A problem people seem to have when reading humidity in greenhouses or grow rooms is that due to the nature of digital hygrometers, that they ALL are ceramic semiconductors that require direct exposure to the air they are measuring, hygrometers cant handle condensating humidity. Fancy ones like inkbird use special membranes or an internal heater so the readings are never true, but they are less likely to see the ceramic sensor get wet, because if it ever did, the sensor would destroy itself through electrolysis.

When growing mushrooms especially, the air gets humidified heavily using foggers, and people who use small or poorly operated systems might end up saturating the environment with fog for a while, in this case, sensor death is inevitable.

There is a solution however, the concept is simple; dilute the air with a known quantity of drier air. odds are the temp inside will be the same as outside so, the math is simple, just average it out.

(outside+inside) /2

as for aquiring this air, the best way would be to modify a peristaltic pump to use 2 small tubes instead of one, that way you could ensure the same amount of air is pumped through. otherwise 2 identical low flow diaphragm pumps will work.

Simply use a hygrometer outside, to read ambient humidity, and combine the hoses, run this though a fairly wide long hose so there is ample time for the gas to properly mix, then direct that into a chamber containing your desired hygrometer. It can be a nice cheap one like a DHT22, but i recommend a BME280. the DHT22 is still suitable for the outside one, it works well enough at low humidity at least. You will need 2, and the BME280 can cost about $4-8 so, no reason to buy 2, so dont.

Once combined, you will read the averaged humidity, if the ratio grows too small because the outside humidity is too high, you can make the pump shut off and operate in a “limp mode” that way the sensor will never die from over humidification, which can happen if the outside RH ever exceeds 90%. If this is the case, odds are the humidification system wont have to run at all anyway. Running it low would be pretty safe in this case, and usually you arent ever supposed to exceed 95% anyway.

The math is simple for getting your inside humidity, just a little bit of algebra, i included the workings out so it makes more sense:

So there you go.

Because humidity sensors are generally pretty accurate outside of extreme high and low humidity, you should get a decently accurate reading out of this that will make up for the loss caused by slight mismatching of your pumps/flow rates and etc, though if you want you can always try also getting flow regulators or making diy flow regulators even to make sure there is identical flow rates.

 

Next post i will show a completed setup. I will also include links to logic (or arduino) controllable mains relays sockets, for safely controlling mains things using arduinos using pre-fab devices, since in some places its illegal to use self-made or modified mains devices, and in most places it probably invalidates your insurance if it causes a fire, using something you made.
I intend to go with one of two methods, either a double tube peri pump, or the perfectly superior, servo motor actuating two side-by-side syringe pistons, pulling in precisely the same amount of air, though this would also require about 4 check valves so i am hesitant to use it, unless i can find or make some cheap ones for the job.

Also i will include arduino code ready to go for anyone to simply copy, paste and compile