Folks, I'm new to this kiln drying process so I appreciate any help or advice you can provide.
I recently built a custom made (insulated) box kiln 4' x 4' x 10' to dry my sawmill lumber in. I'm heating the kiln with 2X 500 watt shop lights. One of which I have plugged into the inkbird controller shown. Even though the actual temp (131deg F) is higher than the target temp (125deg F) this kiln is consistently holding the temp around 125 deg F to within +/-2 deg. I just added a space heater, as shown in photo to help bring the temp up during the cold nights we've been having lately so that's why the temp was high. I also have 3 fans circulating the air and a dehumidifier to remove moisture.
Anyway, I have the kiln loaded with 8/4 live edge cherry. Most of it is between 15-20% MC. It's been running 3 days and my dehumidifier has yet to pull off any moisture. I was concerned that part of the problem maybe that I do not have any vents. Could this be a problem? Or do I need to add patients?
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@dcrice (http://forestryforum.com/board/index.php?action=profile;u=37341)
Your household dehumidifier will probably cut out at 104°F. So you won't get anything at that temp.
That's good to know. Thanks K-Guy!
I, too, have a room dehumidifier in my kiln. Initially. I let it get as hot as 140°F and that temp would destroy the unit. Now, I don't let it get hotter than 100°F and the replacement unit has survived several years. When the wood is 7-8% MC I remove the dehum unit, crank the heat up to heat the wood core to 133°F for sterilization. That has consistently worked for me.
You do not mention any fans to move the air through the lumber pile. Do you have fans or just the DH fan?
Do you also have 3/4" stickers?
How are you measuring MC...pin meter with insulated needles driven 1/2" deep on a face or ??
About how much lumber is being dried in your box?
Quote from: kelLOGg on November 16, 2019, 05:52:34 AM
I, too, have a room dehumidifier in my kiln. Initially. I let it get as hot as 140°F and that temp would destroy the unit. Now, I don't let it get hotter than 100°F and the replacement unit has survived several years. When the wood is 7-8% MC I remove the dehum unit, crank the heat up to heat the wood core to 133°F for sterilization. That has consistently worked for me.
Good to know. I think part of my problem is what K-Guy mentioned. I've been running the kiln at a high temp (125 deg F) and the household DH wouldn't work. I guess I'll need to keep at the lower temperatures for now. It may take longer to dry but it is what it is. How long does it normally take you to dry out Cherry and Walnut to 7-8%? Thanks for the help.....Dr.
Quote from: GeneWengert-WoodDoc on November 16, 2019, 06:32:45 AM
You do not mention any fans to move the air through the lumber pile. Do you have fans or just the DH fan?
Do you also have 3/4" stickers?
How are you measuring MC...pin meter with insulated needles driven 1/2" deep on a face or ??
About how much lumber is being dried in your box?
Yes, I have 3 fans, two in the back and one in front. Yes, I'm using 3/4" stickers.
I'm using a Wagner 220 Moisture Meter.
Currently, I have about 200 to 300 bf of wood in my kiln but it has a capacity of about 400 to 500 bf.
Thanks.....Dr.
the thicker slabs will take longer. you mentioned that at night it requires aux. heat, but you could follow the solar cycle and let the temp drop below 100 at night and let the DH work at that time. what is the RH in the kiln?
I have not yet dried cherry or walnut although I have each soon to go into the kiln. I usually dry oak in the kiln after air drying which will take about 2 weeks to get to 7 or 8%. Operating my kiln in the summer is general not a good idea because the ambient temp is close to the max operating kiln temp. So, when the weather is hot drying in the kiln takes longer. Now is a good time to use my kiln and it is busy.
Quote from: doc henderson on November 16, 2019, 08:31:49 AM
the thicker slabs will take longer. you mentioned that at night it requires aux. heat, but you could follow the solar cycle and let the temp drop below 100 at night and let the DH work at that time. what is the RH in the kiln?
