You will need an analog input card for you ph, ec, and co2 sensors to do dosing, analog cards are not cheap so you will want to limit the amount of inputs you need, for the heating and cooling its cheaper to use thermostats and run the contact to a plc input for control.
I control my dehumidifier and fans, the inline blowers, the E & F pump for the babies, the ballast power, flip relays, PH up or down, nutrient ppm, CO2 PPM, Feed pump for the main system, and the chiller, and it keeps the RO water res and the main res topped off.
Nutrient and ppm is controlled according to the schedule on the HMI, realtime ph, ec, and co2 levels are displayed and charted on screen, Co2 levels can be changed on the HMI.
There are fault conditions programmed in for ph changeing to quickly, ph to high, ph to low, co2 low, co2 high, ppm low, ppm high, co2 bottle empty, RO res tank low, main res system low which also kills the pumps. I also have a reminder once a week to check my pump filters, when any of these faults occur the PC sends an email (or text message but i prefer email) to my phone with current ph, ec levels and the fault condition. It also automatically changes the water every 2 weeks.
Future additions:
Water sensors on the floor in every room to detect leaks.
Amperage monitoring on ballast circuits and controller supply.
R/H and temp probes and transmitters paired with process controllers or possibly analog input card for the plc.
Parts needed for mine:
3 water level sensors
12 relays of various load capacities
10 diaphragm valves (7 gravity feed suitable, 3 need pressure)
a ph and EC probe with transmitter (see below)
2 co2 sensors with transmitter built in
12VDC power supply
24VDC power supply
allen bradley micrologix 1500
analog input card
24VDC output card
lots of cords recepticles, terminal blocks, fuse holders, blah blah
Hannah industrial grade probes and transmitter
HI 3001 - Conductivity Probe w/NTC Sensor for ATC
HI 1001 - pH Electrode w/Double Teflon Junction & BNC
HI 98143-04 - pH & EC Transmitter with Isolated Output 0-4V
honeywell CO2 sensor - C7632A
My understanding, so far, is that SSRs work for digital because of the soft start gently ramping up power not as big of a surge. DPDT contactors for magnetic (240v). Flipping without powering down causes arcing, reducing the life of the contactors.I could be wrong but don't digital ballast have "soft start" control built in to ramp up power when igniting? I am not sure if the ballast would stand up to the amperage surge when you flip to cold bulbs. I know someone who does flip without powering down but he uses magnetic ballasts. PLC would be the best way to do it because everything would be very accurately synchronized. If you dont have experience working with PLC's you could just use a relay. You run the power for your ballasts through a relay on normally closed contacts and use a cycle timer to power the coil for 5 minutes or whatever while you do your flip, just make sure your timers stay in sync.