Is Absolute Humidity More Important Than Relative Humidity?

J

jochhe1998

Active Member
Hey guys, so my questions is what is more important Absolute Humidity or Relative Humidity?

For those of you who don't know the difference here is a short breakdown.

- Absolute Humidity is the total mass of water vapor per volume (typically expressed in grams per cubic meter)
- Relative Humidity is expressed as the percentage of water mass that can be stored per volume of air at a specific temperature (see chart below)

Absolute vs Relative Humidity Chart.png
Image 1: taken from https://socratic.org/questions/how-is-relative-humidity-related-to-the-dew-point

While this chart may not be terribly accurate it serves a general purpose.

Now, in terms of pest and fungal development, shouldn't absolute humidity be the measure by which growers go by? My thinking is that a fungus/mold/pest needs 'X' amount of water vapor to survive and procreate (could be wrong). Considering that the total amount of water at 100% humidity (20 C) is almost the same as 50% humidity (30 C) isn't RH (relative humidity) a pointless measure - especially considering most grow rooms experience 5 degree swings regularly?

All discussions are welcome but please state how you came to your conclusion!

Thanks for the help :)
 
Renfro

Renfro

Well-Known Member
absolute-relative-humidity.png

My favorite chart for humidity. Take a RH% number like 50% @ 80.6F and you get absolute humidity of 11.34 grams of h2o per kg of air. Now take that to a lower temperature like 69.8F and your absolute humidity number puts you somewhere between 70 and 75% RH.

Cooling air raises it's RH% assuming the moisture content (absolute humidity) remains unchanged.
 
Hot Diggity Sog

Hot Diggity Sog

Well-Known Member
Renfro said:
View attachment 4745860

My favorite chart for humidity. Take a RH% number like 50% @ 80.6F and you get absolute humidity of 11.34 grams of h2o per kg of air. Now take that to a lower temperature like 69.8F and your absolute humidity number puts you somewhere between 70 and 75% RH.

Cooling air raises it's RH% assuming the moisture content (absolute humidity) remains unchanged.
Click to expand...
Link to a larger image...
 
Kerovan

Kerovan

Well-Known Member
I don't worry about humidity. They humidity in my house in the winter (the only time I grow) is around 40 @ 70°f. As long as the plants are watered they don't care.
 
J

jochhe1998

Active Member
Hot Diggity Sog said:
Link to a larger image...
Click to expand...
Hey Hot Diggity - thank you for the link :)
Renfro said:
View attachment 4745860

My favorite chart for humidity. Take a RH% number like 50% @ 80.6F and you get absolute humidity of 11.34 grams of h2o per kg of air. Now take that to a lower temperature like 69.8F and your absolute humidity number puts you somewhere between 70 and 75% RH.

Cooling air raises it's RH% assuming the moisture content (absolute humidity) remains unchanged.
Click to expand...
Hey Renfro thanks for the help man. That graph looks far more helpful than mine- making a print now.

Also, just to confirm, you believe we should be measuring Absolute Humidity instead of Relative Humidity? If so, is their a specific range of Absolute Humidity you like to stay in for veg/flower?
 
J

jochhe1998

Active Member
Kerovan said:
I don't worry about humidity. They humidity in my house in the winter (the only time I grow) is around 40 @ 70°f. As long as the plants are watered they don't care.
Click to expand...
Hey Kerovan, that must be nice. Is there any particular reason you only grow in the winter? (just curious).

Unfortunately, I live in the Caribbean and one of the downsides is that the humidity here is a MONSTER.
 
BBQtoast

BBQtoast

Well-Known Member
Relative humidity is not a simple thing, take how clouds form. You see chuck in LA at 30% doing just as well as Frank in China at 70%. A humid day where there were no clouds? Weird science for sure.
 
Star Dog

Star Dog

Well-Known Member
Relative humidity is my guide, absolute humidity is irrelevant to me.
I'm not looking to complicate growing rh% works great for me I've no need to change my method.
 
