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Old 06-30-2020, 09:25 PM
Draikan Draikan is offline
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Default Re: Precise environmental control

I started this project by looking for a product I could use to achieve what I wanted rather than having to build something myself. The only thing that came close were the Apex aquarium controllers. They have a great web interface, but they don't really support the hardware necessary for terrariums, and they are super expensive. They also do not have a humidity sensor.

The humidity sensor is the single most important part of this system. All the hardware control (fan/misters/etc.) ends up being based off the humidity measurement. I have used a bunch of cheap in-terrarium sensors, and they all broke eventually. I've also worked with a couple humidity sensing systems at my job for more industrial applications, but even those had some issues.

There are really only two types of humidity sensor that are widely used, and they both have significant drawbacks:
-Capacitive, these sensors are tiny integrated circuits that measure how the capacitance of a polymer membrane changes as it absorbs water from the air. This is what pretty much all affordable humidity sensors including handheld meters, the govee, and other terrarium products are based on. Unfortunately these are not waterproof, if water gets onto the sensing element it can permanently damage the sensor. Obviously this is bad if we are spraying mist around. This type of sensor is also not able to withstand prolonged exposure to humidities above about 80%. In a high humidity environment the sensor output will drift up, causing the sensor to report a humidity that is erroneously high.
-Chilled Mirror Hygrometer (CMH), this is a precision optical instrument that uses a reflectance sensor to chill a gold plated copper mirror until condensation forms, and then measure the mirror temperature. The temperature at which condensation begins to form is the dew point of the air. These can be very accurate and robust, but typically cost $5k-10k.

I am trying to adress the shortcomings of the capacitive sensor, so it can be used in terrarium conditions. If that does not work, I may try to build a cheap CMH.
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