I have not checked the RH. I was planning to add a wet bulb to the kiln but haven't got around to it.
not as industrial, but I have had good luck with the 14 dollar accurite temp and humidity monitors with a remote. I can read from inside a metal container from 200 feet away in my shop. I also got the sensor push that @YellowHammer (http://forestryforum.com/board/index.php?action=profile;u=11488) uses and recommends. they both read about the same. I have got the accurite monitors from WM or amazon. if the RH is too low, it will create more stress depending on the MC of the slabs. if you cycle it like solar it will relieve stress at night when the RH comes up in the air, near the wood surface.
Was the cherry air dried before it went into your chamber, or was it green? IF AD; what was the MC% when it went in?
Quote from: scsmith42 on November 16, 2019, 10:48:57 PM
Was the cherry air dried before it went into your chamber, or was it green? IF AD; what was the MC% when it went in?
I was air dried to around 20%.
Quote from: doc henderson on November 16, 2019, 08:01:13 PM
not as industrial, but I have had good luck with the 14 dollar accurite temp and humidity monitors with a remote. I can read from inside a metal container from 200 feet away in my shop. I also got the sensor push that @YellowHammer (http://forestryforum.com/board/index.php?action=profile;u=11488) uses and recommends. they both read about the same. I have got the accurite monitors from WM or amazon. if the RH is too low, it will create more stress depending on the MC of the slabs. if you cycle it like solar it will relieve stress at night when the RH comes up in the air, near the wood surface.
I'll have try one of those. Pardon my ignorance, but how low of a RH is too low and whats a good stable level?
well if the wood is air dried, it is starting low. the degrade starts early in the drying process. each thickness and species will have its own schedule of temp and RH or wet bulb depression. If the relative humidity is low, it is the same as drying in a house or airconditioned shop with heat. Lets see what @GeneWengert-WoodDoc (http://forestryforum.com/board/index.php?action=profile;u=20498) and others have to say. @Southside (http://forestryforum.com/board/index.php?action=profile;u=24297) @YellowHammer (http://forestryforum.com/board/index.php?action=profile;u=11488) @customsawyer (http://forestryforum.com/board/index.php?action=profile;u=1861) @WDH (http://forestryforum.com/board/index.php?action=profile;u=4370) and others
@dcrice (http://forestryforum.com/board/index.php?action=profile;u=37341) if you have a sealed unit and no leaks, and the DH is not running, then heat is the only thing dropping your RH, so it will go back up when the temp drops. If it is high, you need to get some out. if it is low then you have leaks or you are opening it every day, and loosing the water that way. good luck. there is lots the has been written, and you first have to see where you are in the schedule. I am a student of wood drying, but the afore mentioned guys do it every day, and or are experts in the field! good luck
As @doc henderson (http://forestryforum.com/board/index.php?action=profile;u=41041) has said, it's important to know the wet bulb or relative humidity to know what is really going on in the chamber.
When running the kiln the more information you can get the better off you are.
Question about room dehumidifiers, if you keep a dehumidifier in your wood dryer during cold weather, and it gets down below freezing, will it ruin your dehumidifier? Looked in my trailer this afternoon and it was above 80 degrees, but I took out the dehumidifier when it started freezing at night.
Quote from: farmfromkansas on November 17, 2019, 04:59:08 PM
Question about room dehumidifiers, if you keep a dehumidifier in your wood dryer during cold weather, and it gets down below freezing, will it ruin your dehumidifier? Looked in my trailer this afternoon and it was above 80 degrees, but I took out the dehumidifier when it started freezing at night.
I killed one from letting it freeze up in winter. It built up so much ice it got into the electronics in the top and shorted it out.
The dont work very well if the temp is down below about 10C anyway. So may as well turn it off unless you have some heat in there.
If you have a hot box or kiln that has no air leaks and vents are closed tightly, and if the DH is not running, how will any moisture get out of this kiln? Unless the floor is cold, which will condense the vapor, the moisture will stay in the chamber and gradually build up to 100% RH.
It is difficult to get really accurate readings in a kiln with the surface meters, versus the pin meter. Neither work well above 30% MC.
The best humidity at any specified moisture content of the wood is given in a table called a kiln schedule. The schedule also has maximum temperatures. Kiln schedules are species and thickness specific. The moisture content can vary from piece to piece, heartwood to sapwood, thick to thin, and quarter to flatsawn. For the wettest, usually look for heartwood, thick, quartersawn. There is a book DRYING HARDWOOD LUMBER that has some key schedules. Google it. In this book, convert the wet-bulb depressions to RH with the tables on p. 16-17.
