Discussion: Vapor Pressure Deficit/Differential (VPD) during drying & curing

MacroLogos · Jun 11, 2022

Considering how effective VPD obbservation and managment is proving to be during growth, I can only presume it will prove to be as effective during drying & curing (once optimal ranges are established).

Why?

Because Vapor Pressure Deficit/Differential is the measurement of how saturated with water vapor the air is at specific temperatures. This effects how the Law of Diffusion opperates on the h2o in the fruits/air and the speed at which diffusion occurs.

During drying, a higher VPD would cause fruit to dry quicker and vice versa.

That is the prolipsis (first or assumed principle) of VPD theory applied to drying & curing.

That said, I am interested in first hand expreiances using VDP observations & managment in the drying & curing process. Additional thoughts based on theory are welcome as well.

It would be nice to have a range of temperatures & humidity combinations to effectivly dry & cure in.

Keeping temps 70° or less in the south west USA is expensive, and if the VPD resulting from the 60°-70° @ 45%-50% humidity ranges can be created at higher tempratures, is their any reason not to dry at higher temperatures?

Heywood · Jun 11, 2022

Interesting thoughts. I am coming up on harvest soon and thought about VPD for a minute.
I didn't think VPD really applied after chopping and defoliation... no leaves = no stomata = no transpiration?

I am not saying that is correct. I am only saying that is why I dismissed it in my own brain.

Regarding drying at higher temps, we have a recent discussion on that very topic here:

Suggestions when having to dry at higher temps

Hey friends. So I am coming up pretty quickly on harvest time for my Blueberry Muffins. I expect to chop them in the next week or so. In anticipation of drying them, there is just no way I am going to be able to get temps down to 60. It will actually be challenging to get down to 70 and...

www.thcfarmer.com

MacroLogos · Jun 11, 2022

Thanks for the link, but I'd like to keep this thread focused on VPD theory applied to drying & curing.

You're a bit off in how you're thinking about VPD (from what I have been reading).

While related, VPD doesn't require plants or stomata or transpiration to be observed & managed... and.. for context.. VPD will effect the rate/speed at which 5 gal of distilled water completely evaporates out of a container into the air.

@ 1.0 kPa (more saturated air), the 5 gal of water will diffuse into the air slower than 5 gals of water in an environment @ 2.0 kPa (less saturated air).

You are correct that VPD has an effect on the 'indirect diffusion' occuring via transpiration: Rather than the water diffusing into the air directly, it diffuses via the plant (water moves from the soil -> roots -> into plants vascular system -> leaves -> out via stomata and... into the air). Transpiration is simply the h2o 'taking a walk around the block to get next-door'.

So, since VPD is directly related to how the Law of Diffusion is going to function (that law paraphrased... in case people need a reminder: areas of high concentration diffuse to areas of lower concentration until equilibrium occures), VPD will be relevant to how fast/slow h2o in the buds is going to diffuse into the air, in THEORY.

A quick look at various VPD charts, and the recomended temp/humidity recommendations on different sites would give a kPa range between 1.0 and 1.7 (with almost every site recommending temps/% for drying towards the 1.7 (dry side) of the range... and curing with a VPD @ 0.95-1.0 kPa (60°f / 50% humidity).

Heywood · Jun 11, 2022

MacroLogos said:
Thanks for the link, but I'd like to keep this thread focused on VPD theory applied to drying & curing.

Cheers.

growsince79 · Jun 11, 2022

Mold grows best over 75f and you lose terps. Not good idea to use vpd for drying.

MacroLogos · Jun 11, 2022

growsince79 said:
Mold grows best over 75f and you lose terps. Not good idea to use vpd for drying.

Mold grows in h2o saturated air (low VPD), and you get that by ignoring VPD.

However, the point regarding treps is worth considering... but based on an untested presumption.

