I am looking for a simple device that dispenses liquids that is computer controlled via a USB or serial port. Nothing fancy but I just need to control the amount of a liquid that get dispensed in a bottle. For example when you go to Home Depot and get a certain color paint, they put the can of white paint underneath a nozzle, enter in the color on the computer and the computer dispenses a few different colors to create that paint. I am looking for that kind of dispenser. I would write the software. Any ideas? Sorry is this is the wrong group to post this in but I am trying to figure out where to start.
A small flowmeter and a computer controlled valve come to mind, but that intuitively doesn't seem very precise for an application like mixing paint. If you can be guaranteed that the liquid you're using always has a constant pressure or flow rate you can time how long a valve is opened for.
I don't like the timing based approach, for these very reasons. I'd prefer to measure in terms of quantity if I did this project myself - the loss of weight in the source tank as the liquid is pumped out for example, or perhaps an ultrasonic or optical method of measuring the level of the liquid surface (that would limit the speed you could pump at, of course, to avoid waves or splashing throwing off the reading...
I think most liquid dispensers are based on positive displacement pumps, essentially running in reverse. The (relative) incompressibility of a liquid works well this way. Essentially, fill a cup (piston, cavity, volume, what have you) turn it out, count one micro unit of dispensation, repeat.
All the manual paint mixers I've ever seen in the last 40 years or so work this way. Don't know about the automatic ones, but I assume they're similar. The paint is first measured into a cylinder that will accept only X amount, depending on the depth setting. The paint is then pressed out of the cylinder. IOW, the paint is not measured in "real time."
This is my first Usenet posting using google groups, so hope this goes OK.
I thought of something along these lines recently while coming up with ideas for a cheap dosing pump for my marine aquariums. It would need something that is not prone to clogging as the calcium solution used causes deposits to build up quickly. Most existing solutions use peristaltic pumps. They can be pretty expensive and hard to find though. My idea was to use a calibrated length of tubing to output a fixed volume. Hope I can explain this without having to resort to ASCII art :-)
Fluid to be dispensed is in a *sealed* bottle with three openings in the lid. One opening admits a tube that ends in the fluid, the second is a tube that ends in the air above the fluid and the third opening is a line to an air pump.
The fluid tube goes into a 2 way valve (or 2 valves in a manifold) that switches its output between fluid and air. The output of this valve leads to the calibrated length of tube. Let's say we want to measure out 100mL of fluid: we put an optical sensor on the tube (tubing is transparent at that wavelength) at a distance from the valve output corresponding to 100mL.
To dispense 100mL we switch the valve to the "Air" position and turn on the air pump. Air flows into the bottle and out of the air tube, through the valve and past the sensor. This ensures that the line is completely clear. Now we switch the valve to the "Fluid" position and monitor the opto sensor for an air->fluid transition. WHen we see the transition, it means we have 100 uL of fluid in the tube, now we switch the valve back to the "Air" position and look for a fluid->air transition at the sensor. We have now dispensed a fairly accurate 100mL of fluid!!! Repeat as necessary until you get to the desired volume.
Haven't built this yet as the only valve manifold I have is a bit leaky, but I plan on trying it. Aquarium air pumps are cheap and a single pump can run many of these contraptions, tubing is almost free and solenoid valves can be found on the surplus market. It should be possible to build a gravity-fed version that does not require an air pump.
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