Turning a small chest freezer into a low & slow bud dryer

[bluter]

you can get threaded pipe in different sizes in larger sections if you want to custom cut your own cable channels. it's easy to add some o-rings for water and airtight installation.

edit : i've actually got a pipe threader to make my own. you can make your own and do different sizes with a good pipe thread set as well.

 

[cbdhemp808]

Do they make slightly bigger cable glands?

This is just lamp pipe, used to repair lamps and for ceiling lights, etc. It comes in a variety of sizes, and these are 3" long, which is perfect. So, I could have gotten M14 which is slightly bigger (+2 mm) internal diameter, but wound up with M12.
I looked for actual cable glands that would be long enough for the fridge lid but couldn't find any. So, there are glands that accommodate larger cable, but long ones I think are hard to find.

you can get threaded pipe in different sizes in larger sections if you want to custom cut your own cable channels. it's easy to add some o-rings for water and airtight installation.

I couldn't find any threaded pipe (external threads) except for this lamp pipe. Actual cable glands seal around the cable, but any DIY setup using threaded tubing won't, and that's why I'll use some silicone.

I'm totally fine with 3 of these DIY "lamp pipe" glands.

 

[bluter]

This is just lamp pipe, used to repair lamps and for ceiling lights, etc. It comes in a variety of sizes, and these are 3" long, which is perfect. So, I could have gotten M14 which is slightly bigger (+2 mm) internal diameter, but wound up with M12.
I looked for actual cable glands that would be long enough for the fridge lid but couldn't find any. So, there are glands that accommodate larger cable, but long ones I think are hard to find.


I couldn't find any threaded pipe (external threads) except for this lamp pipe. Actual cable glands seal around the cable, but any DIY setup using threaded tubing won't, and that's why I'll use some silicone.

I'm totally fine with 3 of these DIY "lamp pipe" glands.

it's not hard to drill through a fridge. the 3 should even work out cleaner than one larger one.

the o-rings i referenced would go around the outside of the pipe between the nut and the wall of the fridge. kind of like a bulwark fitting.

edit : watch out for the insulation. it can mess up drill bits.

 

[cbdhemp808]

it's not hard to drill through a fridge. the 3 should even work out cleaner than one larger one

yeah, I figured. I am going thru the lid... probably today first hole.

the o-rings i referenced would go around the outside of the pipe between the nut and the wall of the fridge. kind of like a bulwark fitting.

yeah, I know. but hopefully I have a drill bit that will make this a close-ish fit, and maybe I'll throw a washer on there, and then there won't be any air getting through. the silicone in the hole on the top side should seal it well.

edit : watch out for the insulation. it can mess up drill bits.

I think you mean fiberglass. do you think this is fiberglass or foam? I'll find out soon enough. If it's fiberglass, I'll probably just drill out the metal top and metal/plastic bottom, and then go through the fiberglass with something else just to move it out of the way.

 

[cbdhemp808]

Tonight I started an initial test of the Inkbird temperature controller.

I drilled the first hole in the fridge lid, back right corner...

I started with a pilot hole all the way through using a 5/32" bit. The insulation is foam and very soft. Then I followed with a 1/2" bit. After carefully cleaning up the burr around the hole on the topside with a Dremel grinder bit, I inserted the cable gland and finger-tightened the nuts. Then I put the Inkbird temperature probe through the cable gland. I stuffed a bit of paper napkin in the top-side hole, to seal it up. (In the finished system, this will be accomplished with silicone.)

Underside view of the temperature probe cable coming through the cable gland.

I plugged the Inkbird into power, and the fridge power plug into the Inkbird. I set the target temperature to 63.0°F. LEFT: Initial temp. inside the fridge, 72.3°F, and "set value" of 63.0°F, which is the temperature that the Inkbird will turn off power to the fridge. MIDDLE: The temp. came down to below 63° and power to the fridge shut off. RIGHT: The temperature continued to drop after power shut off. Eventually it settled at 58.6° before going back up again.

This has been a good test so far, but I'm surprised and a bit baffled as to why the temperature continued to drop so much after reaching 63°. I could understand a degree or two, but it dropped a further 4.4°. I must admit, I've never run a fridge this way before, so I'm guessing this is perfectly normal and is an artifact of the cooling mechanism and/or physics.

