humidity raises over 25% with lights off ?

[benjones]

a few days ago my timer that turns the lights on and off broke and the lights didn't come on at 8am as usual . walking into the flat i could tell something was different as the sitting room was far more pungent than usual . as i walked into the grow room to turn the lights on manually , the humidity meters read 68 and 62 .

i have an air in and out fan on 24/7 . air con in the grow room on 24/7 . the air that comes from the living room into the grow room has the air con on 24/7

humidity in the living room that brings the air into the grow room is always around 39% ( no lights or grow stuff in there )

i have 4 mars hydro fce4800 in a room about 25sq meters . the flow of air in and out is done but 2 6inch mars hydro fans

the only thing i can think that might be changing things is either the heat from the ballasts ( is that the name of them ? ) which is pretty minimal compared to air flow in/out , or that my sensors are light sensitive

i'd appreciate any feedback

 

[BigBlonde]

Humidity is affected by air temperature. Humidity increases as the temperature decreases, so it's normal for the humidity to increase when the lights are off. If it's a problem, a dehumidifier might help. For my small tent, I only have a humidifier and that seems to work okay for me.

 

[elusiveshame]

Without the heat of the light + the light, the evaporation rate of water in the air slows down, allowing more to collect. You need a dehumidifier until you get your environment fixed. You don't want to hit the dew point as you can start getting mildew and mold much quicker.

 

[benjones]

Thank you @BigBlonde for replying

My air con comes on and off to keep the temperature at around 23c , I can’t program it for different temperatures with lights on or off . The temperature outside is 25c plus day or night , I’m in Thailand . Given that the temperatures are about the same with lights on or off ,what could explain the big difference in RH %

 

[benjones]

Without the heat of the light + the light, the evaporation rate of water in the air slows down, allowing more to collect. You need a dehumidifier until you get your environment fixed. You don't want to hit the dew point as you can start getting mildew and mold much quicker.

Thank you mate , I have ordered a dehumidifier and I hope it will be here soon . Anything I could do in the mean time ?

 

[freezeland2]

Thank you @BigBlonde for replying

My air con comes on and off to keep the temperature at around 23c , I can’t program it for different temperatures with lights on or off . The temperature outside is 25c plus day or night , I’m in Thailand . Given that the temperatures are about the same with lights on or off ,what could explain the big difference in RH %

Plants release a lot of water in to the air when the light goes off. The plant has stopped transpiration when they go out. It’s like the last big exhale for them until the lights come back on and the leaves stomata reopen and the plant begins to transpire again.

 

[elusiveshame]

Thank you mate , I have ordered a dehumidifier and I hope it will be here soon . Anything I could do in the mean time ?

If it's during your day time for the plants, you can open your tent a bit to let more air in/let the humidity out, unless the room the tent is in is also humid. You mention it's 39% in that room, so you'll want to pay attention to how quickly the humidity drops.

Where my tent is, it's on the opposite side of my basement away from my pellet stove. Right now it's 28% humidity in here and 48% in my tent. If I open my tent, it drops to 40% in about 1-2 minutes.

 

[benjones]

If it's during your day time for the plants, you can open your tent a bit to let more air in/let the humidity out, unless the room the tent is in is also humid. You mention it's 39% in that room, so you'll want to pay attention to how quickly the humidity drops.

Where my tent is, it's on the opposite side of my basement away from my pellet stove. Right now it's 28% humidity in here and 48% in my tent. If I open my tent, it drops to 40% in about 1-2 minutes.

damn it's crazy . i am growing in a room mate not a tent . the air being drawn in is 23-24c and around 40 RH% , i also have AC in the grow room that keeps it at 23-24c . i never could have imagined that with the airflow i have and the AC that keeps RH fairly low that RH% could go that high when lights are off
i do have a dehumidifier on the way so we will se how it goes and i'll update you

 

[BigBlonde]

Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer. What we "feel" outside is the actual amount of moisture (absolute humidity) in the air.

https://www.weather.gov/lmk/humidity

 

[elusiveshame]

damn it's crazy . i am growing in a room mate not a tent . the air being drawn in is 23-24c and around 40 RH% , i also have AC in the grow room that keeps it at 23-24c . i never could have imagined that with the airflow i have and the AC that keeps RH fairly low that RH% could go that high when lights are off
i do have a dehumidifier on the way so we will se how it goes and i'll update you

Ah, then that's a bit more tricky. Yeah, having a dehumidifier will help, just make sure it's sized for the room. A small little unit won't do much for a big room. I have a fairly large one for my basement as the moisture down here during the summer months can get pretty high, but it takes care of 900 sq ft without issue (I also have it drain to a sump pump so I never have to worry about emptying it).

 

[benjones]

Ah, then that's a bit more tricky. Yeah, having a dehumidifier will help, just make sure it's sized for the room. A small little unit won't do much for a big room. I have a fairly large one for my basement as the moisture down here during the summer months can get pretty high, but it takes care of 900 sq ft without issue (I also have it drain to a sump pump so I never have to worry about emptying it).

