Controlling water volume applied

I need help automating a process we have done by hand over the past few years. We need to "spray" tap water into a material mix at different
rates. The typical volume of water applied ranges from 25 ml to 1000 ml. The typical flow rate should be approximately 100 ml per minute. The accuracy does not need to be that great, probably less than 5%.
We have done this in the past using an approximately 15 psi pressurized clear plastic column filled with water having a scale affixed. We would turn the valve on and off manually after monitoring the volume change on the scale.
I need to automate this process with user defined volumes of water applied. I have very little experience with control systems and am looking for as close to "off the shelf" as possible.
I had thought of using a flow meter and a timer hooked to a solenoid. If I knew the flow rate I could calculate the time to achieve the desired volume and set the timer to activate the solenoid when the calculated volume has passed. But I would need to add a regulator to verify the psi applied to the system is always the same.
I also have thought of using a micro totalizer to do the same job but I don't have any experience with these devices.
But I basically need a technician to enter the volume in ml and hit start. Any ideas on how I can accomplish this task economically?
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RG wrote:

Closed-loop control may be more sophistication than needed. One or a binary series of (relatively) inexpensive flow-control valves can be selected in parallel to set the rate, and your timer-solenoid combination to set the total amount. So long as the pressure is within rating (say 25 - 125 psi) inexpensive valves will hold to about 10%. They are better over a narrower range. A pressure regulator off the mains set to 25 or 30 psi should make the bank very repeatable. The valves at http://www.plastomatic.com/fc.html have too much flow for you, but I'm sure you can find others.
Jerry
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RG wrote:

I would be thinking in terms of metering pumps. They discharge a fixed volume per rotation or stroke. Either you count strokes or run them at a fixed speed and for a controlled amount of time. You check the calibration with a graduated cylinder.
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I was just looking at these pumps. http://www.coleparmer.com/catalog/product_view.asp?skut51910
They rate them at certain psi but is that the input or output? I need enough pressure to get a spray pattern out of a nozzle.
I have seen Dark Room timers that shut off an appliance at intervals of a minute. I will need to look for some digital timer that controls a 115 circuit
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Hello RG,

Isn't that a stroke pump? Then the volume of fluid per minute should be fixed so you could calculate (or measure) the pressure that results with a given spray nozzle and this should not change much. Of course you would need to equalize for the strokes if it runs really slow. In medical we do that with compliant tubing but that won't lilely work at the pressure levels you are using.
Regards, Joerg
http://www.analogconsultants.com
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Joerg wrote:

You can use gear and vane pumps at a slight sacrifice in precision. Their discharge is essentially continuous and they too are very nearly constant displacement. To quote Tom Leherer, "To think of all the marvelous ways they're using plastics nowadays ..."
Jerry
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Jerry Avins wrote:

P.S. A replacement pump for an espresso machine might be suitable. Belt drive it with cone pulleys for changing speed.
Jerry
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Hello Jerry,

A client has a wonderful espresso machine, makes excellent coffee. But every time the pump or something else breaks it costs an arm and a leg just for the parts ;-)
Regards, Joerg
http://www.analogconsultants.com
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Joerg wrote:

I got one for under $100. (All right: $99. Plus tax). I don't think they've gone up much. "Procon."
Jerry
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Hello Jerry,

Just for the pump? That would be a lot, considering that they aren't meant to run for too many hours in their lifetime.
We took the easy route after our espresso machine blew. Bought one of those Saltons for $60 or so. That's for the whole machine. Ok, the coffee from a Cimbali is certainly better but there is also 13dB price difference.
Regards, Joerg
http://www.analogconsultants.com
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Hello RG i would like you to visit www.ab.com and contact their people of your country. They can give you the best solution for this. Thanks
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