When this bud dryer is fully operational, I'll be running it at 68° for the initial 3 days, and then 64° after that. I'm going for 64° for this test, but as you can see, it's not so easy to land on 64°. I actually don't understand the dynamics yet here. There are two cases: 1) The initial cool-down, going from room temperature to 64°, and 2) after the cool down, temperature coming back up to 64° and then returning to below 64° again. I will call these two cases the "initial cool-down" and the "normal cooling cycle". (The normal cooling cycle is actually a bit more complicated, because the temp. comes back up to 64°, but there's a setting called "cooling differential", and I have set that to 2°. So, 66° must be reached before the power is turned back on by the Inkbird.)

I'm guessing that in the case of the initial cool-down, the "momentum" of cooling is greater, because of the larger initial temperature drop. The fridge is running for longer, and so, cooling continues for 4.4° after power goes off. In the normal cooling cycle, the fridge would be ON from 66° to 63°, so when the power goes off, I'm guessing cooling would continue for maybe 2°, and reach 61°.

So, I've got some adjustments to make, but I'll leave this running overnight. My guess is that the solar power will easily handle this additional load. The fridge is reading 59° right now, and I'll take a wild guess that it will only increase by maybe 1° per hour, which probably means the fridge will run only once or twice in the night. I expect that the temp. in the morning will be between 62° and 66°.

In the next test, I will add two 1-gallon glass bottles of water to the fridge, to serve as thermal mass, to decrease power usage.

 

[cbdhemp808]

Update on the Inkbird temperature controller overnight test...

As expected, in the morning the temperature in the fridge was between 62° and 66°... it was 62.7°.

Below is a chart I made of the temperature over time, with the fridge going ON and OFF according to the controller settings. ON is at 65°. OFF is at 63°. Keep in mind, this chart is in the context of the "initial cool-down" having already happened – i.e. going from room temp. down to 63°. The chart shows one full normal cooling cycle: ON at 9:53am, OFF at 10:15am, cooling continues until 11:19am, warming begins, and then ON again at 2:45pm. The timing and temperatures of this cycle should be quite consistent (i.e. it repeats indefinitely).

......62.7° - 6:52am
....63.0
..64.0
64.8 - 9:36
65.0° - 9:53 - ON
..64.0 - 10:05
....63.0° - 10:15 - OFF
......62.7 - 10:18
........62.3 - 10:35
..........62.1 - 10:54
............61.9 - 11:19
..........62.1 - 11:53
........62.3
......62.8 - 12:39pm
....63.1 - 12:55
..63.7 - 1:26
64.7 - 2:28
65.0° - 2:45 - ON
..64.0
....63.0° - 3:07pm - OFF


Observations from the chart:

1) When the fridge goes on, it cools by 2° in about 22 minutes.

2) When the fridge goes off, cooling continues for about 1 hour and drops about 1.2°.

3) So, when the fridge goes on, cooling happens for about 1 hour 22 minutes total, and the temp. drops about 3.2°.

4) It takes about 3 hrs 25 min. for the fridge to warm from the lowest temp., 61.8°, to the ON temp. of 65° (a gain of 3.2°).

5) Cooling takes 82 minutes. Passive warming takes 205 minutes. Cooling is 2.5x as fast as the warming.


Time for the next test...

 

[cbdhemp808]

Inkbird temperature controller overnight test #2...

This will be the same as test #1 (above), with the target temperature being 64° and the ON temperature being 65°. This time OFF temp. will be 64° (instead of 63°). I will also add a 1-gallon glass bottle of water to the fridge, to serve as thermal mass, i.e. a cold battery (two bottles won't fit... need to leave space for the mini dehum). This should lengthen the time a bit between the lowest temp. and the ON temp., thus saving some power.

With the OFF temp. set at 64°, I'm guessing 65° to 64° will only take about 10 minutes. The lowest temp. reached will be something above 61.8°... I'll guess 63.2°.

With this configuration...

• The temp. will not exceed 65°.
• When 65° is reached, temp. will quickly return to 64°.
• Temp. will drop to 63.2° (guess), before it then returns to 65°, over a period of between 2.5 and 3 hrs (guess).
• Temp. will be between 64.5° and 63.5° probably over 95% of the time (guess).

TEST STARTED!

 

[cbdhemp808]

Temperature controller overnight test #2 is complete. I let it run all day Wednesday as well. I'm just now still watching the lowest temperature reached in the cycle, and it has settled at 63.3° and is staying there for a long time... well over an hour.

