Relative Humidity - Where to measure?

I grow in a 4x4x8’ tent using LED lights. I don’t think I have a humidity problem, but, I may depending on where and when you measure.

On top of the canopy (SCROG) where it’s a warm 82°F the RH is 47%, below the canopy in the shade it’s 73°F and the RH is 68%. I have constant 24hr ventilation going and an oscillating fan below the canopy.

One other reading of importance; when the lights go out at night (12/12) during the dark period the tent temp drops to the room temp of 70°F. The RH at both the canopy and below max out at 81% humidity.

In the spring/summer/fall I run a window A/C unit that takes the humidity out of the air. I actually run a humidifier. But right now when the air is cool and humid outside, the A/C unit barely runs.

Yes I can run a dehumidifier, but what RH am I looking for and where do I take the reading from?

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I don’t know, but this is a very good question; I will be interested to know the answer.

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This is a perfect example of RELATIVE humidity. The moisture content of the air has remained relatively stable, but the RELATIVE amount of moisture air can retain goes down with temperature.

Higher humidity is beneficial early on and in veg. You absolutely need to be below 50% during flower and under 40% would be safer still in late bud production.

Personally I would measure fairly close to the canopy height as that is representative of your actual grow.

Let me call @Countryboyjvd1971 John in who is an HVAC expert. …

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I’m going to tag a few more people in. Please forgive me if the tagging is unwelcome…

@Wishingilivedina420state, @bob31, @Covertgrower, @dbrn32, @Ray4x, @FreakyDeekie

Please tag in anyone else that may have experience / knowledge of indoor grows…

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To be completely honest, I’m not sure. I measure at canopy level, and target mid 50% for flowering. It’s wandered up to about the 60% mark without any issues for me. But every setup will behave a little differently.

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You are trying to maintain the RH for the plant, therefore always measure at the plant especially with a lot of airflow and a constant change in air volume. I never measure off the soil area because you will have evaporation from the soil and give a higher reading. Place your meter so the plants are between your fans and meter so you see what the RH is coming off your plants?

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@HappyCamper

I would agree that you should be aiming for 35-50% during flower. I think @bob31 has a chart the shows the ideal RH for each stage of the plant.

I would also tend to think that the RH closer to the canopy is where you should measure. As that is probably a clearer picture of what your tent humidity is. It might be skewed being closer to the soil.

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@HappyCamper I measure my relative humidity 4-6 inches below the top of the colas. You do want higher humidity during veg than in flower. Now from what I’ve read and being a new grower take this for what want. I’ve researched, and experienced as I’m coming up to harvest. When switching the light schedule to 12/12 I would remove as much humidity as possible during flower. I would start at the 50 number, and as flowering progresses to harvest, lowering the humidity along the way more and more until you can get it as dry as possible. The reason for this is make the plant produce more trichomes/resin to protect itself from drying out. Yes the humidity jumps upwards between 5-10% after a watering, but should be on a downward trend after that. I’ve been able to measure this, so it’s not a cause for concern. I hope this answers your question. Here is the data you can see the exact time I watered with the RH humidity going up for about an hour and then on a downward trend:
6041079D-6ABB-45C4-9441-6E0710FC72660E008305-9BF2-42AF-965E-7318EDE98250

I found it @HappyCamper this is a suggestion I found around here somewhere. 89E12A51-E6B4-4779-A637-E97933E03DE9

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What device do you use to measure? Wireless device that reports back to the cloud these numbers? Need to add whatever you have onto my wishlist :slight_smile:

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@Tylan download the app and connects through Bluetooth, but if you get the gateway you can check your tent from anywhere you have internet.

https://www.amazon.com/dp/B01AEQ9X9I/ref=cm_sw_r_cp_api_5653zbFPFAQKQ

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@Covertgrower Awesome, thanks! Gateway kind of expensive so I’d have to start with just the unit first :slight_smile:

@Tylan yes the gateway is a bit much, but you pay the one time fee, and it pays for a cloud service through the company and service to push it to your device with a free account. But it works just fine with Bluetooth too!

Also You can connect multiple sensors, just be sure to name them all! Connects very well!

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I measure rh at the canopy or the net. This is the most important spot in the tent. So I agree with @Myfriendis410 and @Ray4x

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Out of likes @Wishingilivedina420state but well said.

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@Covertgrower sorry I ment to put you n there 2 … but I’m up here with the Hubble right now and that was a lot to read and remember for me right now.

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Hahaha no worries… as long as we aren’t say conflicting terms and suggestions. All is right with the world. @Wishingilivedina420state

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:sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses::sunglasses:

Thanks for all the great advice. It looks like I’ll be shopping for a dehumidifier in the near future.

It seems the consensus is to take readings at the net/canopy level. That’s the place I’ll target my th.

And, @Covertgrower, I love the wireless/charting app. I’m going to look into one of those… :smile:

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@HappyCamper I originally purchased it for myself, but it looks like it’s helping others as well! I’m happy to give someone hard data that helps.

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