I agree that humidity sensors that cost $25 to $30 are adequate for small kiln drying operations. Order at least three and put all three in the same environment (plastic bag) and check that they all read the same. Return those that are off by more than 5% RH.
You cannot run a household DH at 55F or cooler, as the cold coils will be under 32 F and so will ice up and restrict air flow, which means the unit will likely overheat and destroy seals. If it is not running, there is nothing magic about 32 F ambient. However, seals can get brittle when cold, so heat the unit up to 85 F before starting the compressor.
Quote from: GeneWengert-WoodDoc on November 17, 2019, 10:44:33 PM
If you have a hot box or kiln that has no air leaks and vents are closed tightly, and if the DH is not running, how will any moisture get out of this kiln? Unless the floor is cold, which will condense the vapor, the moisture will stay in the chamber and gradually build up to 100% RH.
It is difficult to get really accurate readings in a kiln with the surface meters, versus the pin meter. Neither work well above 30% MC.
The best humidity at any specified moisture content of the wood is given in a table called a kiln schedule. The schedule also has maximum temperatures. Kiln schedules are species and thickness specific. The moisture content can vary from piece to piece, heartwood to sapwood, thick to thin, and quarter to flatsawn. For the wettest, usually look for heartwood, thick, quartersawn. There is a book DRYING HARDWOOD LUMBER that has some key schedules. Google it. In this book, convert the wet-bulb depressions to RH with the tables on p. 16-17.
I agree that humidity sensors that cost $25 to $30 are adequate for small kiln drying operations. Order at least three and put all three in the same environment (plastic bag) and check that they all read the same. Return those that are off by more than 5% RH.
You cannot run a household DH at 55F or cooler, as the cold coils will be under 32 F and so will ice up and restrict air flow, which means the unit will likely overheat and destroy seals. If it is not running, there is nothing magic about 32 F ambient. However, seals can get brittle when cold, so heat the unit up to 85 F before starting the compressor.
Thanks for the info! I downloaded the book and began reading it last night. Very informative! Thanks for sharing.
The floor of my kiln is well insulated. As far as I know it's sealed pretty tight. I found a cheap digital RH monitor over the weekend and have it plugged into the kiln. If the monitor is correct, the RH was reading 70% this morning. My plan is to ramp up the temp of the kiln today to around 125 deg F with the DH turned off and then let the temp ramp down tonight below 100 deg F, then restart the DH to run over night. Kind of like a solar kiln I guess. Doc Henderson recommended doing this so thought I would try it. I also plan to buy a couple more RH monitors just to make sure I know what's going on with humidity in the kiln.
if you could record some temp vs humidity for a 24 hour period, you will see the relationship. the sensor push that @YellowHammer (http://forestryforum.com/board/index.php?action=profile;u=11488) recommends show the daily fluctuations, and each temp spike up, corelates with a RH drop. it costs more so could put that on your Christmas list. :snowball:. many of the cheap monitors will maintain a max and min. temp and RH. early in a schedule, you keep the RH up for a while, but I am guessing you are past that stage due to prev. air drying. did you mention a MC of the wood? this tells you where you are in the schedule.
tried to take a screen shot and add the graph, but it is an extension and I am too old to figure how to change it to a photo on my phone to include here.
clunky but I think I got something.
(https://forestryforum.com/gallery/albums/userpics/51041/DB505BA9-DBE3-4FF4-A379-4B2E366D7434.jpeg?easyrotate_cache=1574201848)
graph on phone from sensor push
Quote from: doc henderson on November 19, 2019, 04:43:39 PM
if you could record some temp vs humidity for a 24 hour period, you will see the relationship. the sensor push that @YellowHammer (http://forestryforum.com/board/index.php?action=profile;u=11488) recommends show the daily fluctuations, and each temp spike up, corelates with a RH drop. it costs more so could put that on your Christmas list. :snowball:. many of the cheap monitors will maintain a max and min. temp and RH. early in a schedule, you keep the RH up for a while, but I am guessing you are past that stage due to prev. air drying. did you mention a MC of the wood? this tells you where you are in the schedule.
tried to take a screen shot and add the graph, but it is an extension and I am too old to figure how to change it to a photo on my phone to include here.
