Kiln Temperature / Humidity - Logging & Insights

[Everest123]

Question Up Front: Can plotting temperature / humidity from the middle of a wood stack in a kiln give an indication of the overall dryness of the load? Are there known statistics / curve data on that subject?


Background: I've recently purchased a few inexpensive but effective wifi enabled temperature / humidity loggers for my kiln.  I have placed several in my kiln and have started gathering data.  The current load is a mix of black walnut and maple between 4/4 and 5/4.  It's been in the kiln for 5 weeks, and this is about my 6th load through this kiln.  I've found that in SUMMER 5/4 stuff dries nicely in about 6 weeks.  Winter / Fall / Spring is highly variable depending on weather, but I generally expect anywhere from 10-12 weeks depending on stock thickness.  3" slabs can take several months even in the dead of summer - they are stubborn in the middle.

A few things I've learned from all these sensors:

1) "My kiln is getting up to 140 degrees" is a meaningless statement unless you specific where exactly in the kiln your sensor is.  Because WOW does it make a big difference, at least for my set-up.  Mid-stack the temperature stays much cooler until around mid-day, but then stays MUCH warmer later in the day.  I presume this to be because it's a big heat sink / thermal battery.

2) My lumber is currently between 11-14% moisture content in the center of my test pieces and 6-8% on the surface 1/4" or so.  Average humidity in the kiln towards the middle / back of the pile is averaging around 13.% with temps ranging from 105 around 3:30 in the afternoon down to 39 in the early a.m. before the sun peaks up high enough to hit the kiln.

3) My vents are currently cracked open just slightly to allow for some airflow.  Humidity inside the kiln drops as the temperature rises as you can see in the curves.

I'm extremely interested to know if I can use this data to fine tune the operation of my kiln or to better understand the correlation between the data and the state of the load in terms of dryness.  Any experience / perspectives would be more welcome.

-Jeff 



 


 

[mike_belben]

That is some excellent data, thanks for sharing.  What hardware and software or apps are you using?

[esteadle]

I built a solar kiln last year and am learning how to operate it by "feel" with a little device called Tempi.fi which is a simple temperature / humidity data logger with a bluetooth wireless interface and App support on iphone. 

I have one set up between two stacks in the center of the pile to get a general read for when the moisture is trending downward. There is a clear pattern that follows the warmth / sunshine of the day where the temperature rises and humidity falls in unison. The plots on the app show the average of the readings over the displayed area. 

At the beginning of the kiln cycle, the 'area under the curve" of the measured ambient humidity tends to stay above this average line. I've noticed that now as I'm getting toward the end of the cycle, the summation of 'area under the curve' is now below the average. 

What I interpret this to mean, is that I'm getting less of the humidity from the transpiration of wood and more of it from the outside air. Which I believe means that a lot of the moisture of my kiln charge has been transferred out of the wood, as expected. I'm close to done. 

This is not nearly as scientific as measuring the weight of a sacrificed piece and comparing it to the oven dry weight of similar wood and computing the difference (which provides the exact amount of moisture content). But I suppose I will give that a try next and see how it compares to my "area under the curve" intuition. 

[Everest123]

That is some excellent data, thanks for sharing.  What hardware and software or apps are you using?

It's a Govee H5179 wifi / bluetooth module.  $35 on Amazon and so far working extremely well.  It does need to be within range of your 2.5Ghz Wi-Fi for it to send data so you can continuously refresh and monitor from wherever you happen to be with internet connectivity.  Lots of reviews on Amazon complaining about the connectivity but half of them are from people who don't understand how Wi-Fi actually works.  So far it's working great for me.
I don't think we can do links here, but if you search on Amazon it will pop right up.
-Jeff

[Everest123]

I built a solar kiln last year and am learning how to operate it by "feel" with a little device called Tempi.fi which is a simple temperature / humidity data logger with a bluetooth wireless interface and App support on iphone.

I have one set up between two stacks in the center of the pile to get a general read for when the moisture is trending downward. There is a clear pattern that follows the warmth / sunshine of the day where the temperature rises and humidity falls in unison. The plots on the app show the average of the readings over the displayed area.

At the beginning of the kiln cycle, the 'area under the curve" of the measured ambient humidity tends to stay above this average line. I've noticed that now as I'm getting toward the end of the cycle, the summation of 'area under the curve' is now below the average.

What I interpret this to mean, is that I'm getting less of the humidity from the transpiration of wood and more of it from the outside air. Which I believe means that a lot of the moisture of my kiln charge has been transferred out of the wood, as expected. I'm close to done.

This is not nearly as scientific as measuring the weight of a sacrificed piece and comparing it to the oven dry weight of similar wood and computing the difference (which provides the exact amount of moisture content). But I suppose I will give that a try next and see how it compares to my "area under the curve" intuition.

When you refer to the average line, I presume you mean the average humidity over the whole run?  That would make sense.... Humidity relative to temperature is going to decrease inside as the wood dries out.  I have an outstanding moisture meter, so I've been reading regular readings.  At some point there has to be correlation between these readings and the moisture content of the wood.  
I have read all the postings that this is totally unnecessary to do with a VT solar kiln, and I'm sure that is true - but I can't resist. This is fun!!
-Jeff