Overall I think this is looking great... fridge ON at 65°, rapidly cools to 64° and OFF. Continues down to 63.3° and hangs out there for over 1 hr., then returns to 65°, repeat. The return time I'm guessing is around 3 hours... I haven't measured it exactly yet. I'll be able to get exact measurements of these intervals when I hook up the temp./humidity monitor and data logger. So, I'm guessing about 4 hours for one cycle, i.e. 65°, cool-down, and back to 65°. That means the fridge will go on ~6 times in 24 hours, and each time it goes on will be for only about 10 min. Roughly three cycles will be during daylight hours when the most power is available, and three cycles will be at night. I feel like this is very energy efficient.

Next up...

After that I'll do a humidity controller test w/ the dehum installed and both humidity probe cable and dehum power cable going through the one cable gland.

 

[cbdhemp808]

Here's the results of today's humidity controller test. I decided to also monitor the temperature, using the temp. controller, since the dehum creates some heat and I wanted to observe that.

I put the dehum power cord through the cable gland, but for the humidity probe and temperature probe I just laid the cables over the edge. I closed the lid and the seal was fairly good... good enough for the test.

Close-up of the above. I left the bottle in there from the temperature test.

I powered up both Inkbirds, but the fridge wasn't plugged in, so there was no cooling involved in this test. Here's a time sequence of the humidity (top) and temperature (bottom). The starting humidity was above 90%, and starting temperature was somewhere around 76°F. Sequence: (A) - dehum running and RH coming down to the set value of 55; (B) - set value reached and dehum goes OFF; (C) - RH immediately begins to rise quickly; (D) - 57 RH is reached and the dehum goes ON; (E,F,G) - RH coming down to 55 again. Notice when the dehum is ON, temperature increases.

Next in the sequence... dehum still running and RH is almost back down to 55. Power monitor shows dehum is pulling 19.4 watts. Temperature is 83°.

Observations...

At the beginning of the test, the temp. in the fridge was about 76°, and with the dehum running the temp. increased to a max. of 83°F, so an increase of 7 degrees. By the way, the best operating temp. for dehumidifiers is in the range 60°F to 85°F, so this is perfect for the operation of this bud dryer w/ target temp. of 64°F.

At the end of the test, the dehum's reservoir had about 2 ml of water in it, so that represents the approx. amount of water that results from 5 cu ft. of air going from 90+% RH to 55% RH.

I was surprised by how quickly the RH came down with the little dehum running, and even more surprised by how fast it went back up with the dehum off, which I actually don't understand; however, it makes me think that the position of the dehum is important, and also the position of the humidity probe, and that a fan to circulate the air is important. The reason being, when the dehum is running, it is blowing warm, dehumidified air out the top. (These same dynamics I think also apply to temperature and temperature probe.)

I think in the next test, I should include a small DC fan to constantly mix the air (which I have on hand), and change the location of the humidity and temperature probes, in relation to the dehum. I need to come up with strategic locations for these things, and perhaps also include a baffle.

I think the next test also needs to include cooling, in order to be a realistic test of the system, because the dehum will behave differently with the temperature down around 64°F. When the dehum runs, it will raise the temperature, which will in turn cause the fridge to run. When I start up the system, at first I will let only the fridge run for the initial cooldown. Once 64° is reached, I can then enable the dehum controller. Then the dehum will run to bring down the RH to 55%, all the while the fridge also will be running (due to heat generated by the dehum). At the end of that sequence, temp. will be somewhere around 64° and RH somewhere near 55%. At that point, the system should be fairly stablized, with temp. rising very slowly (passive warming), but RH remaining the same. With a live run of the system, as the buds dry, moisture is released into the air and the dehum will run, but that should be very gradual; however, keeping in mind that for the first 3 days of running the system, the target temp. will be 68°F. That will remove something like 60% of the moisture from the buds.

I'll need to do some prep work for this next test...

 

[Azimuth]

And of course your thermal mass of water will take a while to get down to the ideal temp zone.

Interesting project @cbdhemp808 !

 

[cbdhemp808]

Thanks, Azi. I was just looking at another couple parts that I need... a power adapter for the fan (USB, 3-pin), and fan cable extension... will need to order and wait for those to arrive. Found this one that has a coupling for going through the cable gland.

 

[cbdhemp808]

Bezos delivery was too slow, so I used eBay. Got a couple parts today in the mail, for the fan power supply.

