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Chapter 10. Water Flow Gauge

Chapter 10. Water Flow Gauge

Determining the consumption of a resource that is measured in units of volume can be more tricky than it sounds. Use of resources 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. This means there is the danger of underreporting usage if your sample rate is slow and usage rapidly fluctuates or spikes. Measuring cumulative volume makes it easy to determine total consumption 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. If your recording interval isn't brief enough, any short-term spikes in usage will be averaged across the recording interval and might not show clearly on the graph.

Flow gauges typically output a series of pulses proportional to the instantaneous flow rate, which means that to interpret them it's necessary to implement a simple frequency counter. This is actually the same way many car speedometers work: a wheel sensor outputs a pulse for each rotation of a wheel, which means the pulse frequency varies proportionally to the vehicle speed. The speedometer then displays a scaled version of the current pulse frequency to show instantaneous speed, while the odometer displays a scaled cumulative pulse count to show distance traveled. Both pieces of information are based on the same underlying data, but they are recorded and displayed in different ways.


  

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