The project halted for a bit, but only because I won an extra week working offshore, so have not been able to work on the actual hardware yet.
At the moment I'm trying to decide between two viable candidates for measuring and adding the drops of chemicals to the optic cell. It's hard working only from limited internet research since I can't physically power up a pump to see how finely I can control the flow rate etc. Hopefully next week that will change.
At first I imagined a peristaltic pump with a PWM controller, enabling low flow rates. The idea is to have one end of the piping in a container with the chemical, and the other end right above the optic cell. A needle or other orifice can be used to try and control the drop size, while a [laser, red LED or IR LED] and [photo diode / photo resistor / photo transistor] detects and counts the falling drops. As soon as the drop falls, the pump reverses a tiny bit to stop rouge drops from falling too.
In concept this sounds good, and thus I've started on the code for that, but then I realised that there is a good chance that controlling the pump at such low volumes might be hard (we are talking about dispensing 50 microliters here). This will be my first option to try, since you can leave a large storage container full of chemical onboard, so you only need to replenish the fluid bi-monthly or so.
If this fails, I'm thinking of building a small scale syringe pump with the same drop sensing circuit as above. Maybe utilising a 10ml syringe. This will require you to replenish the chemical more often, and will make for slightly more difficulty accessing the chemical storage (take out the syringe, draw up 10ml and put it back in). Okay, a 10ml syringe will still hold enough chemical for a month (40 days, minus 10 for safety). Larger syringes will use more space in the final product, but also will reduce the accuracy of the drop measurements, since a 30ml syringe and a 10ml syringe vary greatly in girth, but only slightly in length. Thus the same linear movement translate to a large difference in dispensed amount.
Another plus side for using syringes, is that I'll be able to add a switch to notify the controller when the syringe is almost empty, enabling it to let you know. Still thinking of a way to do this with the peristaltic pump option, save for just sensing no more drops falling out of the needle.
Any comments or recommendations will be appreciated!