Temperature, relative humidity and absolute humidity - big discussion.

Helmut79

Helmut79

Well-Known Member
Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum (highest point) for that temperature.
Absolute humidity is the water content of air at a given temperature expressed in gram per cubic meter.

The point of this thread is that I was having a humidity issue at night and I was planning to lower it by increasing temperature of the room by heating the room with my wood pellet heating system.
I was having 60% at 67F and I wanted to get RH down to 50%.

In my case I have a descent ventilation, so humid air is constantly drawn into the building from outside. Increasing ventilation definitely doesn't lower humidity in the growroom. Increasing room temp by heating definitely lowers RH and these are the facts.

I have seen with my own eyes how RH drops when temperature is increased. That's why I started a thread to know if people approve my idea: http://rollitup.org/t/higher-temp-at-night.924991/page-2

Huckster79 calculated with an online calculator: if RH is 60% at 67F, then absolute humidity would be 9.8g per cubic meter, but if temp would be raised to 77F, then RH would drop to 42%, but absolute humidity would stay unchanged. It would be still 9.8g per cubic meter (Please give me the address of this calculator!).

This is a good example to explain what I've seen - a drop in RH, but thanks to Huckster79 now I know that humidity doesn't actually drop this way (absolute humidity).

Illinois Enema Bandit said that he doesn't see how a passive wood heating system is going to lower the humidity.

What would be the difference in passive vs active wood heating system and which one do I have?

Here comes the big question: Would absolute humidity REALLY stay the same if room is heated to higher temperature?
Doesn't heating lower humidity? We already know RH drops, but does absolute humidity also drop or not?

We all have felt it with our own body how when we heat the room (house or car or what ever), then at one point we feel how our eyes or mouth or nostril gets dry. We feel how air gets dry, don't we?

In my case I have a hot water calorifier as an output for the wood sourced heating system. It's basically like a radiator in cars, but in cars it serves a purpose of cooling as a radiator, but a calorifier is doing the opposite - heating.

Are you sure it's not reducing absolute humidity besides RH when it's heating the air? I'm asking, because I'm feeling with my own body how it dries the air. Wouldn't mold feel it too?

The main goal here is to reduce the risk of mold, but a dehumidifier is out of the topic. Please try to stay on the questions that I've raised!
 
RM3

RM3

Well-Known Member
Guess you're in luck since plants respond to Relative Humidity much the way we do and since it measures a temp/humidity relationship (another one is the heat index)

It is expressed as the amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature. Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal.

and as long as your temps don't drop below the dew point you'll be fine. But you need to understand you are trading one problem for another by having your lights out temp swings in the wrong direction.

I'm guessin you are aware your true solution is a dehumidifier but for some reason you can't get/run one

Good luck
 
S

Shugglet

Well-Known Member
Ive come across a lot of interesting information lately regarding VPD and increased RH being actually quite beneficial to growing. It would also seem that the solution to this problem lies not in lowering RH but rather increasing the amount of airflow in and around the plants.

Also, the only way to lower absolute humidity is to actively take moisture out of the air and sequester it outside the grow.
 
budman111

budman111

Well-Known Member
Helmut79 said:
The point of this thread is that I was having a humidity issue at night and I was planning to lower it by increasing temperature of the room by heating the room
Click to expand...
Most dehumidifiers today have a setting of 45%-55% humidity target and switches off when this desired range is met. The one I bought was £100, 200W, pretty quite and pumps out a litres a day. A must in my Scottish winter as far as I am concerned.
 
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HydoDan

HydoDan

Well-Known Member
You lower the humidity by moving the air not by heating it.. an oscillating fan will help...I got a $90 dehumidifier from Amazon keeps my tent at 50%..
 
Huckster79

Huckster79

Well-Known Member
Shugglet said:
Ive come across a lot of interesting information lately regarding VPD and increased RH being actually quite beneficial to growing. It would also seem that the solution to this problem lies not in lowering RH but rather increasing the amount of airflow in and around the plants.

Also, the only way to lower absolute humidity is to actively take moisture out of the air and sequester it outside the grow.
Click to expand...
I agree with you on air flow for helmut79

Yea im playing w vpd, sems to make sense, but so does low humidity especiallybat the very end. so heres the hybred method iv been doing. roughly following vpd till about 6th week of flower then take a week to drop down bit by bit (for me this is temp, co2, and rh) then week 7,8,9ish forget vpd and get it dry! So im trying to get the growth benefits of vpd while the benefits of giving her low rh stress at end... not sure its the right way but what im playing with right now.
 
ttystikk

ttystikk

Well-Known Member
Huckster79 said:
I agree with you on air flow for helmut79

Yea im playing w vpd, sems to make sense, but so does low humidity especiallybat the very end. so heres the hybred method iv been doing. roughly following vpd till about 6th week of flower then take a week to drop down bit by bit (for me this is temp, co2, and rh) then week 7,8,9ish forget vpd and get it dry! So im trying to get the growth benefits of vpd while the benefits of giving her low rh stress at end... not sure its the right way but what im playing with right now.
Click to expand...
I think this is a good general strategy.
 
