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

[MacroLogos]

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 (mc) 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.

Then, fine tuning the tent's mc is achieved by adjusting the mc RH of the shop.

 

[Perrin6363]

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

I was planning on placing my 2 little fans on the floor feeding each other to kinda push the air around into a vortex but not face them up. Is that too much air? They're like 8 inch fans.

 

[Aqua Man]

I was planning on placing my 2 little fans on the floor feeding each other to kinda push the air around into a vortex but not face them up. Is that too much air? They're like 8 inch fans.

1 on low is plenty

 

[Perrin6363]

1 on low is plenty

Great, thanks

 

[MacroLogos]

Min Temp: 65°f RH: 16% VPD: 0.6 kPa
Max Temp: 85°f RH: 77% VPD: 1.7 kPa
Ave. Temp: 77.1°f RH: 63% VPD: 1.21 kPa

So the VPD average has been 1.21 for about a week (a bit to the dry side during the day 1.2-1.4 kPa, nights are on the moist side 0.9-1.0), and a nice frost has formed on the sugar leaves. No molds or funk happening.

Under a 480w MARS, these buds are a bit small due to some nutrient issues during flower, but for a summer crop in the heat of the Arizona, it'll smoke. ;-)

 

[Mr_Wight]

Min Temp: 65°f RH: 16% VPD: 0.6 kPa
Max Temp: 85°f RH: 77% VPD: 1.7 kPa
Ave. Temp: 77.1°f RH: 63% VPD: 1.21 kPa

So the VPD average has been 1.21 for about a week (a bit to the dry side during the day 1.2-1.4 kPa, nights are on the moist side 0.9-1.0), and a nice frost has formed on the sugar leaves. No molds or funk happening.

Under a 480w MARS, these buds are a bit small due to some nutrient issues during flower, but for a summer crop in the heat of the Arizona, it'll smoke. ;-)

How did they turn out? I ask because I am starting to dry mine using this method. Thus far everything I have read in this thread makes perfect sense. I would be interested in the final outcomes of others as well.

 

[MasterWatererJeff]

Night transpiration is a consequence of allowing respiratory CO2 to escape at sufficiently high rates through stomata. Respiration fuels night-time leaf expansion and requires carbohydrates produced during the day. As carbohydrate availability and growth are under the control of the plants internal clock, so is night-time transpiration. The cost of night-time transpiration is that water is lost without carbon being gained, the benefit is.. a higher efficiency of taken up water for use in leaf expansion? This could provide an unwanted stress response.

 

[Week4Bytch]

Nice topic here, about to hang a plant or two in my 4x2 and have the AC infinity controller set at a constant 3 at the moment with one open port at the bottom. It's sitting in a 10x10 room which will be at about 60% humidity. the temps vary from 70-75 degrees. Live in the desert. Sucks for drying, but it's been working out. No complaints (yet) about the quality of the smoke. I was drying at 60% in the closet of this room and goofed jarring it to soon the last grow, that was in the winter where temps were 65-70 (from memory) so way to soon, I had to be on top of that cure big time till things settled. See how this current dry plays out at 75F and 60% humidity.