Flow Meter for < $100?

I'm designing a low-cost open-source flow meter. The Arduino-based system utilizes an Inertial Measurement Unit, housed within a positively buoyant pipe (PVC), to log absolute orientation (euler vector) data at 2Hz when anchored. I imagine being able to attach the pipe to an eye bolt anchored to a cinder block and have the device log data for about a month or so on one charge (~6Ah).

In order to have everything fit in a 1" PVC pipe, I'll be using the Arduino Nano. While not the smallest microcontroller on the market, the Nano's integrated USB port for serial communication and large number of programmable I/O pins reflect a nice compromise between size and user friendliness. The circuit consists of the following (mostly plug and play) components:

• Arduino Nano

• BNO055 Absolute Orientation Sensor

• DS3231 Real Time Clock

• MicroSD Card Breakout

• 24LC256 EEPROM (or larger, or multiple)

You could use an array of these to compare relative flow dynamics, or calculate absolute flow intensity by incorporating coefficients for the buoyancy and drag exerted on the rig. This figure shows some of my initial data. Location of the points indicate absolute position in reference to an anchor point, arrows indicate orientation of the device, and color indicates the passage of time (from cold to warm).

This next figure is the same data, but projected onto the XY plane (as if you're looking down on the device). I can use data like this to determine net flow direction over time - it would be fascinating to know if certain directional forces are more prominent at certain times, or under other sets of conditions such as climatic, tidal, or even el niño patterns.

Calibration in a lateral flow chamber, as well as in proximity to in situ ADCPs may ultimately yield a robust and inexpensive device with data quality comparable to more expensive options that typically run ~$30,000. I will update this page as the project progresses. For now I'm designing the waterproof housing, and figuring out how to make the circuit able to withstand a little more abuse.