Is it temp ALONE that damages treps, OR is it a combination of factors that includes temp?

freezeland2 · Jun 11, 2022

I’m drying in my tent right now. I have a ac infinity 6” cloudline running on a 67 controller. It’s the newer controller with Bluetooth etc. it gives a vpd readout. I have leaf temp offset set to zero and fan is running on 3 at minimum and 8 on high. The same controller is also running my t4 supply fan. My settings are maintaining 60% rh and temp is currently 68 degrees. The controller is giving a vpd reading of .99 kpa

My temps and rh are constant and the buds are currently 4 days from chop. I checked them for the first time today and they are drying nice and slow. No hay smell in the tent.

growsince79 · Jun 11, 2022

freezeland2 said:
I’m drying in my tent right now. I have a ac infinity 6” cloudline running on a 67 controller. It’s the newer controller with Bluetooth etc. it gives a vpd readout. I have leaf temp offset set to zero and fan is running on 3 at minimum and 8 on high. The same controller is also running my t4 supply fan. My settings are maintaining 60% rh and temp is currently 68 degrees. The controller is giving a vpd reading of .99 kpa

My temps and rh are constant and the buds are currently 4 days from chop. I checked them for the first time today and they are drying nice and slow. No hay smell in the tent.

Good place to be. At 80f and 64%- not a good place.

MacroLogos · Jun 11, 2022

freezeland2 said:
I’m drying in my tent right now. I have a ac infinity 6” cloudline running on a 67 controller. It’s the newer controller with Bluetooth etc. it gives a vpd readout. I have leaf temp offset set to zero and fan is running on 3 at minimum and 8 on high. The same controller is also running my t4 supply fan. My settings are maintaining 60% rh and temp is currently 68 degrees. The controller is giving a vpd reading of .99 kpa

My temps and rh are constant and the buds are currently 4 days from chop. I checked them for the first time today and they are drying nice and slow. No hay smell in the tent.

Thank you for the first hand testimony.

That controller can give the VPD average over a span of time, could you post the max/min/average for those days? No need to export to a file, it's just 3 numbers.

freezeland2 · Jun 11, 2022

growsince79 said:
Good place to be. At 80f and 64%- not a good place.

Yes I know, I’m not using vpd to dry. I just offered what the vpd is in my tent while drying at my set temp and rh.

freezeland2 · Jun 11, 2022

MacroLogos said:
Thank you for the first hand testimony.

That controller can give the VPD average over a span of time, could you post the max/min/average for those days? No need to export to a file, it's just 3 numbers.

Min temp 67.2
Max temp 70.9
Avg temp 69.3

Min rh 56%
Max rh 62%
Avg rh 59%

Min vpd .88 kpa
Max vpd 1.13 kpa
Avg vpd 1.0 kpa

Aqua Man · Jun 11, 2022

VPD is essentially the same reasoning during dry and cure as it is for growing… its used to control the rate of the movement of water through the plant tissue

During dry and curing we use it to slow the water movement that allows certain processes to continue that would stop if moisture gets to low.

First 3 days are usually lower humidity higher temps to lower the moisture on the exterior of the bud to prevent mold, bacterial or fungal infection. Then we go higher RH and lower temp to prevent the outside from getting to dry and allow the moisture to evenly be drawn out of the bud.

This slow even draw of moisture is very key to allow the breakdown and conversion of chlorophyll, starches etc. through chemical and biological reactions and processes. If it dries to fast it stops

Thats the basic idea and youll find varying opinions on thia subject.

MacroLogos · Jun 12, 2022

Aqua Man said:
VPD is essentially the same reasoning during dry and cure as it is for growing… its used to control the rate of the movement of water through the plant tissue

During dry and curing we use it to slow the water movement thatGreenhouse Condensation Control: Understanding and Using Vapor Pressure Deficit (VPD) allows certain processes to continue that would stop if moisture gets to low.

First 3 days are usually lower humidity higher temps to lower the moisture on the exterior of the bud to prevent mold, bacterial or fungal infection. Then we go higher RH and lower temp to prevent the outside from getting to dry and allow the moisture to evenly be drawn out of the bud.