I ran the DH through the night last night. Checked this morning and the RH was at 57%. The DH had not removed any moisture. Temp was around 95-100 deg F. I check the MC of the wood a week ago, it was averaging between 12% to 20% MC. I don't know, at this point I'm beginning to doubt my moisture meters readings. I have a new acu-rite RH meter on order. I'm not sure I understand what the sensor push is. Thanks for the graph.
I calculated your Equilibrium Moisture Content value at 57% RH and 100F. It is 9.8%. Thats where you wood will try to dry to under these conditions.
The sensor push is a small device that interfaces with a cell phone to provide remote and historical trends of both Temp and RH. Basically, when I walk by my kilns and get into Bluetooth range, it automatically uploads all the data to my phone, where not only can the instantaneous reading be seen, but also the historical trends.
Robot Check (https://www.amazon.com/SensorPush-Wireless-Thermometer-Hygrometer-Android/dp/B01AEQ9X9I/ref=sr_1_1_sspa?hvadid=78340248137414&hvbmt=be&hvdev=c&hvqmt=e&keywords=sensor+push+hygrometer&qid=1574277354&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFJNkdEUE4zQ0JOSVomZW5jcnlwdGVkSWQ9QTAyMjg0NjE3R1pSQ0hHSE1ESFQmZW5jcnlwdGVkQWRJZD1BMDYyMjI2MDM0MEsyM0UxQ0Y2VE0md2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl)
Quote from: YellowHammer on November 20, 2019, 02:14:24 PM
I calculated your Equilibrium Moisture Content value at 57% RH and 100F. It is 9.8%. Thats where you wood will try to dry to under these conditions.
The sensor push is a small device that interfaces with a cell phone to provide remote and historical trends of both Temp and RH. Basically, when I walk by my kilns and get into Bluetooth range, it automatically uploads all the data to my phone, where not only can the instantaneous reading be seen, but also the historical trends.
Robot Check (https://www.amazon.com/SensorPush-Wireless-Thermometer-Hygrometer-Android/dp/B01AEQ9X9I/ref=sr_1_1_sspa?hvadid=78340248137414&hvbmt=be&hvdev=c&hvqmt=e&keywords=sensor+push+hygrometer&qid=1574277354&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFJNkdEUE4zQ0JOSVomZW5jcnlwdGVkSWQ9QTAyMjg0NjE3R1pSQ0hHSE1ESFQmZW5jcnlwdGVkQWRJZD1BMDYyMjI2MDM0MEsyM0UxQ0Y2VE0md2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl)
Thanks for link YellowHammer.
Being new to all this I decided to start with a small kiln (4'x4'x10') just to try and understand a kiln environment. This info you guys are providing me with has really opened my eyes to the wood drying world. It's interesting and I'm staying positive. Maybe I need to start with some pine as a baseline instead of air dried cherry. Do you recommend running freshly cut "green" wood through the kiln or air dried?
Enjoy the learning process. Before I started drying lumber I had no idea what was involved. I quickly learned that it's a deep and wide subject. And then some.
Thankfully there are a lot of knowledgeable folks on here willing to share what they know. Digging around and reading threads on here can provide a lot of practical info. There also many references to books and articles that really help.
The more I learn about drying, the more I realize I don't know. But at least now I'm getting good results and learning more all the time. It has been a rewarding and enjoyable process, and there's still a lot of learning ahead.
You're in a good place here on the FF to move forward. Have a good trip!
@YellowHammer (http://forestryforum.com/board/index.php?action=profile;u=11488) do you have more practical info on the sensor push. I got a good pleth at first. I have to open the app to get info on my phone near the container. now it seems to have a strait line over several days instead of the minute to minute. I have done several uploads, is there a reset or something. not getting the same graph as before.
I don't know, that hasn't happened to me. What happens if you move it into a known environment, such as in your house to see if it responds? I'm wondering if the sensor hole is covered with sawdust or otherwise obstructed?
thanks I will check. the instructions are rather technical. the lines are so straight it seems like maybe it is broke. I thought maybe it needed reset.
