VPD and drying - An overview

[HandyGringo]

I just thought I'd share this here since it's useful to me, so maybe it will be valuable to others too. Sometimes it's hard to have the ideal 60/60 environment for drying, I personally only have a furnace room for drying where the temperature is a bit too high, so I wanted to ensure that my dry was still done as well as possible.

I'm basing it mostly on this presentation:

The Science of Drying and Curing - By Allison Justice, PhD

And this study

Post-Harvest Operations to Generate High-Quality Medicinal Cannabis Products: A Systemic Review

But VPD for drying has been talked about by many more people, for instance, the creator of the Cannatrol also speaks of it and mirrors these same findings.

Anyway. In the above presentation, two interesting charts are presented.



This chart, briefly summarizes which temps and humidities work for drying. Based on this, it seems like 0.8 - 1.1 kpA (VPD) is best for drying. Not too fast, not too slow.

Followed by this chart:



Which shows you some combinations that keep you in that ideal range. The only issue I have with this chart is that it's too narrow, too few combinations accounted for and I don't entirely understand how they calculated the VPD. Because for instance 25% RH and 13c give a kPa of 1.12 not 0.99. So if anyone can tell me why their VPD calculations are different that would be helpful.

I made this chart, to expand it and I'm using the standard VPD calculations, highlighting the aforementioned 0.8 - 1.1 kPa range. Yellow are the values that are almost okay, but still, should aim for green. Red is a bad time all around. The HIGHER the kPa the quicker the dry.



A final note on terpenes. Based on the various studies on terpenes and their evaporation, 21c/70f is the temp where a few of them start evaporating (This is also spoken of in the presentation I linked above) - But a majority of terpenes don't start evaporating until 38c/100f.

It also seems like most of the studies into terpenes recommend drying in the 16c to 21c range (61f - 70f) - I'll be honest, I have no idea why I keep seeing a minimum temp like 18c or 16c mentioned. If anyone can elaborate on this and explain if there's any negative effect from drying at below 16c/61f I'd like to know. Because I can't seem to find any reasoning.

Perhaps it's because it's just cheaper and easier to dry at 16c to 21c, so that's what they tested with?

Anyway, thanks for reading, hope it can be useful for someone else than me. I also made a quick tool to input temp and humidity, will return the VPD and tell you if it's good for drying or not. But I feel like it's a bit redundant with the chart. But here's the link DryingVPD.

Please help me by correcting any mistakes or misunderstandings, because I'm no expert on any of this, this is just stuff I made for my use to help me dry.

 

[conor c]

I just thought I'd share this here since it's useful to me, so maybe it will be valuable to others too. Sometimes it's hard to have the ideal 60/60 environment for drying, I personally only have a furnace room for drying where the temperature is a bit too high, so I wanted to ensure that my dry was still done as well as possible.

I'm basing it mostly on this presentation:

The Science of Drying and Curing - By Allison Justice, PhD

And this study

Post-Harvest Operations to Generate High-Quality Medicinal Cannabis Products: A Systemic Review

But VPD for drying has been talked about by many more people, for instance, the creator of the Cannatrol also speaks of it and mirrors these same findings.

Anyway. In the above presentation, two interesting charts are presented.

View attachment 5410923

This chart, briefly summarizes which temps and humidities work for drying. Based on this, it seems like 0.8 - 1.1 kpA (VPD) is best for drying. Not too fast, not too slow.

Followed by this chart:

View attachment 5410922

Which shows you some combinations that keep you in that ideal range. The only issue I have with this chart is that it's too narrow, too few combinations accounted for and I don't entirely understand how they calculated the VPD. Because for instance 25% RH and 13c give a kPa of 1.12 not 0.99. So if anyone can tell me why their VPD calculations are different that would be helpful.

I made this chart, to expand it and I'm using the standard VPD calculations, highlighting the aforementioned 0.8 - 1.1 kPa range. Yellow are the values that are almost okay, but still, should aim for green. Red is a bad time all around. The HIGHER the kPa the quicker the dry.

View attachment 5410924

A final note on terpenes. Based on the various studies on terpenes and their evaporation, 21c/70f is the temp where a few of them start evaporating (This is also spoken of in the presentation I linked above) - But a majority of terpenes don't start evaporating until 38c/100f.

It also seems like most of the studies into terpenes recommend drying in the 16c to 21c range (61f - 70f) - I'll be honest, I have no idea why I keep seeing a minimum temp like 18c or 16c mentioned. If anyone can elaborate on this and explain if there's any negative effect from drying at below 16c/61f I'd like to know. Because I can't seem to find any reasoning.

Perhaps it's because it's just cheaper and easier to dry at 16c to 21c, so that's what they tested with?

Anyway, thanks for reading, hope it can be useful for someone else than me. I also made a quick tool to input temp and humidity, will return the VPD and tell you if it's good for drying or not. But I feel like it's a bit redundant with the chart. But here's the link DryingVPD.

