I have been working for several days on an inexpensive but reliable means of dosing 2-part and other additives. After several attempts I have come up with this design. It has been running for a couple of days now and seems to work quite well.
Parts Required:
Airline
Check valves
Airline "T"
Airline pinch valve
Timer
Air pump
Additive reservoir
The premise of the design is that a given length of airline will contain a given amount of liquid, which can be pushed out of the airline by an air pump. If the airline is cut to the appropriate length, the desired volume will be delivered. By using a siphon to refill the airline and a timer to periodically pump out the liquid in the airline, a dosing scheme can be acheived.
The design is shown below:
Once a siphon is created out of the reservoir, the coil of airline will fill up with liquid until it reaches the height of the liquid in the reservoir. When the air pump is turned on by the timer, the liquid in the coil will be pushed into the sump. The pinch valve creates some backpressure to prevent liquid from siphoning out of the reservoir once the airline has been purged and only air is flowing through the line. This is important if you are using a 15 minute increment timer. The top of the discharge line must be above the height of the liquid in the reservoir to prevent continuous siphoning.
So far I have been able to achieve delivery volumes of 5, 10 and 20 ml of liquid per cycle.