Quote from: GeneWengert-WoodDoc on November 17, 2019, 10:44:33 PMYou cannot run a household DH at 55F or cooler, as the cold coils will be under 32 F and so will ice up and restrict air flow, which means the unit will likely overheat and destroy seals. If it is not running, there is nothing magic about 32 F ambient. However, seals can get brittle when cold, so heat the unit up to 85 F before starting the compressor.
Why heat the unit as high as 85 F? In a domestic room the temp will probably never be that high.
Folks, I'm sort of at a dead end with drying my cherry slabs. I bought a $35 General pin moisture meter at Lowes just to get a second opinion and the
slabs are reading 5-6% MC. I check them with my pricey Wagner 220 pinless meter and the MC is everywhere (11-24%). I removed the cherry from the kiln for now. I plan to load it with some maple next. I'm hoping for some improvements so we shall see.
I bought my first moisture tester from MLCS, and it would read about 6 percent on air dried lumber. Bought a mini Ligno, and it reads from 11 to about 13 on air dried lumber. A cheap meter is about worthless.
All moisture meters measure an electrical property and relate that to MC. So, the accuracy of any meter depends on the calibration (electrical property to MC) used by the manufacturer. Typically, foreign-made meters have a different calibration than US made meters, so foreign made meters are not as accurate, so by 5% or more. You get what you pay for.
No electric meter, pin or pinless, can measure accurately above 30% MC. Pin meters cannot do well under 6.5% MC. Pinless can go down to 4% accurately.
PINLESS
The pinless meter requires care in its use. If you measure two 1" thick boards and get a readings on each separately, with an air space underneath each one when you take the reading, and then put the two together, face to face so they act like one 2" piece, you will get a higher MC than the individual.
You can read the MC on one face and then turn the lumber over and measure again, you will sometimes get two different MC readings by several %MC
The pinless meter also responds to density differences...a heavy piece of oak, fast grown, will pave a higher MC than a slow grown lower density piece of the same species and same oven-dry MC.
PIN
To be fair, the pin meter also has issues that must be accommodated, including wood temperature, depth of pins, internal MC gradient, species and pin insulation, to name a few.
My requirements on a meter are lower than most. I just want to know if it is fully air dried or not and maybe how much more it needs to go. With that in mind and no experience in my pocket I bought a really cheap 2 pin one on fleabay for $16.00 or so. It would max out if the wood was over 30% which is fine, that's just 'green' and all I need to know. But for firewood and air drying boards, my sense is it was fairly close. It also seemed to be 'off' on KD lumber. Then I had some slabs I needed to plane and a fellow FF member offered to do that for me on his brand new Grizzly IF the MC was below 15%. Well I checked them and my meter said 12%. I brought the slabs to his place and he pulled out his fancy expensive meter to double check. I figured it was the moment of truth where I would come up looking stupid and and his meter said .......... 12% 8)
At that point I thought I had a pretty good 'first glance' meter. It worked good until my wife ran it through the washer and dryer last week. It is destroyed now. A new one, slightly different brand, should arrive today. We will see if that is close enough.
Here is a thought, if using a Dehumdifer during the summer when the outside humidity and temp is high and that dosn't work. How about replacing that with a portable LG 12,000 BTU AIR CONDITIONER. Just thinking outside the box.
it would help. the idea of a kiln is to speed the drying process, so increased temp over the normal household temp is better, and a way to remove the moisture. the ac unit would pump the heat outside. you could put the unit inside entirely so the heat and cold mix. that is kind of what a dehumidifier is. my 70 pint dehumidifier was about 300 bucks. comes with a water bucket and an adapter to a garden hose to discharge the condensate water outside. if your plan is to mimic indoor conditions, then you can just put the wood in an air-conditioned space.
Consider a dehumidifier kiln...it is a box with lumber and air. It may have a vent that operates when the unit gets to hot, but basically it is a closed system, so the humidity outside has no effect on the humidity inside the kiln.