So, moving forward with the project, probably this weekend.

I'll post some photos tonight of the fan hooked up to the USB power supply.

 

[cbdhemp808]

Low-power fan powered by USB 12v adapter. 3-pin wiring is in 2 ft lengths, giving 5+ ft total length. Fan draws very little power and will be constantly ON during the drying process.

Close-up of USB adapter. Converts 5v DC to a voltage range selected by the thumb screw. Also has ON/OFF button.

Looking forward to setting up the next test, hopefully this weekend...

I think in the next test, I should include a small DC fan to constantly mix the air (which I have on hand), and change the location of the humidity and temperature probes, in relation to the dehum. I need to come up with strategic locations for these things, and perhaps also include a baffle.

I think the next test also needs to include cooling, in order to be a realistic test of the system, because the dehum will behave differently with the temperature down around 64°F. When the dehum runs, it will raise the temperature, which will in turn cause the fridge to run. When I start up the system, at first I will let only the fridge run for the initial cooldown. Once 64° is reached, I can then enable the dehum controller. Then the dehum will run to bring down the RH to 55%, all the while the fridge also will be running (due to heat generated by the dehum). At the end of that sequence, temp. will be somewhere around 64° and RH somewhere near 55%. At that point, the system should be fairly stabilized, with temp. rising very slowly (passive warming), but RH remaining the same. With a live run of the system, as the buds dry, moisture is released into the air and the dehum will run, but that should be very gradual; however, keeping in mind that for the first 3 days of running the system, the target temp. will be 68°F. That will remove something like 60% of the moisture from the buds.

 

[HyperRoller]

very nice project!! Also i have situation that i need to figure out where i dry . Nice info and good job ! Will def follow this thread

 

[Azimuth]

Here's a video related to your project...

 

[cbdhemp808]

Thanks, Azi. That video was packed with a lot of information.

For those who didn't watch it, it's David Sandelman of Vermont being interviewed and talking about his commercial drying & curing system called the Cool Cure, base price $1,600. It's somewhat similar to what I'm building, which is also based on an insulated box with a door, as David says, plus temperature and humidity control. The main difference with the Cool Cure system is they are controlling vapor pressure.

The fact that he mentioned vapor pressure immediately jumped out at me, because I could relate it to what I inadvertently discovered when I put a small jar of green buds in the fridge with a moisture packet. The lid was so tight due to pressure differential that I needed to use a big wrench to get it off.

What I understand about the Cool Cure system is that it's an air-tight box, and they are controlling the atmospheric pressure inside the box. The system is running at 68°F (default), which happens to be exactly the same temperature I plan on running my system at, at least for the first 3 days (as per Rosenthal). At this temperature, the buds are releasing water vapor (and to a much lesser extent, terpene vapor) at a specific pressure. You could call it the water evaporation pressure, corresponding to the rate at which water is evaporating from the buds. If the buds were not in an enclosed space – say just sitting on racks in a greenhouse at 68°F with ambient humidity of 55% – then the water evaporation pressure would be affected by the local atmospheric pressure (i.e. counter pressure). At sea level this pressure is "one atmosphere" or about 760 mm Hg. At 2,000 ft. elevation, the atmospheric pressure is about 7% less, or 706 mm Hg. So, if the greenhouse was located at 2,000 ft. elevation, the moisture in the buds would evaporate off quicker, because there is less atmospheric pressure "pushing back" against the pressure of the water evaporating from the buds.

Now back to the Cool Cure system. They are controlling the temperature, keeping it as steady as possible at 68°F. They are also removing the moisture – it's a closed system, so the moisture from the buds has to be absorbed and captured somehow. But why then also control the ambient air pressure inside the box? That's a good question. When you control the ambient air pressure, you are doing one thing: either increasing or decreasing the rate of water evaporation from the buds. You are also doing the same for the terpenes – if the ambient air pressure is too low (push back), then terpenes will off-gas too much. If you bring it low enough, for long enough, the terpenes will vanish almost completely, which is what I theorize happened with my inadvertent discovery.

So, there's 6 things going on simultaneously in any closed-system bud dryer: 1) rate of water evaporation from the buds, 2) rate of terpene evaporation from the buds, 3) air temperature (and bud temperature), 4) amount of water vapor in the air, 5) removal of water vapor from the air, and 6) stirring of the air using a fan. There's also another dynamic at play: warm air is lighter (less dense) than cold air and consequently exerts less pressure, which also contributes to evaporation rate, on top of the fact that warmer buds will release water quicker. Also, humid air is less dense than dry air, contrary to what you'd think, which gives rise to this dynamic: as the buds warm, they release moisture into the air, which in turn reduces the pressure of the air, which in turn encourages more evaporation.

