Here is a picture of our ultra-cheap and easy water flow sensor fully built and ready to use:

Fully assembled water flow meter

The rotor is nothing more than a cheap propeller purchased from an RC toy shop nearby mounted on the end of a stick. We then attached 3 small magnets between the blades and mounted it loosely on a screw to an aluminium stick.

Front view of rotor assembly

Finally, we mounted a hall effect sensor to the stick and waterproofed it with glue. The hall effect sensor is mounted so that it can sense the magnetic field of the magnets as the rotor spins.

Side view of rotor assembly

The pieces partially disassembled

The hall effect sensor is powered by a 9V or 12V battery and a 5V regulated power supply on the “dry side” of the stick, and the output wire from the hall effect sensor is connected to a cheap multi-meter that has a frequency measurement mode (Hz). When the water moves over the rotor blades, it spins the magnets causing the magnetic field to change at 3 periods per rotation. By reading the Hz from the meter and dividing by 3, you get the rotations per second of the rotor blades.

I calibrated it by moving it through water and measuring the number of rotations per distance travelled. This value was pretty consistently about 9.5cm per rotation. Thus, 3Hz on the meter equals 1 rotation per second of the rotor and corresponds to 9.5cm/sec water flow rate. So if X is the reading on the meter, then the water flow rate = (9.5/3)*X cm/sec.

You can compare this device to something like this:
http://www.vernier.com/products/sensors/flo-bta/

I asked for a quotation for a similar device available here in Thailand, and they quoted me at 180,000THB (about $5,000). Obviously, this is way too expensive for my little experiments with water flow. I can only imagine that this thing has some kind of precision calibration. I imagine mine is +/- 10% accuracy.

Here is a pricelist for the main parts of this little device:

• Propeller: 80B (~$2) for pack of 2
• Hall effect sensor: 17B (~$0.50)
• 5V regulator: 5B (~$0.15)
• 3 small magnets:  15Bx3 = 45B (~$1.30)

Technically, you could wire this up with just the above mentioned 5THB 5V regulator to power the hall effect sensor from a battery, but we happened to have a bunch of these LM2596 DC-DC switching power supplies laying around so we just used one those.

The scrap piece of aluminium shaft, tape, glue and a screw to mount the propeller are considered free.

Note that the multimeter is not permanently attached to this device. We connected it with alligator clips so you can use it as a multimeter most of the time and just connect it to this device when you need to. But if you need to buy one, this one was about 1000THB (<$30).

Total price for our little flow meter is… 147THB (~$4). This is if you have a cheap multimeter laying around. Otherwise it is about 1150THB (~$33) if you need to also buy a multimeter for it.

It might be less precise than the device I got a quote for, but we also spent about 1200x less on it. 🙂

And now to use it to measure the flow rate of our prototype circulation pump…