A dehumidifier in the kiln is a heat pump and is essentially identical to an air conditioner, except both hot and cold coils are inside the kiln, compared to operating as an air conditioner where the hot coils are outside. By putting the hot coils in the kiln, we have the energy needed to evaporate water.
If the kiln overheats, that means that energy is not being used for evaporation. At the same time, if you exhaust heated air, the incoming air will be cooler and fairly dry.
i have started running a dehumidification kiln this winter and I initially had the same issue dcrice is having with the dehumidifier not taking any moisture out. So, I took mine apart and confirmed the pump was working but the coils barely got cold. I then went and bought the cheapest dehumidifier the local home store had and that one started taking out a gallon a day. Maybe your dehumidifier is weak.
My kiln is 4x4x11, insulated and the inside is sealed with plastic sheeting so its sealed pretty well. My dehumidifier will easily keep my kiln about 110 degrees if I let it but I keep turning it off so as not to burn it out. I have been running my last batch for about 10 days with approximately 300-350 bf in the kiln and have removed about 15 gallons of water. I use a Lignomat Mini pin type meter. I have been measuring both external and internal MC daily. Surface MC this morning was 6 % lowest on meter but internal was 12%. I have fans running 24/7 to keep air moving but only run the dehumidifier about 12 hrs a day. It has started to remove much less water so I am planning on running the temp up to about 145 degrees for about 2 days or until the internal temp of the thickest board 8/4 is 140 for at least 6 hours but probably 12 hrs. I will then run the dehumidifier again when the temp drops to 100 because the high heat brings out more internal moisture. I then plan another couple more days with the dehumifier and slowly let the kiln cool with the fans running. which will take about 3 more days. Last run 17 days total.
I really appriciated reading all the MC comments since I'm new to kiln drying.
Southview1, can you share what meters/sensors you are using
I bought the temp sensor/controller for the kiln off amazon, its a Bayite temp controller 1650W BTC211 this has worked perfectly to control the heat. I have the heater controlled by this unit when turning the heat up to 145. I have the sensor located in the middle of the stickered lumber so i get a better reading of what temp is circulating around the lumber.
The moisture meter is a Lignomat mini. This is a push in pin type that was recommended in one of the wood working magazines I get. I drilled a couple of small holes in a couple of boards near the bottom and drove in two nails, insulated around the nails to keep heat and moisture out. When i touch the pins to the nails i get a read of the internal MC.
i have drilled a couple holes in two different 8/4 boards added plugs to them and when i am checking the internal temp of the lumber I pull out the plugs and use an instant read meat thermometer to check temp. Not perfect system but its working and I have some planned modifications after this run. I plan to add a RH meter for thr kiln box and an internal temp sensor that I can inbed into the thickest lumber. I also keep learning from the experienced contributors to this forum so I'm sure there may more.
If I understand you, your are measuring the temperature of the wood. With green lumber, the temperature will be the wet-bulb temperature and then as the wood dries, the temperature will be very close to the dry-bulb. We really do not need these values, however, as there are no instructions on what temperature is the correct value. That is, we just measure the air temperature. For the pins, or nails, it takes 20 F difference to make. 1% MC difference, so accurate wood temperatures are not needed.
Almost every kiln schedule we have uses the average MC of the wood, not shell or not core, to determine the corrected or desired maximum dry-bulb. This is obtained by driving the tips of the pins 1/4 of the lumber's thickness deep.
If you predrill the holes for the nails, make sure that the holes are shallow, so that the nails are driven into fresh, up drilled wood for at least 1/8".
The main reason I measure the the temp of kiln box is mostly to make sure I'm in the acceptable range for the dehumidifier to work properly because if I let the DH run to long it will raise the temp over 120 degrees, not good for DH. The reason I measure the internal temp or the lumber is because I have ppb and want to make sure the internal temperature gets high enough to kill them.
I didn't know much about the average MC measurement but assumed the MC of the wood was an average because everytime I measure MC in multiple places whether shell and internal there seems to be variation.
I measure the internal MC to make sure it has come down to 14% on my meter so when I turn the heat up I don't case harden the wood. I don't remember where I read that it should be 14% or below or that could happen. So, when I turned the heat up this time the shell MC was 6% or below, on my meter, and internal about 14%. When the temp was at 145 deg. the shell was 14% MC and internal read 20% MC or more. As the temp came down so did the MC but not as low as when I turned heat up so I ran the DH another day or two and then let the kiln cool with just fans running.