It's a dynamic, complex system, with a lot of things going on at the same time. But again, why also control the ambient air pressure? The only answer that makes sense to me is, to help make the system resistant to temperature and humidity fluctuations**, both of which change the rate of water evaporation from the buds. For example, if during the cooling cycle the temperature goes from 68° to 66°, then the air pressure inside the box can be adjusted down – i.e. a slight vacuum – to maintain the evaporation rate corresponding to 68°. Likewise, when the amount of water vapor in the air reaches a peak – i.e. high humidity – the air pressure inside the box can be adjusted upward to maintain the evaporation rate corresponding to lower humidity. (**EDIT: This matches what they say on the product website: "...the [Cool Cure] system was able to maintain more stable environmental conditions, minimizing spikes in temperature and humidity".)

Sounds great, right? But is it really necessary to control the ambient air pressure? Another good question.

First, the main goals of a bud dryer are:

1. dry the buds as quickly as possible
2. don't dry them too quickly
3. maximize retention of terpenes
4. prevent mold from growing

While the system is running, let's say at 68°F and 55% RH, with temperature and humidity being controlled by sensors, and both refrigeration and dehum activated as needed, there will be peaks and valleys in both temperature and humidity. Here's the key: as long as these peaks and valleys don't significantly impact the main goals 1-4, then the answer is no, you don't need to control ambient air pressure.

Let's look at both temperature and humidity swings. As long as the temperature doesn't go above 68°, the system will work just fine; however, using air pressure control could speed up overall drying time some. Humidity swings are a bit more tricky. I think small dips below 55% RH would not be a problem. The question is whether or not the dehumidifier can keep up with the water evaporation from the buds, because if it can't, then the humidity of the ambient air in the box could possibly become too high. With air pressure control, this humidity spike could be mitigated by increasing air pressure, and thus turning down the evaporation rate. But, is it really a bad thing if the humidity of the air in the box spikes for a period, before it settles back down? Another good question. Terpene evaporation sort of rides on the back of water evaporation, because warmer, humid air causes increased evaporation.

I think the answer to the humidity spike question is, keep the RH as close to 55% as possible, meaning turn on the dehum at 56%. That will cause the smallest spike possible above 55%. But I think there's a more fundamental principle going on here. Looking again at the main goals 1-4, what does a humidity spike really mean? Certainly a humidity spike helps goals 1 and 4, i.e. dry the buds quickly and prevent mold from growing. That leaves goals 2 and 3: i.e., don't dry too quickly, and retain the terpenes. In my mind, temperature is the main factor for these two goals, not humidity. Or to put it another way, temperature is driving the magnitude of the humidity spike. I plan to run my system at 68°F for the first 3 days, which should remove about 60-70% of the moisture from the buds, and then run for an additional period at 64°F, probably at least a week.

My overall best guess is that a well-tuned system like the one I'm building will have a similar performance as a Cool Cure system, in terms of reaching the goals 1-4. They gray area is in curing – the Cool Cure apparently dries the buds in 4 days, and cures in another 4 days; however, the 4-day curing claim is the absolute minimum, and they admit that some strains could take a month to properly cure. The system I'm making may arrive at 85% removal of moisture in a total of 10 days (or less), leaving 15% moisture content, which is apparently ideal for vaping, which is what I'm shooting for. For storage, I plan to use mason jars and moisture packets.

Other factors are cost, energy use, and drying space. My system, with it's custom rack insert and 8 drying racks, will cost way less than the $1,600 Cool Cure. Main costs: chest freezer: $200, controllers: $73, dehum: $45. Energy use of the Cool Cure is 70 watts (I'm guessing that's max and that it fluctuates). Energy use of my mini-dehum is 19 watts. The fridge pulls about 63 watts when running. With both running, 82 watts. In terms of drying space, it looks like my system will be on par with the Cool Cure... hard to say without knowing the exact spacing of the racks in the Cool Cure. Their system comes with 6 racks about the same size as the racks I plan to build. Their racks are perforated metal, while mine will be stainless steel #5 mesh. I can see the mesh as providing better air circulation.