Afgan King

Afgan King

Well-Known Member
Huckster79 said:
I agree with you on air flow for helmut79

Yea im playing w vpd, sems to make sense, but so does low humidity especiallybat the very end. so heres the hybred method iv been doing. roughly following vpd till about 6th week of flower then take a week to drop down bit by bit (for me this is temp, co2, and rh) then week 7,8,9ish forget vpd and get it dry! So im trying to get the growth benefits of vpd while the benefits of giving her low rh stress at end... not sure its the right way but what im playing with right now.
Click to expand...
Low rh is where it's at I run 35% whole life cycle plants love it lol
 
ttystikk

ttystikk

Well-Known Member
Helmut79 said:
Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum (highest point) for that temperature.
Absolute humidity is the water content of air at a given temperature expressed in gram per cubic meter.

The point of this thread is that I was having a humidity issue at night and I was planning to lower it by increasing temperature of the room by heating the room with my wood pellet heating system.
I was having 60% at 67F and I wanted to get RH down to 50%.

In my case I have a descent ventilation, so humid air is constantly drawn into the building from outside. Increasing ventilation definitely doesn't lower humidity in the growroom. Increasing room temp by heating definitely lowers RH and these are the facts.

I have seen with my own eyes how RH drops when temperature is increased. That's why I started a thread to know if people approve my idea: http://rollitup.org/t/higher-temp-at-night.924991/page-2

Huckster79 calculated with an online calculator: if RH is 60% at 67F, then absolute humidity would be 9.8g per cubic meter, but if temp would be raised to 77F, then RH would drop to 42%, but absolute humidity would stay unchanged. It would be still 9.8g per cubic meter (Please give me the address of this calculator!).

This is a good example to explain what I've seen - a drop in RH, but thanks to Huckster79 now I know that humidity doesn't actually drop this way (absolute humidity).

Illinois Enema Bandit said that he doesn't see how a passive wood heating system is going to lower the humidity.

What would be the difference in passive vs active wood heating system and which one do I have?

Here comes the big question: Would absolute humidity REALLY stay the same if room is heated to higher temperature?
Doesn't heating lower humidity? We already know RH drops, but does absolute humidity also drop or not?

We all have felt it with our own body how when we heat the room (house or car or what ever), then at one point we feel how our eyes or mouth or nostril gets dry. We feel how air gets dry, don't we?

In my case I have a hot water calorifier as an output for the wood sourced heating system. It's basically like a radiator in cars, but in cars it serves a purpose of cooling as a radiator, but a calorifier is doing the opposite - heating.

Are you sure it's not reducing absolute humidity besides RH when it's heating the air? I'm asking, because I'm feeling with my own body how it dries the air. Wouldn't mold feel it too?

The main goal here is to reduce the risk of mold, but a dehumidifier is out of the topic. Please try to stay on the questions that I've raised!
Click to expand...
You've already defined relative humidity and absolute humidity, then you go on to contradict your own conclusions.

To dehuey is to heat. Period.

Your pellet stove will generate heat and thus lower the relative humidity value in the space. If you feel it, so will the plants.

Who gives a fuck how much moisture is in each cubic meter of air? The plants are concerned with transpiration, which is a function of RH. VPD, or vapor pressure differential, is RH vs temperature.

Standalone dehumidifiers are basically AC units that condense moisture and ADD HEAT, which is what actually dehumidifies the space.

You need to add dry heat to reduce RH and your pellet stove or hot water radiator will both work.
 
somebody1701

somebody1701

Well-Known Member
Absolute humidity isn't relevant. That's the total amount of water in the air. Plants care about RH which is how much water is in the air relative to it's saturation point (which increases with temperature). Whether you raise RH with heat or lower it (and absolute) with a dehumidifier, neither the plant nor you should care about AH.
 
HydoDan

HydoDan

Well-Known Member
ttystikk said:
Point blank not true.

Dehumidification is achieved with heat. Period. It's in every textbook on meteorology.
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Thanks for the correction.. Don't know what the hell I was thinking.. My wood stove drys my house out better than my forced air furnace.. Duh
 
somebody1701

somebody1701

Well-Known Member
If you are bringing in cold outside air and heating it, it will be very dry (RH) without actually removing any water from the air.
 
ttystikk

ttystikk

Well-Known Member
somebody1701 said:
You can dehumidify and cool. Most air conditioners do this and most dehumidifiers (cool coils) do as well.
Click to expand...
No, but you can run several devices together which when summed together will create this result.

In this case, the AC unit is cooling and removing moisture. It is NOT dehumidifying anything. That job is being done by your lighting, because they're supplying the necessary heat. The two working against one another is what results in the readings on your thermometer/hygrometer.
 
S

Shugglet

Well-Known Member
ttystikk said:
No, but you can run several devices together which when summed together will create this result.

In this case, the AC unit is cooling and removing moisture. It is NOT dehumidifying anything. That job is being done by your lighting, because they're supplying the necessary heat. The two working against one another is what results in the readings on your thermometer/hygrometer.
Click to expand...
If it is removing moisture from the air it is, by definition, dehumidifying... even without lights on, the humidity will drop with an AC chugging away.

The thing is, dehumidifiers are AC units that simply run the air back over the heating coils before putting it back into the room. This is so you can drop humidity without cooling the room as well. Its not like you cant remove humidity and cool the room at the same time...
 
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