Humidity / Fan / lights on / lights off question

Mr. Bakerton

Mr. Bakerton

Well-Known Member
So I'm just a tad confused and hoping a quick post will clear up my foggy brain.

I have a tent thats sitting in a room that is pretty much closed off from other air sources. A slight exchange with the rest of the house and maybe 30 to 60 mins a day to outside air with an open door, no forced movement. The humidity in that space is hard 58%, like never changing somehow and if it does its just a few points or over a period of time.

The tent exhaust is based on high temps and the fan is always running somewhere between 1 and 3 (out of 10). During lights out the humidity is somewhere around 70% in the tent. During Lights on its 40%. The fan runs @ 1 at night and bounces between 1, 2 and 3 during lights on.

What I don't understand is how can the humidity be so much lower than the room the tent is sitting in? Does the movement of air itself reduce the humidity? Is it decreased with lights on and heat? Possibly a combo of both?

I don't want to run a humidifier and I don't want the lung room to get any higher than 60%, so how could I raise the tent humidity during lights on? I am considering relocating the LED drivers out of the tent to reduce temps which will reduce Lights on fan speed.
 
coreywebster

coreywebster

Well-Known Member
So what your reading on your hydrometer is relative humidity.

Key being relative to the temperature.
So when the temp is higher the air can hold more water, when it's lower it can hold less.
So the absolute humidity is around the same but the relative humidity shows higher or lower due to the potential at the given temperature.
 
Mr. Bakerton

Mr. Bakerton

Well-Known Member
coreywebster said:
So what your reading on your hydrometer is relative humidity.

Key being relative to the temperature.
So when the temp is higher the air can hold more water, when it's lower it can hold less.
So the absolute humidity is around the same but the relative humidity shows higher or lower due to the potential at the given temperature.
Click to expand...
So which do we care about in regard to maintaining a healthy environment?

I should take a moment to read up on the measurement devices I have and see if they point to reading one type or the other, if I understand that correctly.
 
coreywebster

coreywebster

Well-Known Member
Mr. Bakerton said:
So which do we care about in regard to maintaining a healthy environment?

I should take a moment to read up on the measurement devices I have and see if they point to reading one type or the other, if I understand that correctly.
Click to expand...
They all read Relative humidity and it is important , I'm just explaining to you why the readings are higher or lower than your room itself.

If you want higher RH then your on the right track with remote mounting your drivers which will lower the temp and slow your extraction down.

You can also have a bucket of water with a rag hanging out like a wick and air on it to increase it a bit.

Personal I don't worry about it because its not far off where i want to be.
But if you want to chase ideals then some form of humidifier is needed
 
coreywebster

coreywebster

Well-Known Member
RH will also climb as your plants grow, more plant more transpiration.
Also more air circulation (not exchange) will increase plant transpiration
 
LeastExpectedGrower

LeastExpectedGrower

Well-Known Member
Mr. Bakerton said:
So I'm just a tad confused and hoping a quick post will clear up my foggy brain.

I have a tent thats sitting in a room that is pretty much closed off from other air sources. A slight exchange with the rest of the house and maybe 30 to 60 mins a day to outside air with an open door, no forced movement. The humidity in that space is hard 58%, like never changing somehow and if it does its just a few points or over a period of time.

The tent exhaust is based on high temps and the fan is always running somewhere between 1 and 3 (out of 10). During lights out the humidity is somewhere around 70% in the tent. During Lights on its 40%. The fan runs @ 1 at night and bounces between 1, 2 and 3 during lights on.

What I don't understand is how can the humidity be so much lower than the room the tent is sitting in? Does the movement of air itself reduce the humidity? Is it decreased with lights on and heat? Possibly a combo of both?

I don't want to run a humidifier and I don't want the lung room to get any higher than 60%, so how could I raise the tent humidity during lights on? I am considering relocating the LED drivers out of the tent to reduce temps which will reduce Lights on fan speed.
Click to expand...
Relative humidity is a function of temperature and moisture. So as the temp rises (lights on) the humidity falls in relation to that heat. During lights out, the temps are down, so the RH is higher...also plants transpire differently in the dark/cool than they do in light/warm.
 
LeastExpectedGrower

LeastExpectedGrower

Well-Known Member
Mr. Bakerton said:
Hearing it from different perspectives is helpful in filling in the gaps in my understanding.

Thanks for the time and input here! @coreywebster @LeastExpectedGrower
Click to expand...
In veg I pretty much let the tent do whatever for humidity up in the 60s-70s range and just focus on controlling heat. I do have an exhaust fan that's running all the time at low levels that then kicks up if triggered by excess heat or super high humidity range, and I have a couple circulation fans so there's always airflow.

In flower I clamp down on the humidity a bit...low 50%'s with lights, up to about 60% in the dark the same fans going all the time. So far I haven't had any PM problems which would be my main concern. My VPD is generally averaging right around 1, and swings from about 1.3 down to .7 depending on time of day.

Your concerns are keeping the VPD in a reasonable zone for plants to transpire effectively but also keep powdery mildew away. So far I haven't had any PM issues.
 
Star Dog

Star Dog

Well-Known Member
Rh can be difficult to get your head around, I think good way to understand rh% is... If you take a sample of air in a jar (the moisture content can't change) but if you heat or cool the sample the rh% will change.

The readings are relative to temperature, change temperature you change the amount of moisture that can be held.

Dew on the grass in the evening is cooler air dumping water because it no longer has the ability to hold moisture as a gas.
 
Mr. Bakerton

Mr. Bakerton

Well-Known Member
I haven't had issues with PM indoors yet. I know that there isn't an exact temp/humidity but if the below quote is favorable, my setup isn't hitting the mark.

"The temperatures in which PM thrives is between 50-65 degrees Fahrenheit, with a relative humidity of 80-90 percent. " https://hightimes.com/grow/how-to-avoid-powdery-mildew/


Typically I'm 68 degrees/ 70% @ lights out and 80 degrees/40-50% lights on. As outdoors humidity starts to rise, my dehumidifier will need to start running and thats going to dry things up a bit while possibly pushing temps a tad higher.

I just had the best dry/cure at about 65degrees/60%, thats a seasonal result and we work what we have. Gotta remember to try and time future grows to hit that window.
 
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