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 ambient air 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. (Provided of course that temperature and humidity were basically the same at both elevations.)
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:
- dry the buds as quickly as possible
- don't dry them too quickly
- maximize retention of terpenes
- 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. (This is a little counterintuitive, but if the humidity in the air spikes, it means the moisture in the buds decreased, which is exactly what you want to inhibit mold.) 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, which are 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 think the mesh will provide better air circulation.
