We had covered flow sensors in last year's sensor section. The I/O units would be gravity fed valves. The idea being that the valve opens and the the fluid runs until sufficient has been detected by the flow meter.
Upon researching the valves I discovered that gravity fed valves are the not the normal method. The simplest devices use the pressure of the fluid to open the diaphragm, such as this one from Adafruit and only £6. Valves that open with 0 pressure are called latched valves or zero pressure valves and are slightly more expensive, starting at about £25. Here's a selection of all kinds.
The second issue is that the ultrasonic flow meters I knew about are £187 each which is prohibitively expensive when I need at least 6 of them. So I considered using the same flow meter for each input.
This is a possible configuration. It does rely on the flow meter being sensitive.
I found one for 0.5-15 l/m at RS; it uses a rotary mechanism.
The next option I considered is one without a flow meter but instead uses timed delivery. This has been the stalwart of liquid delivery systems.
Before I make any decisions I need to estimate the actual volumes of fluid for each input because I anticipate needing a range of accuracy /control for each input rather than homogenous.
Ok I need something like 10ml of feed per 10 litres of water for both Solutions A and B. The Adafruit pump does 100 mL/min so 6 seconds is 10 mL. Of course I could pre-dilute it so if I did it 50/50 that 12 seconds is the same 10mL concentration but I'd have to mix 20 Litres of output.
So, in conclusion, I need three peristalic pumps to add solution and pH fixer. The water input pump, output pump and also the recycler pump can be the 9L/min pump and use a "cheap" flow meter.
This leaves me with the following block diagram:
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