I'm currently working on a new project using the arduino for a true aeroponic setup. I've worked out all the details except the final one. I'd like to be able to sense the mist/moisture in the root chamber of the aeroponic system and base misting cycles on demand rather than the far more typical timed cycles.
The trouble i'm running into is finding an appropriate way to sense/measure this variable and this is where i need your help and suggestions. Let me run through the options I've considered and my concerns with them.
Humidity and Temperature Sensor - RHT03 (sparkfun). I don't think this would hold up in the environment.
Optical Dust Sensor - GP2Y1010AU0F (sparkfun). Same as above
Sunkee Rain Sensor Module. Requires waiting for condensation formation to bridge the contact pads and reset time would be very difficult (once wet in this environment it would always read as wet).
Environmental Specific concerns.
The root chamber of an aeroponic system is subjected to nearly 100% humidity 100% of the time. Not just with water, but with a cocktail of nutrient solution in water, very high in conductive salts (not the most electronics friendly environment).
The roots of the plants must be kept in total darkness, so a simple optical sensor (led and photocell) or laser based systems would not be options.
The particle size of the mist being used is in the 20-100 micron range (mostly around 50 microns).
I've considered trying to use ultrasonic distance sensors, but I'm unsure if they would "see" the mist or shoot right through it given the particle sizes. So that's where I'm at. Any and all suggestions are welcome. Thanks in advance.
For my purposes consider the root chamber as a more or less sealed 5 gallon bucket. Nominally at room temperature ~68 F. 3 Fuel injectors are tapped into the bucket to provide the nutrient mist. The system operates around 60psi and control of the injectors is in the millisecond range. Excess mist collects in the bottom and is carried into a drain line but this exceptionally wasteful. Currently I use timed cycles to mist the root chamber periodically, but it occurred to me that a far better system would be based on demand. The mist solution tends to suspend in the chamber for a period of time due to particle size before condensing on the walls of the chamber (this is unavoidable and expected). There will always be some waste, but if i could figure out an appropriate way to sense the "density of the mist/fog" for lack of better term it would be far more efficient. Alternatively way to sense the moisture level within the root mass would work. However, given the rapid root development in systems like this, any such sensor would likely be occluded in short order by the plant roots.
It does to a point, depending on the stage of plant development, from cloning/root formation, vegetative growth and flowering/fruiting being the major 3 changes. While specific nutrient content of the mist varies with each stage, the content of the mist for any given stage is fairly static. The waste runoff is highly variable in content and concentration which is why it must be discarded rather than recirculated.
Hmm.. As long as the dielectric properties of the mist stay relatively stable it would be possible to use a capacitive sensor to measure the density of the mist. However this does appear to be the case.
Light scattering at right angles is commonly used as a measure of (small) particle size and density, and the optical system could obviously be a small chamber completely isolated from the electronics and the roots. Google "aerosol measurement light scattering" for more.
light scattering is indeed common, and i believe is the basis of the sharp "dust" sensor i alluded to from sparkfun. My concern with it is that rather than a dry dust, I'm dealing with a wet environment. Not only are such sensors generally not waterproof, the diffraction differences between water and dust are very different. If you're aware of a water/weather rated module I'd love to take a look, as it would probably be ideal.
I may have found a suitable sensor from sonbest.com (also carried by adafruit) they have a temp/humidity sensor that works over i2c encased in weather tight mesh screen (supposedly capable of full submersion). I'll order one and let you all know how it works out. Thank you for the suggestions.