Please help me by correcting any mistakes or misunderstandings, because I'm no expert on any of this, this is just stuff I made for my use to help me dry.

They don't recommend it lower maybe incase of mould however Ime lower and slower if everything's good is best also it annoys me there's not one mention of in the dark as you should be keeping it away from light as it's drying and curing for the sake of the micros that are involved in the process as well as preserving the monoterpenes also another black hole in our current research is the ferment cure like Malawi cobs I don't think anyone's done any scientific research into that if I'm wrong feel free to correct me folks but I ain't seen much if anything on it

 

[Roy O'Bannon]

Is there a temp diff or wet bulb of the plant itself?
Growing vpd charts ask for a leaf temp to calculate that.

In this case I'm thinking the less of a temp difference the dryer the bud would be.

 

[Avering]

I know the thread is months old, but I’m looking at the last chart you posted and wondering if drying in the late 70s and 80s temp/humidity is okay

 

[FmSwayze]

I just thought I'd share this here since it's useful to me, so maybe it will be valuable to others too. Sometimes it's hard to have the ideal 60/60 environment for drying, I personally only have a furnace room for drying where the temperature is a bit too high, so I wanted to ensure that my dry was still done as well as possible.

I'm basing it mostly on this presentation:

The Science of Drying and Curing - By Allison Justice, PhD

And this study

Post-Harvest Operations to Generate High-Quality Medicinal Cannabis Products: A Systemic Review

But VPD for drying has been talked about by many more people, for instance, the creator of the Cannatrol also speaks of it and mirrors these same findings.

Anyway. In the above presentation, two interesting charts are presented.

View attachment 5410923

This chart, briefly summarizes which temps and humidities work for drying. Based on this, it seems like 0.8 - 1.1 kpA (VPD) is best for drying. Not too fast, not too slow.

Followed by this chart:

View attachment 5410922

Which shows you some combinations that keep you in that ideal range. The only issue I have with this chart is that it's too narrow, too few combinations accounted for and I don't entirely understand how they calculated the VPD. Because for instance 25% RH and 13c give a kPa of 1.12 not 0.99. So if anyone can tell me why their VPD calculations are different that would be helpful.

I made this chart, to expand it and I'm using the standard VPD calculations, highlighting the aforementioned 0.8 - 1.1 kPa range. Yellow are the values that are almost okay, but still, should aim for green. Red is a bad time all around. The HIGHER the kPa the quicker the dry.

View attachment 5410924

A final note on terpenes. Based on the various studies on terpenes and their evaporation, 21c/70f is the temp where a few of them start evaporating (This is also spoken of in the presentation I linked above) - But a majority of terpenes don't start evaporating until 38c/100f.

It also seems like most of the studies into terpenes recommend drying in the 16c to 21c range (61f - 70f) - I'll be honest, I have no idea why I keep seeing a minimum temp like 18c or 16c mentioned. If anyone can elaborate on this and explain if there's any negative effect from drying at below 16c/61f I'd like to know. Because I can't seem to find any reasoning.

Perhaps it's because it's just cheaper and easier to dry at 16c to 21c, so that's what they tested with?

Anyway, thanks for reading, hope it can be useful for someone else than me. I also made a quick tool to input temp and humidity, will return the VPD and tell you if it's good for drying or not. But I feel like it's a bit redundant with the chart. But here's the link DryingVPD.

Please help me by correcting any mistakes or misunderstandings, because I'm no expert on any of this, this is just stuff I made for my use to help me dry.

Have you/or are you using this method currently? My aim here within a few weeks is to set my controller up via VPD to mimic the dew point numbers. My understanding is that is what the new tech is using canatrol* . 68f 61rh dew point 54

 

[formularacer]

Volume enters into equation. I use my shed which has a hanging rack and a 6 tier drying rack. The temp swings at this time of year from 40 to 80 it takes two to three day before dry to the touch. At this point I bin or bucket the contents. The humidity at start shows low 50's after a few hours in sealed bucket it climbs to 70's. Room humidity is in 50's. The buckets are opened for a few hours at a time humidity monitored. After 2 or three days where they are sorted for size popcorn goes in popcorn bucket. When the buckets hit 63 I glass jar the buds.

I have nice tasting cannabis no complaints from any testers decent looking buds and a workable kief collection system.

So there are many ways of skinning a cat. But the trick is putting in the time to monitor. I have thought about doing an automated system but the better quality comes from putting the time in.

 

[HandyGringo]

lower and slower if everything's good is best

I think so too, but I don't know enough about when and how mold forms to put it into words. I believe when you're drying at like, 10c, the chances of mold are minimal, so lotus drying might be the best way to dry, other than freeze drying.

