** UNDER DEVELOPMENT **
A positive displacement (PD) water meter measures the flow of water by allowing a specific volume of water to pass through the meter and then counting the number of times this volume is filled and emptied. This ensures that the meter accurately measures every gallon (or another volume unit) that passes through it.
Here's a basic idea of how it works:
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Chamber Filling & Emptying: The PD meter typically has a chamber or compartments that are mechanically filled and emptied with a precise volume of water. As water flows through the meter, it fills these chambers, and once a chamber is full, it triggers a mechanism to empty and then refill.
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Counting Mechanism: Each time the chamber fills and empties, the meter's counting mechanism registers an increment, whether it's a rotation of a gear or some other movement. This movement is then translated into a water volume measurement which is displayed on the meter register.
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Accurate Low Flow Measurement: The design of PD meters allows them to accurately measure low flow rates, which some other meter types might miss.
There are various designs of positive displacement meters, with the most common types being:
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Oscillating Piston Meters: These have a piston that moves within a chamber. As water flows through the meter, the piston oscillates, and each oscillation represents a fixed volume of water.
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Nutating Disk Meters: These have a disk mounted on a sphere inside a chamber. As water flows through, the disk wobbles and this movement is counted.
- Diaphragm Meters: Also known as a rotary displacement meters, these meters have two or more diaphragm chambers. As water enters the meter, it fills one of the diaphragm chambers. This causes the diaphragm to expand. Once this chamber is full, the flow of water is directed to the next chamber, causing the first diaphragm to contract and the next one to expand. This continuous movement of filling and emptying chambers pushes gears or levers attached to the meter register.
Positive Displacement meters are commonly used in residential settings because of their accuracy at low flow rates, which are typical in households. They are reliable and have been in common use for many years.