The Story 

RG3 positive displacement meters have used an SR® style oscillating piston chamber since the 1980's after the patent ran out on the design. While we have enough respect for the SR® measuring chamber to copy it, the SR® style chamber will start to lose low flow due to scaring on the piston and cylinder. An SR® with 1 million gallons of use will typically test almost perfect on high and medium flow while showing low flow loss. Many customers get 25 years of faithful use out of these meters, but nobody wants to lose low flow in the process. 

The low flow drop comes from the piston rubbing against the cylinder wall as it moves around the chamber. The piston is designed to be lubricated by the water and never touch the cylinder wall, but unless the bolts are perfectly tightened at exactly the same time and stay in exactly the same place, the force from the bolts pushes the bottom plate into the chamber which bends the chamber wall just enough to cause the piston to rub. The rubbing wears away at the piston and cylinder wall until a micro water bypass has been created reducing low flow over time. 

Tightening or loosening bolts on any meter will greatly affect it’s accuracy. When torqueing the bolts to secure the bottom plate, every meter has a sweet spot that will provide the best test results but with temperature changes and time, bolts tend to back off or tighten up at uneven rates. The only reason that torqueing a bolt down more could change the meter’s accuracy is because the bolts compress the bottom plate and the bottom plate then compresses the measuring element (chamber). Bending the meter chamber causes the cylinder wall to slightly push into the piston causing the rub. The only way to solve the wear problem caused by tightening the bolts is to remove them from the design and make sure the chamber is compressed exactly the same all the way around with 360° of even compression at the perfect amount of force so you always stay in the accuracy sweet spot. 

That is how the Perpetual meter came into existence. The patented brass main case design applies 360 degrees of uniform compression loading on the SR® style oscillating piston measuring chamber. The chamber is cradled in 100% silicone on top and bottom to prevent the bottom plate from pushing the cylinder walls into the piston when the meter is assembled. The design doesn’t allow the piston to touch the cylinder wall which prevents wear spots from being created. We also added an oversized 100% silicone outlet gasket to create a superior seal against the machined brass body to ensure no bypass occurs. We use 100% silicone because it doesn't break down and crack like the nitrile rubber other manufacturers use when exposed to water additives like chloramine for years. 

The Perpetual PD design reduces drag and wear which improves long term low flow accuracy and increases longevity. That is why Perpetual PD meters have twice the new meter accuracy warranty of any other mechanical meter.