplexer) and possible attenuation, if any, then I'm facing a harsh environment.

I also saw postings of wind tunnels being used to devise a better formula for the results from the sensor, so a work in progress. I recall 300 CFM is near 3-5 MPH, so not a very fast wind, but I have not seen anything to suggest that slow winds are any less detectable by the Wind Sensor.

Thank you for your time reading this.

Re: Wind Sensor for Wood Drying Kiln

Posted: Mon Dec 28, 2020 11:13 am
by paul
JohnLPoole,

A lumber yard bought a lot of sensors from me to try the same thing. People don't report on their projects usually though, so I can't vouch for the results.

I would suggest spraying the sensors with a coating for the high humidity - or we could coat them for you for a few dollars a sensor - we have some acrylic conformal coating that we use on another product. The sensors are very good at sensing low speeds and are very repeatable, however even the Rev P has some temp sensitivity so it would be a good idea to calibrate them with another sensor at the conditions you are using.

I also recommend cleaning the sensors from time to time as build up of dust on the hot element will affect the readings.

A newer version of the sensor is about to drop that has improved ambient sensitivity. It's really just a change in some resistors, so you might want to wait for the new version which I will post about.

The take home:
The sensors will work but will require some fussing - I'm only recommending the Rev P.
They may not last forever but are fairly inexpensive to replace, compared with your expensive cargo anyway.

Re: Wind Sensor for Wood Drying Kiln

Posted: Thu May 20, 2021 8:41 am
by JohnLPoole
paul wrote: Mon Dec 28, 2020 11:13 am
A newer version of the sensor is about to drop that has improved ambient sensitivity. It's really just a change in some resistors, so you might want to wait for the new version which I will post about.
re: Wind Sensor Rev. P $24.00

Has a newer version become available?

Re: Wind Sensor for Wood Drying Kiln

Posted: Thu May 20, 2021 10:47 am
by paul
JohnLPoole ,

Yes, shipping Rev P6 now. Very little change in output over temperature.
But I'm remiss for not having posted about it. Working on documentation

Paul Badger