It’s always fun to see someone ask a weed question and see the bushel of different replies on how to do the same thing.
5 Likes
After watching this video, I found this post lol. The science of How the steps of drying and processing plant matter can affect the outcome has been around for a long time. devils in the details, just the explanation of the plant stress response, enzymes, and fermentation/oxidation in the drying process effect the outcome. Just like if bud is dried too quickly, the enzymes and cells are “killed off” too quickly, you’re left with that green flavor.
8 Likes
I don’t vent. I let the buds keep the humidity up then use a humidifier for later stages.
I do have a fan blowing indirectly just for air movement.
5 Likes
10 Likes
8 Likes
I use those drying racks they work great. These temps and RH seem to be working well out in my shed. 14 days dry Clip nugs into a a hand basket and rotate once a day for a couple days. Into 1lb grove bags. Worked like a champ last year.
I don’t even use a hang line anymore these hold a lot of flower.
16 Likes
I saw the post and figured it was you. Very distinctive style posting screenshots of PDFs. Summarizes most of what I have found although their VPD calculations don’t quite line up with mine. I’ll have to do some more digging.
Thanks for posting this 
5 Likes
I find that using VPD in this fashion is an odd metric as the leaf temperature differential would be practically zero (I believe) and the leaf humidity will be decreasing (no longer 100% and moisture balance to measure). RH and temperature ranges should be sufficient.
7 Likes
So nowadays there are people with a PhD doing conferences about the best way to cure weed. I’d never thought I would see that in my life! 
Thanks! @Dirt_Wizard, bookmarked for a reading later in the bed.
6 Likes
It’s different than VPD as calculated using leaf temp for growing.
I just treat leaf temp as equal to air temp, there’s no illumination. Vapor pressure deficit (or differential) still is the primary variable affecting moisture removal RATE. That’s the knob that controls how long the dry takes, it’s how water evaporates - pressure differential.
Of course, it’s a function of T and humidity. It’s analogous to using your tachometer to control your speed in the car. Speedometer (VPD) is closer to what you are trying to control but you can also get where you want to go using the tach (humidity) and knowing what gear you are in (temp)
3 Likes
As someone who used to drive a manual Volkswagen with a broken speedo but a good tach, this makes total sense. Like many things it’s about internalizing the logic and math of the system either intuitively through practice and observation or by understanding math better than me, or by the nice infographics people like that make. I was able to use the highway mile markers and the seconds hand on the analog dash clock to check real speed and eventually I just knew by gear and pedal feel (a wire through the firewall directly to the accelerator lever and spring, god that was nice), and by the speed of the dashes in the road passing at a given tempo.
5 Likes
Hi FieldEffect. Thank you, I agree and hear what you are saying.
To help clarify what I was noting regarding VPD being an odd metric, the difference between using VPD on a live plant vs drying plant matter hides a piece of information inherent in the calculation.
Leaf temperature, as you’d noted, is one. Leaf temperature meaning plant matter in this context. The temperature differential is practically zero for this scenario (I believe). So we ignore that.
The other is humidity of the leaf. The humidity of the leaf (plant matter) for VPD is simplified such that the leaf is considered 100% saturated. 100% humid. It’s a simplification and you’ll see most online calculators gloss over that value.
Here are a couple of the preliminary calculations for VPD (full algorithm implementation detail can be found here):
vapor pressure atmosphere = Rh * Es
vapor pressure leaf = (1) * Es
The (1) in the above indicates 100% humid. As the plant dries, the plant matter has to become less humid otherwise it would never dry. The humidity term introduces error as the plant humidity decreases. That is, unless the moisture content is measured and inserted into the calculation. You’d need to use a moisture balance or some other technique.
In the end, it doesn’t really matter. Either way, you are setting the humidity and temperature (or VPD) to a range that is effective and safe for drying … and keeping the range balanced through the drying cycle. You’d end up in the same place either way. 
6 Likes
Totally! That abstraction is about as far as my knowledge/comprehension had allowed me to go.
The relationship between the leaf “humidity” or leaf vapor pressure is a difficult one to find a relationship for. The paper linked by @Dirt_Wizard and other plant/packaging science literature I’ve read don’t provide an analytical insight into that.
The water activity level target of 60-62% seems well established, but I believe the detail lies in the sorption isotherm relating the moisture content to that water activity level or eventually, after equilibrium is reached, the RH.
One of the things I haven’t seen specifically for cannabis in any literature were these plots and table of datapoints in the presentation dirt wizard linked:
This is the first data demonstrating the sorption curve for cannabis in particular. I imagine we can back out the detailed internal leaf vapor pressure from this?
I’ve got my mind in other places but appreciate you bringing this up, I stopped thinking about it a year ago content with the simplification. No longer 
4 Likes
@FieldEffect I found this equation for it, I think this is what you’re looking for?
If the air was also at 25 C, but only 50% RH, the vapor pressure (VP) of the air would be:
0.50 * 3.167 = 1.584 KPa
The gradient in VP between the leaf and air represents the true driving force for diffusion of water vapor out of the leaf, and is called the Vapor Pressure Deficit (VPD).
VPleaf - VPair = 3.167 - 1.584 = 1.584 KPa
To relate transpiration (E) to VPD, we use the following equation:
E = [VPD/BP]*gsw
E is the transpiration rate in mmol H2O m-2 s-1
VPD is the vapor pressure deficit in KPa
BP is the barometric pressure in KPa (101.3 at sealevel)
gsw is the conductance to water vapor, usually the total or stomatal conductance
https://www.appstate.edu/~neufeldhs/pltphys/transpirationbasics.htm
5 Likes
mmol meter^2/seconds
Took me a minute so I figured I would clarify for anyone after 
One thing that would seem obvious is the target water content, what remains after the dry stage. It isn’t really since temps are seldom 60 degrees for most of us and so the target RH should fluctuate with temperature.
If drying at room temp of 74 RH would seem to need to be about 65% to achieve the same water content and at 74/58 would be too dry.
I am not certain a constant RH or vpd target serves us best. We may wish to pull a lot of moisture initially and much less later.
6 Likes
I endorse this, my experience so far has been a hard initial dry followed by a slow walk down to curing RH is ideal
3 Likes
I’m going to do some thinking and reading tomorrow on this. I gotta learn about transpiration. I wasn’t searching for the right term
My favorite way to dry is fixed VPD (simplified) for a week ~0.75kPa followed by a week of 62% RH to cure. Based on my experiments with my dry cabinet (which I’ve posted lots of my hypotheses and my setup in that thread) I think that’s the way. As pointed out, if you have solid control of both variables it really doesn’t matter set it and forget it. I have to dry in my shed so the temperature is quite variable (±5F) and there’s some consequence to the method chosen. If I learn something I’ll change the firmware and give it a shot 
4 Likes
8 Likes
2 Likes
I ran a couple of samples through a moisture balance. A moisture balance is what amounts to a precision scale that has an infrared heating element. The heating element bakes off the moisture and the before/after difference is calculated as a percentage moisture.
I ran two samples at ~0.4grams each. First sample is a fresh sample that is close to being stabilized. The second sample is 12 months old.
| Sample | Moisture | RH | Age (months) | Run Temp |
|---|---|---|---|---|
| 1 | 18.62% | 67.4% +/- 3 | 0 | <100C |
| 2 | 15.07% | 66.5% +/- 3 | 12 | <100C |
The plant material moisture content is higher than some of the recommendations that I’ve seen. Although, I’ve haven’t noticed any problems across ~10 gallon containers, some of which are 24+ months old. Some figuring to be had.
10 Likes