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 
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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.
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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.
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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)
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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.
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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. 
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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 
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@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
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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.
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I endorse this, my experience so far has been a hard initial dry followed by a slow walk down to curing RH is ideal
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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 
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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.
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Hey guys. I usually wet trim. I can’t see for shit and small leaves right next to the buds after drying is too much for me. However, i have an auto I took down this last week as a dry trim test as I haven’t done it that way in about 10 years. My last wet trim dry I had to do a bunch of stuff to keep the bud from drying out too fast. had to buck at day 4 and put into brown paper bags, waking up at night to check them. it was not nice and I think the paper affects the smell. to keep it from drying in like 3 or 4 days, I can start the tent at 65rh and work down a pct each day. by day 4 the plants can’t even keep the tent at 60rh. by 55% rh it’s ready to jar and then i have to burp over the next month.
I am at day 4 of drying a whole hung plant. I started at 62% on the controller. one day the humidity was higher than 60% in the house. i had to run humidifier for a bit. already the rh level is 57% in the tent and I am guessing I might make it to day 5 before i have to buck and trim. since i have like 20 plants to trim, i was hoping i would have freedom to pull out plants or parts of plants when I felt like it because the wet trim would ‘extend’ the dry process so much. some people it seems are pre drying their crop on the vine before chop just to dry in 14 days. I can’t make it a week and i water just before chop.
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What’s the temp? Even if your RH is around 60 temperature makes a big difference. 80F will go about twice the rate 60F will
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temp is 60 to 67ish. it’s good. i’ve dried before. best was in the spring during series of rain storms. humidity was so high i ran a dehumidifier. it seems like the lung room needs to be 60, not the dry tent per se. i just setup a humidifier in the lung room to get the rh back up to 55ish. that way i don’t have to pull it down. 55 is ok, below that is not in my experience. but I don’t know much…
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So what does the science say? How do we lose the hay smell and retain that potent weed smell? Am I really doing more harm than good jarring my buds with a bit of extra moisture and burping daily?
Usually takes about 4-5 days (depending on bud density) to “dry” to my liking, and then I jar it and burp occasionally until the buds feel right.
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i got some hay, green, herbal stuff. i package all my bud into quarters in mylar packs. they are sealed. after about 2 months in a bag the bud smell good. it had about 50 or more days in a jar before going into the mylar bag. it’s way different now. not sure that would fix it all, but some bud needs a long time i guess. i have other bud, most of it, that is best the earlier you smoke it. perhaps dessert buds need less time.
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For me, if I can get a 10-14+ day hang dry, my terps are wild. With a quick dry, I’ve got some grassy shit, no coming back from that.
I also get a better result, with, the dryer the better before the jar.
Friend handed me some weed to ‘test’, i smelled it and asked how long he dried it for. 2-3 days 
$0.02
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