where to put probes for accurate humidity-readings

mannitu68 · Apr 28, 2022

Hello and good day to you all.
I am in mid flower now and want to make sure im in the right humidity range, but my meters show different values at different measuring spots. Ive got 1 in the middle of the tent, hanging down 2-3 inches into the plants. Thats the one showing the lowest humidity. Another one is fixed to a rod in the corner, it reads slightly higher. However, theyre somewhat close enough, i dont really care as long as im somewhere between 30-55%. But the third probe, the one i am holding directly to the exhaust, right on the filter, shows 10-20% more. Often around 70%. It is the spot of choice to get an accurate reading, at least some people say and believe that. I have no clue. Ive cross-tested the devices to make sure its not device-related, but it really seems to be the exhaust-spot, that shows readings much higher. Anyone has an idea why that is or even more important, where to put my probe?

Answer appriciated

Moe.Red · Apr 28, 2022

Hello and welcome to the farm.

I would tend to discount the one in the exhaust, but before I go too far can you tell us what probes you are using and maybe post a pic of the way you are holding it in the exhaust? If it is on the lung room side and it is a lower temp, RH will be higher. Same amount of water in the air but lower temp = higher RH.

mannitu68 · Apr 28, 2022

nah, my filter is standing outside directly on the tent. And im holding the sensor right against it. The open side of the pistol, where the mesh is and the sensor is under...thats what im holding against the filter. So the air is warmer than the average in the tent. Its only showing 40-something now because its dark in the tent. With the 2 other devices, i just leaned them against the filter and let them stay there. Tried to get the sensor as close as possible to the filter. They all showed somewhat the same, maybe 3-4 degree difference. They take a bit longer until the value is settled. Ive winded an addiotional active-carbon sponge around the filter and the lady tights holds it in place. Before you ask.

From my intuition i would say, the readings inside the tent do reflect the truth more. It just doesnt make any sense to me, that the exhaust shows 70% while i really have a lot of air moving in that small 2x2. The meters inside the tent are probably closer to reality. But why does it show so high values at the exhaust?

edit: Maybe these meters do need air that is still standing or only moving slow. When you blow air right into the housing, like in this case..maybe there are too many molecules hitting the sensor and you can forget an accurate reading.

so where shall it put a probe? Right next to a bud? Or deep into the canopy?

Moe.Red · Apr 28, 2022

I've never seen a probe used like that before.

Don't you think the stocking will hold moisture? Like breathing thru a mask?

I try to keep my probes in the area with the most photosynthesis. Canopy level which changes thru the grow.

freezeland2 · Apr 28, 2022

I place mine at top of canopy.

N1ghtL1ght · Apr 28, 2022

rH rises naturally when temperatures drop, I'd venture to say the "air" is hotter in the vicinity of your light.
Actually direct light exposure can mess with the internal sensors when a casing is heated up without proper airflow inside, giving off non-true readings.
And you always want to take temps alongside rH to better envalue the locale situation

A good way to assess what is happening in the growroom is to take the temp/rH at the air intake & outtake (in the pipe) and then see the difference, as the total air mixes there best. And a few others inside the canopy, above the lamp, and on the bottom (or soil probe) to see how it can fluctuate locally.
And shield the probe from direct strong light exposure with an alu foil shield if possible.

Moe.Red · Apr 28, 2022

N1ghtL1ght said:
A good way to assess what is happening in the growroom is to take the temp/rH at the air intake & outtake (in the pipe) and then see the difference, as the total air mixes there best.

That's interesting. You could get a rough measurement of water uptake if you knew CFM.

BorealCuring · Apr 28, 2022

I certainly hope you have a fan in your tent. If you do, I would point it above your weed and never on it. This would normalize the humidity evenly in your grow space. Something else that's important is a calibrated meter.

If you think about it, putting your meter in the center of your space, you will get a tru(er) reading than you would where the air velocity is higher, like next to a fan, simply because you're moving more humidity into your meter by virtue of much more air.

Moe.Red · Apr 28, 2022

What we are after is the temp and RH of the most important part of the tent. Where the action happens. The Canopy.

All other data is interesting, but you would not want to tune for those numbers like input and output. You tune for what is happening at the buds.

If I had multiple sensors, I would place them evenly across the canopy and use that data to steer fans, add/remove water via dehuy, ac, lights, etc.

delps8 · Apr 28, 2022

Moe.Red said:
What we are after is the temp and RH of the most important part of the tent. Where the action happens. The Canopy.

All other data is interesting, but you would not want to tune for those numbers like input and output. You tune for what is happening at the buds.

If I had multiple sensors, I would place them evenly across the canopy and use that data to steer fans, add/remove water via dehuy, ac, lights, etc.

Agree 100%.

If you want to know the temperature and RH in the canopy the best place to put the sensors is in the canopy.

I use a Pulse One, AcuRite sensors, and AC Infinity tent fan and inline fan. There's also a sensor of the Inkbird RH controller and, when I need to heat the tent, a sensor of the Inkbird temperature controller. The main driver for my decision to use this location is the directions for where the Pulse One should be placed.

ChrisSoiCy203 · Nov 9, 2023

delps8 said:
Agree 100%.

If you want to know the temperature and RH in the canopy the best place to put the sensors is in the canopy.

I use a Pulse One, AcuRite sensors, and AC Infinity tent fan and inline fan. There's also a sensor of the Inkbird RH controller and, when I need to heat the tent, a sensor of the Inkbird temperature controller. The main driver for my decision to use this location is the directions for where the Pulse One should be placed.

View attachment 1240150

Does it matter if your prob shakes from the fan wind as long as it’s over the canopy for an accurate temperature reading?

Gasman5.0 · Nov 24, 2023

ChrisSoiCy203 said:
Does it matter if your prob shakes from the fan wind as long as it’s over the canopy for an accurate temperature readi

ChrisSoiCy203 said:

Does it matter if your prob shakes from the fan wind as long as it’s over the canopy for an accurate temperature reading?

Click to expand...

Not at all.. just sampling a larger area of the grow canopy for better average rh measurement..,probably better imho with single point measuring.

where to put probes for accurate humidity-readings

where to put probes for accurate humidity-readings

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