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Sensors, Actuators, and Robotics » Dispensing Water

December 25, 2010
by dkilmer
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My project is a coffee maker, which needs to dispense water into a reservoir in measurable and reproducible amounts. Originally, I was just considering turning a water spout on for a defined amount of time, but this doesn't seem like it will be very accurate. I've read some threads on avrfreaks and elsewhere, and there are lots of interesting ideas, but I wanted to see what people here thought about it. It seems like there are three basic options:

  1. Use a pressure sensor under a leg of the reservoir.
  2. Use an air pressure sensor with a tube submersed in the reservoir (so as the water rises, the air pressure in the tube will rise)
  3. Use some sort of simple conductive thing (wires/resistors) submersed in the reservoir.

Option #2 seems like the most fun (and expensive), and option #3 seems easiest but least advisable (is it unwise to send current through the water?). I don't really need to dispense arbitrary amounts of water -- I'd be happy with specific levels like 8, 12, 16, and 24 ounces. Any ideas? Other options? Has anyone worked with pressure sensors and liquids?

December 26, 2010
by Ralphxyz
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Why wouldn't timing be accurate? If you are always using the same amount of water in the same container it will always weigh the same.

A alternate sensor is optical use a led and a phototransistor projecting a beam across the container when the beam gets blocked turn the flow off. Using optics you have none contact.

Ralph

December 26, 2010
by bretm
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Timing might not work becUse faucet water pressure can vary by day and location and temperature.

December 26, 2010
by mongo
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Timing can work if you have a regulator on the water line, set for a certain pressure to give you a known amount over a period of time. We had coffee makers at work that did just that. I think Ice makers in domestic freezers like in your kitchen, also regulate the pressure and just activate the valve for a set time.

December 27, 2010
by dkilmer
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Varying pressure was indeed the reason why I thought simple timing might be inaccurate. I'll have to see if a pressure regulator would be small and inexpensive enough for my project. I was also thinking of having a second reservoir that always fills to the top, and a small water pump that pumps from that into the brewing reservoir. The water pump might provide a more constant flow rate.

The optical sensor is a good idea. I'm worried that it might not work if it's trying to "see" though a glass or plastic container, or that water, being relatively transparent, might not block it properly. Any experiences with that?

December 27, 2010
by Ralphxyz
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I had a liquid level that used two clear pieces of tubing with a length of water hose in between.

I think it used a IR Led and Phototransistor. Radio Shack part # 276-0142.

One would have to play to see what worked.

Now if you wanted to get fancy (anal) about your water you would get a Reverse Osmosis water filter, they (a least GE's) come with a 1 gallon storage tank. The water taken from the storage tank would be of a constant temperature and pressure.

I still think timing would work it would not be be more than one or two grams difference from a domestic water supply at best even without using a storage tank.

Again another good reason to do some experimenting (playing around) with electronics.

Ralph

December 27, 2010
by mongo
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There is always the pre-measured kind, like a toilet tank. Fill the tank with a small line and let the tank do the work.

January 31, 2011
by Twarter369
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How bout a float sensor? Say Chip -> turns on water -> Float valve sends an interrupt that shuts the water.

I bought a 1gpm brushless (submersible) CPU cooling pump for around $17. If you used something like that you would get a constant flow and therefore you can use the algorithm time = desired amount / flow rate to find out how long to run your specific pump.

i.e. if you wanted 2 gallons then t = 2/1 or 2 minutes....if you wanted 6 fl oz then first convert the flow to oz (1 gal = 128 fl) so t = 6/128 or .046 minutes

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