Star Dog

Star Dog

Well-Known Member
jochhe1998 said:
Hey Kerovan, that must be nice. Is there any particular reason you only grow in the winter? (just curious).

Unfortunately, I live in the Caribbean and one of the downsides is that the humidity here is a MONSTER.
Click to expand...
The amount of moisture in the air during the winter is significantly lower than in the summer, 60% at 10c is world away from 60% at 20c, the 60% air at 10c gets heated and it's rh% drops by almost half.

I'm in the UK it has a high rh% all year round but when it's winter the air is to cold to hold much moisture so it might say 70/80% but as it is heated it's rh% drops rapidly.

Warm air that has a high rh% is the growers nightmare, I've no doubt you have a constant battle with it?_20201029_002933.JPG
Yes its damp, but cold damp isn't as much a problem as warm damp.
 
J

jochhe1998

Active Member
Star Dog said:
The amount of moisture in the air during the winter is significantly lower than in the summer, 60% at 10c is world away from 60% at 20c, the 60% air at 10c gets heated and it's rh% drops by almost half.

I'm in the UK it has a high rh% all year round but when it's winter the air is to cold to hold much moisture so it might say 70/80% but as it is heated it's rh% drops rapidly.

Warm air that has a high rh% is the growers nightmare, I've no doubt you have a constant battle with it?View attachment 4746360
Yes its damp, but cold damp isn't as much a problem as warm damp.
Click to expand...
Ahhh now I see.

Yes the warm humid air is definitely a constant battle. I run my AC 24/7 now and usually hold approximately 60% - 65% relative humidity between 26 and 30 degrees Celsius.

I am only vegging right now so I'm not too worried but may need to invest in a dehumidifier once they switch to flower.
 
Star Dog

Star Dog

Well-Known Member
jochhe1998 said:
Ahhh now I see.

Yes the warm humid air is definitely a constant battle. I run my AC 24/7 now and usually hold approximately 60% - 65% relative humidity between 26 and 30 degrees Celsius.

I am only vegging right now so I'm not too worried but may need to invest in a dehumidifier once they switch to flower.
Click to expand...
Have you considered an outdoor grow, unless I'm mistaken your part of the world gets approx 12/12 365 days?
 
J

jochhe1998

Active Member
rkymtnman said:
i wouldn't call that unfortunate. lol

invest in fans and keep the air moving. alot of air moving.
Click to expand...
Haha excuse my poor choice of wording - I do love it here but it definitely has its downsides (as does everywhere).

Just installed a 740 CFM inline fan for my 2.5 by 8 and my 2.5 by 4 tents. It seems to be keeping everything under 60% RH @ 29 degrees C pretty comfortably.

Only issue I'm having now is the negative pressure (I know some people say it's good and some people say it's bad but it's a bit too negative right now).
 
rkymtnman

rkymtnman

Well-Known Member
jochhe1998 said:
Haha excuse my poor choice of wording - I do love it here but it definitely has its downsides (as does everywhere).

Just installed a 740 CFM inline fan for my 2.5 by 8 and my 2.5 by 4 tents. It seems to be keeping everything under 60% RH @ 29 degrees C pretty comfortably.

Only issue I'm having now is the negative pressure (I know some people say it's good and some people say it's bad but it's a bit too negative right now).
Click to expand...
make sure you have plenty of air ON the plants too. especially during bloom. will help reduce chance of bud mold.
 
Star Dog

Star Dog

Well-Known Member
jochhe1998 said:
Haha excuse my poor choice of wording - I do love it here but it definitely has its downsides (as does everywhere).

Just installed a 740 CFM inline fan for my 2.5 by 8 and my 2.5 by 4 tents. It seems to be keeping everything under 60% RH @ 29 degrees C pretty comfortably.

Only issue I'm having now is the negative pressure (I know some people say it's good and some people say it's bad but it's a bit too negative right now).
Click to expand...
Negative pressure is only related to smell, if smell isn't an issue you've nothing to worry about, if smell is a problem and leaking out you might want to close an inlet down a bit.
IMHO 29c is a bit hot, are you taking the temperature near the light?
 
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