Mechanism for pouring exact amount of liquid?

I have a cylindrical container(around .75 liters) that is filled with water. I want to accurately pour a certain amount of water in a cup. How can that be done??

My idea (might be complete fail) :
I put a hole at the bottom of the cylinder, and a door mechanism that opens and closes the hole. The door will be controlled by timing. So lets say i want to pour 50ml of water in a cup. Using some physics i can get the time needed for the water to freefall into the cup and i will open and close the door accordingly.  The idea sounds a bit cheesy so I wanted some opinions. Picture is attached.

Some comments I would appreciate:
1. Is my door opening and closing mechanism a complete fail? A small motor will turn the pinion which will in turn move the rack horizontally opening and closing the door. Note that I want the door open/close mechanism to be as small as possible. 
2.If the drink is not water, lets say orange juice or red bull, will the timing vary considerably?

3. Any ideas how to accomplish this in another way other than timing and opening closing a door? I want minimum complexity, and the amount poured doesn't have to be very exact, lets say within a 10% margin. (This is basically my question in the first 2 lines). 

Note: At my disposal i have simple machining methods(lathe, milling, drilling, cutting), but nothing too fancy. I have sufficient knowledge of microcontrollers and electronics.

Thanks in advance!!


Picture of Mechanism for pouring exact amount of liquid?
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Use a solenoid valve.
Chris414 (author)  steveastrouk4 years ago
How would I able to fit the valve in my system? The valve looks a bit big (from google images).
Use a small valve !
frollard4 years ago
Others here have very good suggestions -- if you need a specific volume every single time, do it like a liquor bottle valve spout.

It has a container under your main storage area that has an 'in' valve and an 'out' valve. Both valves move in unison, where one is always open and one is always closed. Activating the set of valves makes it so the previously open valve closes, and the previously closed valve opens. The amount of liquid in the bottom closed section dictates the dosed amount. When the bottom section is drained, the valves move back, closing the bottom of the extra container and opening the extra container to the storage tank, where the liquid fills the lower container again.

Chris414 (author)  frollard4 years ago
Wow that is a beautiful idea! I'm not sure yet if i need a specific volume every time but thank you!
the trick is to have the ability to precisely measure out a certain smaller volume several times to get the amount you want. Not the quickest thing in the world, but one of the most precise.
Kiteman4 years ago
I wouldn't use a sliding door.

Use a servo to turn a ball-valve, or to raise and lower a plate on the inside (put the motor at the top, pulling at the plate with something like monofilament fishing line).

Apart from that, your idea sounds fine.
Chris414 (author)  Kiteman4 years ago
So many useful answers for me. I think I will eventually go with the ball valve. It sounds much simpler and a better idea in general from my pinion/rack thing. Thank you!
RavensCraft4 years ago
Instead of timing the dispensing of fluid,
you could use the weight of the fluid in the
cup to turn a solenoid valve off .
The platform on which the cup rests could have
light springs under it which depress as the cup fills.
Eventually the platform sinks to a level which activates
a micro switch or magnetic switch,which controls
power to the solenoid valve.
Or if you really wanted to get fancy, you could have
the cup on an electronic scale ,and use the scale
output to somehow turn the solenoid off/on, maybe
using a microcontroller.
Chris414 (author)  RavensCraft4 years ago
I like this idea! Could give me quite accurate results if the scale is accurate. I will have to look into the cost though. Thanks!
Vyger4 years ago
Here is a totally different way. One of the most accurate ways to get a consistent measured volume of a liquid is to use a syringe that has an exact displacement. You could adapt this by making a tight fitting plunger that pushes down on your fluid and has a feed hole to allow the displaced liquid to get pushed out. The distance that you push the plunger down will determine the amount of fluid displaced.