Register ICMag Forum Menu Features Search Today's Posts Mark Forums Read
You are viewing our:
in:
Forums > Marijuana Growing > Growroom Designs & Equipment > VPD: Ideal temperature/relative humidity

Thread Title Search
Post Reply
VPD: Ideal temperature/relative humidity Thread Tools Search this Thread
Old 08-21-2016, 07:02 PM #1
MileHighLife
Newbie

Join Date: Jul 2014
Location: Denver
Posts: 14
MileHighLife is on a distinguished road
VPD: Ideal temperature/relative humidity

So you thought your lights were too hot ... maybe you just need a bigger humidifier

Here is a great post by @Kcar:

Quote:
Originally Posted by Kcar View Post
Here is a simpler explanation

Humidity and Vapour Pressure Deficit (VPD)

For years Relative Humidity (RH) has been used as a measure of how much water vapour is present in the air and is probably still the preferred method used by experienced growers. In a greenhouse, the amount of water vapour present has a direct effect on a plants ability to transpire and hence grow.

Another measure called vapour pressure deficit (VPD) is also used to indicate humidity and is felt to be more directly related to a plants wellbeing. VPD combines the effects of both humidity AND temperature into one value and so gives a good indication of plant wellbeing without the need for the grower to do any mental arithmetic. VPD values run in the opposite way to RH values so when RH is high VPD is low.

If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD).

Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms.

It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex.

By measuring the temperature and relative humidity within the crop canopy the calculated VPD is still a useful measure as it combines both temperature and humidity into a single measure in a way that approximates the well-being of the crop. As an example, for many crops it is suggested that RH should be kept between the following limits at the stated temperatures:-


Temperature oC Min RH (apply fogging) Ideal RH Max RH (for disease prevention)
15 - 50 73
20 46 64 80
25 60 73 86
30 70 80 89

You can see from the table that the higher the temperature is the more humidity is required by the plants. The above makes it difficult to specify control parameters as different RH settings are required at different temperatures.

Now look how much simpler this is made by using VPD as the whole of the above table is contained in just three VPD values as follows
Maximum VPD (too hot and dry - apply fogging) VPD ideal

VPD too low (too cool and humid -warm/ dehumidify)
1.25 0.85 0.45

AutoVent 2 and 3 environment controllers estimate the VPD based on the air temperature and humidity in the crop canopy. It will only be close to the true figure for a healthy transpiring crop. The VPD calculator below allows the VPD to be estimated based on both air and leaf temperatures. This clearly shows the possible error in VPD due to just a 1 deg C difference between air and leaf temperature.

As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 KPa

Fogging or other humdification is usually applied at VPDs above 1.25KPa and heating and dehumidification at VPDs below 0.45KPa
VPD Calculator
Calculate VPD on your own using this

Here is a list of temps in Fahrenheit with correlating relative humidities to keep the VPD between 1.25 and .45 KPa

69F - 70F: 50% - 82%
71F - 72F: 53% - 83%
73F - 74F: 55% - 84%
75F - 76F: 58% - 85%
77F: 60% - 86%
78F - 79F: 63% - 87%
80F - 81F: 65% - 87%
82F - 83F: 67% - 88%
84F - 85F: 69% - 89%
86F: 71% - 89%
87F - 88F: 73% - 90%
89F - 90F: 74% - 90%
91F - 92F: 75% - 91%
93F - 94F: 76% - 91%
95F: 77% - 92%
96F - 97F: 79% - 92%
98F - 99F: 80% - 92%
100F - 101F: 80% - 92%

And here's a list of temps in Fahrenheit with correlating relative humidities to keep the vpd between .8 and .95 KPa. This is the sweet spot

69F - 70F: 62% - 68%
71F - 72F: 64% - 70%
73F - 74F: 66% - 73%
75F - 76F: 68% - 74%
77F: 70% - 75%
78F - 79F: 72% - 76%
80F - 81F: 73% - 77%
82F - 83F: 75% - 79%
84F - 85F: 76% - 80%
86F: 78% - 81%
87F - 88F: 79% - 82%
89F - 90F: 80% - 83%
91F - 92F: 81% - 84%
93F - 94F: 82% - 85%
95F: 83% - 86%
96F - 97F: 84% - 87%
98F - 99F: 85% - 87%
100F - 101F: 86% - 88%

A couple of threads on VPD:

https://www.icmag.com/ic/showthread.php?t=311661
https://www.icmag.com/ic/showthread.php?t=153547


Get those foggers pumping
MileHighLife is offline Quote


2 members found this post helpful.
Old 08-27-2016, 02:03 AM #2
cannaisok
Guest

Posts: n/a
its related to veg, right?
cant imagine 80%rh in week 7-finish
just lost 4 main coals 5days ago and it was 78%RH for only 3days at 80-90°F...
Quote


