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DIY kiln controller

Started by perhans, October 19, 2021, 07:49:08 AM

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perhans

Greating from Denmark.

I am looking into building a small scale dehumidification wood kiln and a part of this project is to make a controller that measures and controls temperature and humidity in the kiln. I have read a lot online and also a few books but I am still very much in doubt what the best way is to go about this. So let me ask a few questions and hopefully someone can clear things up for me.

*One thing that confuses me is that a lot of the sources online and in the literature are quite old and I still can't figure out if all the talk about web and dry bulb temperature is because it after all it the best way to measure humidity or just because the electronic humidity sensor was not invented or readily available when the source was written. Is wet/dry bulb temperature still the best way to measure humidity in a wood kiln today and why?

*As far as I understand, one of the dangers with a dehumidification kiln is to dry the wood too quickly. In order to prevent this from happening my idea is to measure the humidity in the kiln and also measure the amount of water coming out of the dehumidifier and compare this to a drying chart I have from a book and stop the dehumidifier if it pulls too much water out. Is this a good way to control the drying rate?

GeneWengert-WoodDoc

Indeed, wet-bulb was used as a way to measure relative humidity starting in about 1909. Other than the requirements for distilled water and good air flow across the wet wick, the wet bulb was very rugged and was a reasonable tool in the dry kiln environment.

After the wet-bulb, Bruner-Hildebrand kilns developed the concept of using a thin wood wafer, beech originally, and then measuring the resistance to measure then moisture content, which was then converted to relative humidity.  Soon thereafter, kiln schedules sometimes included an EMC setting for the wood (paper was also used) sensor readings.  Again, these stood up to high and low humidities and high and low temperatures for at least one kiln run.

Humidity electronic sensors were develop decades ago, but most cannot maintain calibration when exposed to high humidity.  High humidity can occur at start up, when fans reverse, when drying a wood that dries faster than the vents or dehumidifier can remove moisture, etc.  most early sensors were limited to around 120 degrees F maximum, so steam kilns could not use them and some dehumidifiers could not use them.  In addition, the acids released, such as acetic acid, confused the sensors especially after several kiln runs.  With accidental exposure to high humidity such as during a power loss, has limited electronic sensors in kilns, although some warehouse predryers use them.  In predryers, I have seen 10% RH errors.  Further, electronic sensors are usually no better than + or - 5% Rh (some 3%), so their accuracy is an issue.  Plus their long term performance above 120 F is questionable.  Plus there are plenty of devices sold that are inaccurate when made and have no durability.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

perhans

So wet-bulb is still the best way to measure RH in a wood kiln today?

GeneWengert-WoodDoc

The main issue with a wet-bulb is the water supply.

The wood or paper wafers are equal in performance for drying lumber.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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