Relative Humidity

[Mike H]

This thread seems to have died out...since building my greenhouse and moving my orchids inside I've kept track of the humidity and it is ALWAYS above 75% and often in the mid 80's.
As Wade mentioned a few posts back the higher the humidity the more important air movement is. Mounted plants are obviously happier in the higher humidity and I can't see a downside evident with any other plants.

[Dorsetman]

I wonder how you are measuring it ? This cheap dial hygrometers ( human hair based ! ) are accurate plus or minus 100% I think ! They are not far out within a few days of being reset, but if longer than that, of no value. Reset by wrapping in a wet towel for a couple of hours, then adjust the needle to be on the 100% mark exactly.
The old-fashioned and very reliable way is a wet and dry thermometer, but they are a nuisance to remember to keep filling up the little bottle with very pure water, and I have found the wick rots, and again, they are then unreliable.
Currently I use a little hand-held digital one, giving an instant read-out. According to my local Mercedes dealer, using one to check out the climate control system in my car, they are the best.

[Mike H]

I have three sensors in the greenhouse and have questioned them myself as the humidity actually approaches 95% at times.
Two are ACU-Rite digital sensors and one is a Temp Stick. The manufacturer swears the Temp Stick is accurate but I did make a downward adjustment to it to be more inline with the two ACU-RITE sensors.

[Tyrfing]

I wonder how you are measuring it?

I've been waiting for someone to ask a question like this.

I recently got a new weather gizmo. It also has several indoor modules (which measure temperature, humidity and CO₂ levels). Of course I put one of them into my grow tank. This gizmo is wifi connected, so I can access it from the internet (and with a mobile app), so now I'm able to check on the humidity and temperature all the time

The accuracy is supposed to be ± 0.3 °C (0.54 °F), ± 3 % for humidity and ± 50 ppm for CO₂. The temperature and humidity are about the same as my old cheapo digital measurement device displays.

Attached is an image of 24 hours in my tank. My lights are on 0600–2000. The temperature quickly rises to about 22–24 °C (72–75 °F) when the lights are turned on while the relative humidity is reduced when the temperature increases. Both of these are as expected. The CO₂ level is a bit more surprising, I would have expected it to increase during the night and decrease during the day, but it seems to be opposite. It also rapidly increase when the lights (and fan) is turned on at 0600. And then increases again in the afternoon until the lights (and fan) is turned off at 2000. I might want to try to have the fan on 24 hours/day to check if this could be the CO₂ concentrating at the bottom of the tank (where the sensor is).

[raybark]

Even the stationary wet/dry thermometers are suspect. If you really want an accurate reading, you'll need a sling psychrometer. However, that said, how accurate do you need to be?

It seems to me that once you're above 50-60% - and you can feel that - it's "good enough".

I'd rather deal with "too high" than the opposite, and I've never had issues with it being too high.

[Arne]

Sometimes a thread gets better over the years. This one definitely did.

[Mike H]

Tyrfing, my Temp Stick has a similar graphing function, no CO2 sensor though, what is growing in this "tank" or should we not talk about it.

[Dorsetman]

I am curious as to why the CO2 goes up and down. The air cannot change in composition, surely ?

[Tyrfing]

I'm not sure if I understand what you mean by change in composition. Normal fresh air outdoors has roughly about 400 ppm CO₂. Inside in a well ventilated room the levels should be able to approach this. The tank I grow in is not airtight, but it has very limited exchange of air (and possible even less when the fan is off). I think I will try to keep the fan on just to see what (if any) impact that will have.

And for the record I'm growing orchids in the tank, and I would be kind of grumpy if @Mike H tries to smoke them

[Mike H]

now that we have that cleared up Tyrfing...post pictures of your tank please