Use an Arduino and a hall-effect flow rate sensor to measure water usage.
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README

Water Flow Gauge
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Copyright 2009 Jonathan Oxer
Copyright 2009 Hugh Blemings

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| This project is featured in the book "Practical Arduino" by         |
| Jonathan Oxer and Hugh Blemings (Apress, 2009). More information    |
| about the book and this project is available at:                    |
|                                                                     |
| www.practicalarduino.com/projects/easy/water-flow-gauge             |
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Measuring the consumption of a resource that is measured by volume can
be more tricky than it sounds. Usage of materials such as water, gas,
and even electricity is typically measured by gauges that determine
either instantaneous flow rate or cumulative volume over time. Both
techniques have problems: measuring flow rate at frequent intervals
allows you to do time-based reporting and generate a graph of how the
flow rate varied over time, but to determine the total consumption by
volume across a specific time period you then have to integrate the
data and there is the danger of under-reporting usage if your sample
rate is slow and usage rapidly fluctuates or spikes. Measuring
cumulative volume makes it easy to determine total consumption by
volume across a period and is accurate in terms of total usage but to
generate a flow rate graph you then need to calculate the difference
between each sample, and if your recording interval isn't brief enough
any short-term spikes in usage will be averaged out across the recording
interval and may not show clearly on the graph.

Flow gauges typically output a series of pulses proportional to the flow
rate which means that to interpret them it's necessary to implement a
simple frequency counter. This is actually the same way a car speed
sensor works: it outputs a pulse for each rotation of a wheel so that
the pulse frequency varies proportionally to the vehicle speed. The car
speedo then displays a scaled version of the current pulse frequency
while the odometer displays a scaled cumulative pulse count.

This project uses a flow rate gauge containing a hall-effect sensor
that outputs a pulse rate proportional to flow rate, so not only is it a
useful project in its own right but it also demonstrates a very useful
technique that you can use in a wide range of projects that need to
measure the rate at which something happens.