Positive displacement flowmeters, sometimes known as PD meters, have been around for more than 100 years. They are commonly used in a wide range of applications from domestic water measurement to measuring ultra flow rates of chemical at high pressures subsea.
- Rotary Piston: As mentioned above these form the basis of domestic water measurement but the design of the rotary piston that oscillates in a circular chamber with a fixed web has been modified and extended to ultra low flows and high flows, as well as high pressures and for food applications. A good all-rounder.
- Spur gear: The fluid rotates two gears and is forced around the outside of the gears and the inside of the chamber. Depending on the location of the sensor these can yield very high pulses per litre values useful in batching and fast acting processes.
- Diaphragm (or bellows meter): These are common in many people’s home as their domestic gas meters. When the gas flows through it alternately fills and empties bellows causing levers to crank a shaft providing an output. Very useful for wide-ranging gas totalisation.
- Oval Gear: Quite similar to the spur gear where two oval gears mesh together and sweep the chamber. The volume displaced is much larger than the round gear. Fairly low cost and some designs available in plastic.
- Nutating Disc: This meter is the hardest to understand but is effective. The rotor is a circular disc attached to a ball. The shaft on the ball is inclined. As the disc rotates in a spherically sided chamber the disc and therefore the shaft wobble creating an output.
- Helical Screw: Possibly the most accurate PD: meter two intersecting cylindrical bores are fitted with 2 interlocking helical screws. As the fluid passes through they rotate. On standard applications the author has observed differences of just ±0.37 per cent of reading over 50:1 turndown over annual recalibrations over 10 years – quite an achievement. Also common nowadays fitted on petrol pumps.
- Slide Vane: Historically the most accurate of PD meters with the rotating element having a number of moving blades that rotate about a fixed cam. Linearities have been claimed of ±0.02 per cent.
- Others: If we go back to Felix Wankel’s seminal work on rotary machines we see that there are as many designs for PD meters as there are pumps. He explored in a rational way the various shapes of rotor and chamber. While the majority don’t see the light of day in the marketplace this brief essay illustrates the variety in general use, and this is without discussing the Roots meter, wet gas meter and multi rotor designs.