This slow even draw of moisture is very key to allow the breakdown and conversion of chlorophyll, starches etc. through chemical and biological reactions and processes. If it dries to fast it stops

Thats the basic idea and youll find varying opinions on thia subject.

Yep, that's the gist of it...

Here is a very clear scientific paper (a fact sheet, in fact) on using VPD for both fungual and disease control by controling condensation in greenhouses I just dug up for us...

It goes deeper into VPD's effect on when condensation & evaporation will occure, and give insight on management at any temprature.

And here are the important details:

[...]several studies that explore disease pathogen survival at different climate levels reveal two critical values of VPD. Studies show that fungal pathogens survive best below 0.062 psi VPD (<0.43 kPa). Furthermore, disease infection is most damaging below 0.030 psi (0.20 kPa). Thus, the greenhouse climate should be kept above 0.062 psi (>0.43 kPa), to prevent disease and damage to crops.

However, that is *very saturated* air, and I'm thinking a VPD range of 1.0-1.1 is going to be near ideal for drying, and well out of range of disease/mold levels.

Aqua Man · Jun 12, 2022

MacroLogos said:
Yep, that's the gist of it...

Here is a very clear scientific paper (a fact sheet, in fact) on using VPD for both fungual and disease control by controling condensation in greenhouses I just dug up for us...

It goes deeper into VPD's effect on when condensation & evaporation will occure, and give insight on management at any temprature.

And here are the important details:

However, that is *very saturated* air, and I'm thinking a VPD range of 1.0-1.1 is going to be near ideal for drying, and well out of range of disease/mold levels.

This is because you want water movement always exiting the tissue never entering as it then becomes a vector for infection. I have long said that rehydration of your buds is not a good idea for this reason

Aqua Man · Jun 12, 2022

MacroLogos said:
Yep, that's the gist of it...

Here is a very clear scientific paper (a fact sheet, in fact) on using VPD for both fungual and disease control by controling condensation in greenhouses I just dug up for us...

It goes deeper into VPD's effect on when condensation & evaporation will occure, and give insight on management at any temprature.

And here are the important details:

However, that is *very saturated* air, and I'm thinking a VPD range of 1.0-1.1 is going to be near ideal for drying, and well out of range of disease/mold levels.

Also when reaching dew point in the grow its is ideal for infection. Also long said ppl dont pay enough attention to the humidity just after lights out. I say this because the lights go off and the temperature drops fast and RH being RELATIVE humidity (relative to temperature) that means the humidity spikes up fast and can reach dew point quickly that most never even onow this is happening until its to late. When this happens a few days in a row bam infection.

The worst grow condition is dray days and humid nights. This is ideal for spore germination and spore dispersion

Humid days and dry nights are fine butbi wont get into that

MacroLogos · Jun 12, 2022

Aqua Man said:
Also when reaching dew point in the grow its is ideal for infection. Also long said ppl dont pay enough attention to the humidity just after lights out. I say this because the lights go off and the temperature drops fast and RH being RELATIVE humidity (relative to temperature) that means the humidity spikes up fast and can reach dew point quickly that most never even onow this is happening until its to late. When this happens a few days in a row bam infection.

The worst grow condition is dray days and humid nights. This is ideal for spore germination and spore dispersion

Humid days and dry nights are fine butbi wont get into that

Dry days & humid nights is the end of an Arizona Summer... and I am juggling temp/RH with 2 different evaporative cooler systems and 2 micro climates (an uninsulated well ventilated enclosed 10'x20'x15' shop, and a 4x4x7 AC Inf. tent in that shop).

Falling out of range to the 'wet side' is fine so long as it is corrected within about an hour or 2, but to counter the condensation we *must* reestablish evaporative conditions and increase air flow (>x2) for about as long as we spent "wet"... which will require spending a bit of time out of range on the 'dry side' before dropping back into the sweet spot for the night.