My room is 3meters x 3 m X roughly 2.5 - 3 m height , I’ll have to measure . The only dehumidifier I could get here in a short space of time without spending shitloads and having to have it wired in with a breaker was one that does 20-30 Sqm. I hope it will help

@BigBlonde , I always thought that the lower the temperature the lower the RH%

 

[RootsRuler]

My room is 3meters x 3 m X roughly 2.5 - 3 m height , I’ll have to measure . The only dehumidifier I could get here in a short space of time without spending shitloads and having to have it wired in with a breaker was one that does 20-30 Sqm. I hope it will help

@BigBlonde , I always thought that the lower the temperature the lower the RH%

Curious why you run your room at such a low RH? What is your average temp with lights on and off? Also, in general, the lower the temp the lower the RH since low temperatures require a low RH to be able to sustain temp.

 

[benjones]

Curious why you run your room at such a low RH? What is your average temp with lights on and off? Also, in general, the lower the temp the lower the RH since low temperatures require a low RH to be able to sustain temp.

At the moment I only have an old air conditioner so if I set it too 23 c that’s about where it stays . This as you say affects the RH% . I’m 3-4 weeks away from harvest . Living in Thailand my Thailand usual temperature is 30c daytime 27 night time and rh% can be as high as 85% .

I average 23-24c lights in and off .
Rh lights on around 40 at the moment . Was higher a couple of weeks ago . Like I say . I walked in to find that with lights off the rh is a m the 60s
Didn’t seem right to me

 

[elusiveshame]

If you’re this late in flower, your humidity should be between 40-55%. I feel comfortable between 45-49% myself.

 

[RootsRuler]

At the moment I only have an old air conditioner so if I set it too 23 c that’s about where it stays . This as you say affects the RH% . I’m 3-4 weeks away from harvest . Living in Thailand my Thailand usual temperature is 30c daytime 27 night time and rh% can be as high as 85% .

I average 23-24c lights in and off .
Rh lights on around 40 at the moment . Was higher a couple of weeks ago . Like I say . I walked in to find that with lights off the rh is a m the 60s
Didn’t seem right to me

Oops! My bad. I saw 23c and missed the "c". I thought you were running at 23% RH. I see that you're closer to 40%. If you want to be in the sweet spot you need to raise it to at least 55%.

This chart can guide you and let you know where you need to raise or lower humidity in regards to your temperature.

 

[benjones]

Oops! My bad. I saw 23c and missed the "c". I thought you were running at 23% RH. I see that you're closer to 40%. If you want to be in the sweet spot you need to raise it to at least 55%.

This chart can guide you and let you know where you need to raise or lower humidity in regards to your temperature.

View attachment 1310919

Thanks @RootsRuler . I will raise RH . At what point is mold a danger ?

 

[RootsRuler]

Thanks @RootsRuler . I will raise RH . At what point is mold a danger ?

70% and up but if you have good air circulation in your garden you should be able to keep disease from taking a foothold.

 

[benjones]

70% and up but if you have good air circulation in your garden you should be able to keep disease from taking a foothold.

thank you @RootsRuler . i have reasonably good air circulation . extraction fans in and out , 1 large oscillating wall fan covering both tables , 2 small ones for the corners , 1 medium fan on the floor trying to move air under the plants ( i have pruned them ) .

i have turned off the AC from the living room , this is where i bring the fresh air in from and the RH has gone up to around 45%

i have just bought a dehumidifier which i will try to get it to come on at night , any idea on the best place to put it ?

btw, that humidity chart , is that for veg ?

 

[elusiveshame]

thank you @RootsRuler . i have reasonably good air circulation . extraction fans in and out , 1 large oscillating wall fan covering both tables , 2 small ones for the corners , 1 medium fan on the floor trying to move air under the plants ( i have pruned them ) .

i have turned off the AC from the living room , this is where i bring the fresh air in from and the RH has gone up to around 45%

i have just bought a dehumidifier which i will try to get it to come on at night , any idea on the best place to put it ?

btw, that humidity chart , is that for veg ?

Toss it in between your hygrometers. You could also get one of those humidity controlled outlets to have the dehumidifier turn on when it reaches at certain humidity level so that way you don't have to worry about catching it in time.

 

[RootsRuler]

thank you @RootsRuler . i have reasonably good air circulation . extraction fans in and out , 1 large oscillating wall fan covering both tables , 2 small ones for the corners , 1 medium fan on the floor trying to move air under the plants ( i have pruned them ) .

i have turned off the AC from the living room , this is where i bring the fresh air in from and the RH has gone up to around 45%

i have just bought a dehumidifier which i will try to get it to come on at night , any idea on the best place to put it ?

btw, that humidity chart , is that for veg ?

I wouldn't be employing a dehumidifier at this point if you're trying to raise the humidity in your room. A humidifier is what you want.