Old 08-28-2016, 01:16 AM #3
RedBeardy5
Member

RedBeardy5's Avatar

Join Date: Jul 2013
Posts: 546
RedBeardy5 has a spectacular aura aboutRedBeardy5 has a spectacular aura aboutRedBeardy5 has a spectacular aura about
Well a big factor is where you live. If you live in the Midwest, you probably will have mold problems if precautions are not taken. If your out west where its real dry then running 80% humidity is pretty safe. Seal up and get some heap filters.
RedBeardy5 is offline Quote


Old 08-28-2016, 02:20 AM #4
MileHighLife
Newbie

Join Date: Jul 2014
Location: Denver
Posts: 14
MileHighLife is on a distinguished road
Quote:
Originally Posted by cannaisok View Post
its related to veg, right?
cant imagine 80%rh in week 7-finish
just lost 4 main coals 5days ago and it was 78%RH for only 3days at 80-90°F...
Pulling the RH down for the last 2 weeks of flower helps to frost everything up.

Was that your RH and temp during lights on and off? In my flower rooms the RH goes way up during lights off and the temp goes down ... without a dehumidifier during lights out I'd have major problems.
MileHighLife is offline Quote


1 members found this post helpful.
Old 09-04-2016, 08:51 PM #5
Lesterburnum
Member

Join Date: Feb 2013
Posts: 186
Lesterburnum will become famous soon enoughLesterburnum will become famous soon enough
Great post. Those seem spot on.
Lesterburnum is offline Quote


Old 09-04-2016, 09:06 PM #6
Tynehead Tom
Banned

Join Date: Feb 2016
Location: In my magic garden
Posts: 3,032
Tynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant future
good thread, new growers, especially those using small spaces or tents..... this should be the first step to grasp and understand. Media and nutrition are easy, dialing in environment so the plants maximize the light and food is where most new growers overlook the details and question their results.

even experienced growers going from bigger rooms to grow tents, need to totally rethink how they set up environment.... I know because I'm one of them. It took me some convincing to not use an intake fan, just didn't make sense to me. Added the small humidifier and rode out the temps and what do you know. My grow log shows similar numbers as the chart posted above and I was extremely happy with the harvests.
Tynehead Tom is offline Quote


1 members found this post helpful.
Old 09-04-2016, 10:28 PM #7
Hookah79
Member

Join Date: Apr 2014
Posts: 211
Hookah79 will become famous soon enough
Used a dehumidifier once ,did not like it.Temps go up,and your ac kicks on.Intake/exhaust fans with climate controllers is where it's at.

I live in a humid environment in the summer.Fans kick on when lights are off ,some nights humidity climbs to upper 70% with the fans running,but as long as you have the right air circulation you're good.

I have never ever had mold problems.

Besides it's cheaper to run inline fans than ac/dehuy....
Hookah79 is offline Quote


1 members found this post helpful.
Old 09-04-2016, 10:41 PM #8
Tynehead Tom
Banned

Join Date: Feb 2016
Location: In my magic garden
Posts: 3,032
Tynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant futureTynehead Tom has a brilliant future
for me I try and only run in the cooler months of the year so for me that is October to end of june. No AC , no dehumidifier , just a 450 cfm exhaust, passive intakes and a blower right under the 940watt bulb. sitting in a 6x6x7 tent in a 12 x 13 x 8 room with the window open when the lights on and closed when it's off. My exhaust only runs during lights on and blows out thru filters and into the house but the blower under the bulb and a small 6" desk top fan stay on 24 7
I live at 3000 ft elevation in western Canada and we do have a naturally drier air here tho this year has been an extreme exception to that rule with more rain than sun it seems.
I realize what works for me doesn't work for everyone but I was having problems dialing my tent in and it took deleting the intake fan and adding a small room humidifier (lights on only) and everything balanced out for my climate..... for the cooler months of the year.
for me it makes more sense to shut down in july and august and into September so I can do outdoor and go play and have some downtime. Something I didn't do this summer and I'm kicking myself LOL
Tynehead Tom is offline Quote


Old 09-05-2016, 07:23 PM #9
RedBeardy5
Member

RedBeardy5's Avatar

Join Date: Jul 2013
Posts: 546
RedBeardy5 has a spectacular aura aboutRedBeardy5 has a spectacular aura aboutRedBeardy5 has a spectacular aura about
what do you guys keep you night time temps and humidity?
RedBeardy5 is offline Quote


Old 09-06-2016, 12:17 AM #10
MileHighLife
Newbie

Join Date: Jul 2014
Location: Denver
Posts: 14
MileHighLife is on a distinguished road
Quote:
Originally Posted by RedBeardy5 View Post
what do you guys keep you night time temps and humidity?
Around 70 degrees and 65% - 70% RH.
MileHighLife is offline Quote


2 members found this post helpful.

Post Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


All times are GMT +2. The time now is 08:16 AM.




This site is for educational and entertainment purposes only.
You must be of legal age to view ICmag and participate here.
All postings are the responsibility of their authors.
Powered by: vBulletin Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.