That has been my experience with this current Grow, and all is well -- with the exception of a bit of nutritional issues on 2 plants... so I am thinking I'll be able to pull off Drying & Curing too. It is quite the juggling act. :-D

Perrin6363 · Jun 12, 2022

freezeland2 said:
I’m drying in my tent right now. I have a ac infinity 6” cloudline running on a 67 controller. It’s the newer controller with Bluetooth etc. it gives a vpd readout. I have leaf temp offset set to zero and fan is running on 3 at minimum and 8 on high. The same controller is also running my t4 supply fan. My settings are maintaining 60% rh and temp is currently 68 degrees. The controller is giving a vpd reading of .99 kpa

My temps and rh are constant and the buds are currently 4 days from chop. I checked them for the first time today and they are drying nice and slow. No hay smell in the tent.

I've got the same ac controller and fan and will be using it to dry for the first time soon. I'm hella nervous. So much work to screw it up at the end. I almost want to take a week off work so I can monitor it. But I'm going to count on that controller to maintain 60%. My temps should run around 68°.

Aqua Man · Jun 12, 2022

Perrin6363 said:
I've got the same ac controller and fan and will be using it to dry for the first time soon. I'm hella nervous. So much work to screw it up at the end. I almost want to take a week off work so I can monitor it. But I'm going to count on that controller to maintain 60%. My temps should run around 68°.

I found 70f at 50% for 3 days then 60-63 at 60% until ready to jar. Depending on bud size anywhere from 10-20 days. Just what i found to work well for me.

Sensor placement is key. Air movement is key but never directly at the buds

MacroLogos · Jun 13, 2022

Aqua Man said:
I found 70f at 50% for 3 days then 60-63 at 60% until ready to jar. Depending on bud size anywhere from 10-20 days. Just what i found to work well for me.

Sensor placement is key. Air movement is key but never directly at the buds

70f @ 50% is ~1.45 kPa, and 60-63 @ 60% is ~0.8-0.9 kPa

I'd have to use at least 2 different temp/RH combos... one starting a bit after dawn, and the other, after sunset... to sustain that VPD...

Evaporative coolers have a hard limit: they max out at a 40f temp drop from outdoor ambient @ <10% humidity. Also, as humidity rises, this max temp drop goes down.

So I can't drop and hold a ~110f+ outdoor ambient temp to more than about 82-84f in the tent... but I can drive up humidity to bring it to those VPD targets by using a dual micro-climate approach and running the 2nd evap cooler in the shop's ambient (around 100f+ and 30% RH) and venting the tent's humidity back into the shop.

From this point, fine tuning the tent's micro-climate (mc) is achieved by adjusting the shop's mc RH.

Aqua Man · Jun 13, 2022

MacroLogos said:
70f @ 50% is ~1.45 kPa, and 60-63 @ 60% is ~0.8-0.9 kPa

I'd have to use at least 2 different temp/RH combos... one starting a bit after dawn, and the other, after sunset... to sustain that VPD...

Evaporative coolers have a hard limit: they max out at a 40f temp drop from outdoor ambient @ <10% humidity. Also, as humidity rises, this max temp drop goes down.

So I can't drop and hold a ~110f+ outdoor ambient temp to more than about 82-84f in the tent... but I can drive up humidity to bring it to those VPD targets by using a dual micro-climate approach and running the 2nd evap cooler in the shop's ambient (around 100f+ and 30% RH) and venting the tent's humidity back into the shop.

Many ways to skin a drying an curing cat… just need to keep the principal in mind and you will do fine brother. No hard rules just goals to strive for and what works for me

Discussion: Vapor Pressure Deficit/Differential (VPD) during drying & curing

Discussion: Vapor Pressure Deficit/Differential (VPD) during drying & curing

MacroLogos

Heywood

Suggestions when having to dry at higher temps

MacroLogos

Heywood

growsince79

MacroLogos

freezeland2

growsince79

MacroLogos

freezeland2

freezeland2

Aqua Man

MacroLogos

Aqua Man

Aqua Man

MacroLogos

Perrin6363

Aqua Man

MacroLogos

Aqua Man

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