I know there isn't a bunch of science to my method just information I picked up from here and other sources. I'm trying to put it to use on my lumber to disinfect and not ruin the lumber. I actually prefer to work with air dried, darn ppb.
It would seem that you could use inexpensive sensors (like InkBird ala Amazon) to control turning off the DH after a certain temp, but also use one to turn on an exhaust vent during overtemp. Could also work to exhaust RH after the DH no longer works, yet the wood is still releasing moisture.
Both would give you more control.
PA_Walnut, one of my next modifications is a controlled vent when the heat is up to remove moisture. The DH was cheap from home store so when the power goes out it has to be turned on again. Turning the DH on and off is really not the issue because I like checking on the kiln. I also need to get RH meter and get a better understanding of wet bulb and dry bulb along with much more. I will check on the inkbird sensor.
South, I think your vent/fan will help you a lot. As you said, not knowing the chamber's RH is a real debilitating issue. Either you will have to go super-slow, at the penalty of wasted energy, or risk damaging your charge. If you aren't monitoring RH, you can/should test the MC of the wood often. This will give you somewhat of an indicator of what's going on.
On your DH shutting off, maybe you can circumvent the stock controls with your own. I'd imagine it's not difficult as the compressor is either on or off. However, I'm not aware if the temp switch is a safety thing, or the compressor/gas stops working at a certain temp. On my Nyle, I shut the compressor off at 125° or so.
Of course, by the time you engineer all of this (time and dollars) you may be better off buying a real kiln unit used. If your needs aren't that serious, a solar kiln is a great option. I keep saying that I need about 10 solar kilns...may start building one during the bunker-up going on right now. 8)
Engineering it all is half the fun. I constructed most our of used materials. Its small enough I can move with my skid steer with fork attachment. currently its resting on an old hay wagon bed for mobility. I have about $500 into the whole thing, I don't know what a small used kiln goes for. I am going to get an RH meter and power vent. Which will allow me to manage and understand what is going on in the kiln better.
Did dcrice figure out his dehumidifier issue?
I have an acu-rite temp and humidity monitor in my container. the readout sits in my shop 200 feet away, and records the daily high and low for both. It was $14 on amazon and is right on when checked with other methods. the wet bulb, dry bulb does the same thing but it the traditional way and is easier to adapt to switches for a traditional kiln. basically if the dry and wet bulb reading are the same, no difference, your humidity is 100%. the wider the difference in temp, the wet bulb going lower, it reflects a lower RH. i.e. more evaporation in a low RH.
Doc; what model acu-rite temp and humidity monitor do you have?
I have several. one is bigger, back lit screen, with both inside and remote outside temp and humidity. another is just remote temp and humidity. I will get out and get numbers and pics. later. bought some at WM, and some on amazon.
I found the bigger one that I use in my container on amazon. the remote is in my container 100+ feet away. the station is inside my shop. # 00592A4 at $19.98
got a pic in my shop. the top numbers are in the container, with the remote sensor. the date and time are off I see. back lit if needed, and daily high and lows are stored for 24 hours and accessed by pushing buttons at the bottom. you could record everything once a day. the bottom number are inside my shop.
(https://forestryforum.com/gallery/albums/userpics/51041/8BBD92AF-B426-4C42-8C67-939C4847B8A3.jpeg?easyrotate_cache=1585844320)
(https://forestryforum.com/gallery/albums/userpics/51041/5521DC83-0889-4A60-A9B4-EEB82CEFF6CF.jpeg?easyrotate_cache=1585844349)
I have used this one for about 2 years. the cheaper ones are no longer on shelves at WM, and did not see them online. I used to put the remote under plastic if a did a simple solar kiln, and also in my beer cooler so I could glance at it without opening up a door, and know if I had a refrigeration failure.
Keep in mind that around 6% RH change is 1% MC change. So, extremely accurate RH measurement is not needed.
and... the MC of the wood takes time so the MC is determined by the RH average over time. If it was fast, we could dry wood in one day.