What actually is measured to get RH readings?

I’m curious as to what is actually read by my meters readout. I started my tent with 70-80 degree temps. RH seemed to be a steady30% regardless of temp. (This is a basement grow) In an effort to bring up humidity I went with a u-sonic in tent. Maxed out, it went up to 41%. Knowing seedlings and clones like even more, I was surprised I didn’t have enough. Still experimenting, I put a 1.75l coffee/tea water pot/hot plate in. The tent was now exhausting water vapor, I was able to cause condensate on its window, yet RH only registered 52%. To me it looked like a steam room in there.

Please explain what I am misunderstanding. Or is my digital temp/hydro meter. Junk? It’s brand new.

Relative humidity is the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature.

In other words the amount of water vapor that enters (or can enter) a given space at a given temperature.

I’ve raised my RH from 35 to 63 just by having a damp towel in the tent hanging up top and changing it out when it’s dry (usually once a day). I don’t have condensation issues on windows. I am also a basement grower.


That’s kind of why I’m asking. I can actually SEE water vapor hanging in the air as well as venting into a 63F room. It’s not like in venting to a freezing cold area. I’m wondering why my meter isn’t reading higher. Especially when someone like yourself can raise up to 63% RH using a wet towel/sweatshirt. Makes me think I’m misinterpreting something here.

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You can check your meter against the ambient humidity your local weather reports.

The sensor is a resistor made from a hydrophilic polymer that changes resistance with moisture content. More expensive sensors use other means.

They are generally speaking not very accurate but it sounds like you have a bad one.

Also, remember that relative humidity is RELATIVE!


I use this

And this in my veg tent because I only have one of the above

I measure at canopy level because that’s where the plants are I don’t care what it is anywhere else.


I have that same Acurite hygrometer and it works well.

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I agree with @Myfriendis410. I probably have close to a dozen hygrometers around. A couple I’ve had for my cigar hunidor, bought for grows, and some I use for curing . If I set them up in the same place about half of them read within 5% of each other. Then there’s a couple that are steadily about 10% or so higher than what I consider the good ones no matter what. And the last couple just flat out aren’t accurate or even consistent.

You can try sealing it in a bag with a boveda pack. Or there’s a way to make a salt slurry and check them too.

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I love the way people get hung up on RH and Temps somehow I must be an absolute flukey bugger I somehow? pull off harvesting every 4-6 weeks with RH ranges from 20-70% and temps which dip into 50’s and climb into high 80’s during summer.
Actually it isn’t fluke it’s called adaptation if a person is patient you can make plants adapt to your space not other way around, I am certain if I spoiled my ladies with perfect temps RH and I would pull more weight but at what cost? More power use wear and tear on my filters fans etc? If you think outdoor growers have a set temp RH worry it do death I am yet to see a plant outside which has perfect rh and temps whole season :slight_smile: I grow hardy strains slow and gradual letting them adjust to my conditions if they don’t like it I try other strains if they do I clone, I want plants I can neglect and abuse until they are used to it and grow well anyway.
An odd way to think I guess for a person who suggests you should love growing and tending your plants? don’t mistake what I am saying there is too dry and too cold but there are also points between there and just right which plants can learn to thrive in :wink:


Thanks for that @Donaldj , I kinda had that figured as well too. During my first inquiry, I was referred to your very post about your “bat cave” install.
(You diehard you! Man, talk about WANTing something.)
I learned a lot about the variances you dealt with along the way from both Temp&RH standpoints. After the thorough reading many of my concerns were answered. This is just my first grow and I’m shooting for my best chances of success right out of the gate. I know I’m in for many various ailments to learn/ deal with along the way. I figure by getting my “standards” correct, I stand to best chance of troubleshooting problems as they arise.
I have noticed much advice here from the forum members starts with getting the “standards” adjusted firsthand them you folks help us from there. Much to our (noobs) appreciation. Thanks again Don J.


Oh yeah, an update to the original purpose of this post.


I visited the grow store today and explained my predicament, I shot a 10second video to show them the “fog” inside my tent at an indicated 42%RH. I could tell I was an idiot somehow when the guys looked at each other and smirked before answering.

“ The probe only measures temperature, the meter goes inside the tent”.

As Homer says, DOH!


we love our plants to death

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