I know the thread is months old, but I’m looking at the last chart you posted and wondering if drying in the late 70s and 80s temp/humidity is okay

Only downside would be that some terpenes start evaporating at 70f. Will affect smell.

Have you/or are you using this method currently? My aim here within a few weeks is to set my controller up via VPD to mimic the dew point numbers. My understanding is that is what the new tech is using canatrol* . 68f 61rh dew point 54

I mean yeah I'm using it, but my drying room is close to ideal lately. Just adjusting the humidity to fit the temperature. I don't know enough about dew point to help you out there. I'm a simple man.

 

[CWF]

For humidity over 50% (always the case for us), an approximation for calculating dew point temp is:

Tdp = T - (100-RH)/5 , T in degrees Celcius (Lawrence 2005) example: 60%rh 60F, Tdp = 46F.

Since both DP and VPD use the air temp and RH to compute them, either one works as a drying control variable equally well. Also, for measuring VPD using an ACI controller or similar (I use the Sensor Push app), set the leaf temp offset to 0 (since once cut, bud does not transpire, and the air and bud are the same temp when drying). HTH

 

[CWF]

I've got a load drying in a Koolatron 20, the usual setup, and it seems to me to not be able to get the high kPa numbers, at least with a full load of wet weed at 60F. It is slow drying at a low temp, which is what I want, and VPD is creeping up. Initially it was around .45 but now is at .50 with RH dropping to around 71. So far so good. The Koolatron/peltier does not have the thermodynamic "ass" to achieve the green VPD chart numbers, and as we know, the CT has to run at higher temps to achieve their numbers, at the expense of taste IMO.

 

[FmSwayze]

Started drying my current harvest using VPD. Starting at .60kPa with buffer of .10, will stay here for the first week.

*whole plant dry
*no trimming
*left in pot
*probe positioned just below canopy
*day 1 was 11/6

 

[raggyb]

I just thought I'd share this here since it's useful to me, so maybe it will be valuable to others too. Sometimes it's hard to have the ideal 60/60 environment for drying, I personally only have a furnace room for drying where the temperature is a bit too high, so I wanted to ensure that my dry was still done as well as possible.

I'm basing it mostly on this presentation:

The Science of Drying and Curing - By Allison Justice, PhD

And this study

Post-Harvest Operations to Generate High-Quality Medicinal Cannabis Products: A Systemic Review

But VPD for drying has been talked about by many more people, for instance, the creator of the Cannatrol also speaks of it and mirrors these same findings.

Anyway. In the above presentation, two interesting charts are presented.

View attachment 5410923

This chart, briefly summarizes which temps and humidities work for drying. Based on this, it seems like 0.8 - 1.1 kpA (VPD) is best for drying. Not too fast, not too slow.

Followed by this chart:

View attachment 5410922

Which shows you some combinations that keep you in that ideal range. The only issue I have with this chart is that it's too narrow, too few combinations accounted for and I don't entirely understand how they calculated the VPD. Because for instance 25% RH and 13c give a kPa of 1.12 not 0.99. So if anyone can tell me why their VPD calculations are different that would be helpful.

I made this chart, to expand it and I'm using the standard VPD calculations, highlighting the aforementioned 0.8 - 1.1 kPa range. Yellow are the values that are almost okay, but still, should aim for green. Red is a bad time all around. The HIGHER the kPa the quicker the dry.

View attachment 5410924

A final note on terpenes. Based on the various studies on terpenes and their evaporation, 21c/70f is the temp where a few of them start evaporating (This is also spoken of in the presentation I linked above) - But a majority of terpenes don't start evaporating until 38c/100f.

It also seems like most of the studies into terpenes recommend drying in the 16c to 21c range (61f - 70f) - I'll be honest, I have no idea why I keep seeing a minimum temp like 18c or 16c mentioned. If anyone can elaborate on this and explain if there's any negative effect from drying at below 16c/61f I'd like to know. Because I can't seem to find any reasoning.

Perhaps it's because it's just cheaper and easier to dry at 16c to 21c, so that's what they tested with?

Anyway, thanks for reading, hope it can be useful for someone else than me. I also made a quick tool to input temp and humidity, will return the VPD and tell you if it's good for drying or not. But I feel like it's a bit redundant with the chart. But here's the link DryingVPD.

Please help me by correcting any mistakes or misunderstandings, because I'm no expert on any of this, this is just stuff I made for my use to help me dry.

I'm trying to watch this more closely at the higher temps (70F) too in the future. Seems like 0.4 vpd is too low and slow. @ 70F that is 80%H. 0.6 also, @70F that is 75H. I've probably been there at the start.

Does anyone know what oxygen's role is in it? I'd think actually air movement is just an idea of keeping anaerobic molds away. So many say have indirect air movement along with the so often repeated 60F/60rH recommendation. But what I want to know is exactly why is oxygen required? Could we not sterilize the air somehow and then not have air movement?