user21253: After recent flooding of my apartment (pipe burst) I got an idea of automatic water flow watch. I would like to measure or just watch if water flows on main water intake. Arduino will know at what time of day (during nights and when nobody is home) the water flow is a problem and will send me an sms or whatever alarm. Measurement will be better to enable you to go to the bathroom, so the alarm will be raised only after certain amount of water.
The main problem is how to measure the water, are there any water-meter that can communicate with arduino? 

user2881: The utility company probably already placed a meter at your apartment. 
Mine has a spinning dial, that's half metal, half plastic. I've had a photo-reflective sensor attached to it that so I could detect the change in amount of light being reflected (metal vs plastic). Every peak meant one liter. 
Mine also has a rotating magnet, but I wasn't able to read that using a hal-effect sensor, or the magnetometer on my phone.

user19300: I have an electronic water flow meter which uses a turbine wheel inside the pipe. Ostensibly there is a magnet on one or two or all four of the blades, which is read by a hall-effect sensor inside the meter. The specifications require 3 gallons per minute, which may be lower than you wish.
http://catalog.gpi.net/item/chemical-and-water-meters/01-series-electronic-digital-meters/01n31gm-nylon-water-meter?plpver=1001
If the flow rate is acceptable, you could add your own hall-effect sensor outside the pipe and connect an appropriately programmed Arduino to perform the tasks you require.
Alternatively, one could create via 3d modeling software an equivalent design to be printed by an online service or a friend with a printer. For balance purposes, a magnet in each vane would be advised.
Years ago, I had a speedometer for a kayak, about the size of one's thumb. It used the same technology, although the sensor was about two inches away on the upper side of the hull, while the tiny propeller was on the bottom. Apparently small magnets spinning create sufficient disturbances in the force to be read by mass produced sensors, even more than a decade ago.

21253: <p>After recent flooding of my apartment (pipe burst) I got an idea of automatic water flow watch. I would like to measure or just watch if water flows on main water intake. Arduino will know at what time of day (during nights and when nobody is home) the water flow is a problem and will send me an sms or whatever alarm. Measurement will be better to enable you to go to the bathroom, so the alarm will be raised only after certain amount of water.</p>

<p>The main problem is how to measure the water, are there any water-meter that can communicate with arduino? </p>
 2881: <p>The utility company probably already placed a meter at your apartment. </p>

<p>Mine has a spinning dial, that's half metal, half plastic. I've had a photo-reflective sensor attached to it that so I could detect the change in amount of light being reflected (metal vs plastic). Every peak meant one liter. </p>

<p>Mine also has a rotating magnet, but I wasn't able to read that using a hal-effect sensor, or the magnetometer on my phone.</p>
 19300: <p>I have an electronic water flow meter which uses a turbine wheel inside the pipe. Ostensibly there is a magnet on one or two or all four of the blades, which is read by a hall-effect sensor inside the meter. The specifications require 3 gallons per minute, which may be lower than you wish.
<a href="http://catalog.gpi.net/item/chemical-and-water-meters/01-series-electronic-digital-meters/01n31gm-nylon-water-meter?plpver=1001" rel="nofollow">http://catalog.gpi.net/item/chemical-and-water-meters/01-series-electronic-digital-meters/01n31gm-nylon-water-meter?plpver=1001</a></p>

<p>If the flow rate is acceptable, you could add your own hall-effect sensor outside the pipe and connect an appropriately programmed Arduino to perform the tasks you require.</p>

<p>Alternatively, one could create via 3d modeling software an equivalent design to be printed by an online service or a friend with a printer. For balance purposes, a magnet in each vane would be advised.</p>

<p>Years ago, I had a speedometer for a kayak, about the size of one's thumb. It used the same technology, although the sensor was about two inches away on the upper side of the hull, while the tiny propeller was on the bottom. Apparently small magnets spinning create sufficient disturbances in the force to be read by mass produced sensors, even more than a decade ago.</p>
