http://www.amazon.com/Ventamatic-XXDUOSTAT-Adjustable-Thermostat-Ventilators/dp/B00336G1ME/ref=sr_1_1?ie=UTF8&qid=1384376190&sr=8-1&keywords=duo+stat
this is the thermostat/humidistat i was talking about above... srry i thought this thing was alot cheaper than 30 bucks.
this is an example of the other type of thermostat i mentioned. again they tend to be very rugged, and will stand up to moisture and heat etc. all day. the cheap one i mentioned above, it far less... robust. it relies on a bimetal coil to sense heat, and a plastic film material to sense humidity. Ive never had one fail... but they are known to.
W/ respect to the supplementary lighting you see in professional type greenhouses. from what i understand... they are mostly used in upper lattitude greenhouses to supplement lighting during short photo periods in winter months? for heating etc, ive seen that they like to burn cheap biomass in highly efficient forced air boilers .
overcast days perhaps, could be detected with a simple net enabled device retrieving weather information from a weather station....or even perhaps pyranometer data from a local station? IDK if this is possible or not, im really not up on this sort of thing. seens more ideal than a photocell arrangement.
Plz allow me to play devils advocate w/ respect to the arduino/PI controllers...
IMO stay away from arduino/ PI systems as far as PH control goes.
PH controllers are fairly specialized pieces of equipment BUT they very common in industry... because they are so common,
they are very cheap on the used market.
a good example is the GLI 53... it was VERY VERY common... there are 100's of these things on ebay every month. i have personally, about 3 ph controllers and a hand full of conductivity controllers. ive never payed more than 50 bucks for any of them.
they used to drop these things into sewerplants... paper mills... effluent monitoring stations etc. theres so many out there up for grabs.
yokogawa also makes a number of good ph analyzers, exact same form factor, and same sensors.
rosemount makes them... look for "solucomp" analyzers etc.
thermo now makes them... these are heinously expensive usually, full color screen... data logging onto an sd card etc.
a ph controller needs to be able to calibrate and generate a slope, keep this in mind when you consider an arduino set up. you will need a way to enter new calibration information, or have it do this automatically like a traditional ph analyzer unit
Also, MOST if not all of the cheap ph electrodes you get with your arduino PH stamps etc. are IMO junk. IMO you need an electrode rated for 100% duty, and "process" immersion. these sorts of electrodes are necessarily more expensive. with that said, i have not tried using a cheap gellie electrode for continuous usage. it may just work well enough... if you know what i mean? i mean if it lasts 6 months, fine. its probably worth the money, but keep in mind they are not designed for continous usage. they have fairly small reservoirs of electrolyte.
the continuous rated process type electrodes will tend to be refillable,have replaceable salt briges... some even have replaceable glass. most will have a much thicker glass that will hold up to repeated electrode cleanings, and hence can essentially be regenerated many many times, once its fouled... they will also have an integrated ATC thermocouple for very easy temp. correction.
some of these electrodes are rated for absurd operating conditions such as 300 psi and 200+ Celsius, this is of corse not necessary for hydroponics, however many of these electrodes are rated as such as a matter of fact... being that their main usage is for industrial processes that tend to be very very harsh.
brand new these electrodes are heinously expensive, however again they are very very common in industry
and hence can be had for pennies on the dollar on the used market.
look here:
http://www.ebay.com/sch/i.html?_trksid=p2050601.m570.l1313&_nkw=accuglass+&_sacat=0&_from=R40
rosemount makes very good sensors... look for accuglass stuff with appropriate threads, and the correct glass.
GLI also makes a very very good sensor, if you are patient you can get one for under 100 bucks.
yokogawa makes some good sensors altho dont be confused by " reference" electrodes. you do not want a reference electrode!