Read: Differential pressure flow meter
The positive displacement type flowmeter measures the volume flow rate Q directly by repeatedly entrapping a sample of the fluid. It measures the volume by dividing flowing fluid into fixed finite volume and successively fill and empty a chamber having fixed volume.
PD flow meter usually has a chamber, and a mechanism either rotating or reciprocal, that allows the passage of fixed amount of fluid. In order to measure flow using this device, it requires fluid to mechanically displace the component in the meter. The rate of revolution or reciprocation (it can be sensed mechanically) proportional to the number samples passed through the mechanism. It helps to determine the total volume.
The total volume of the liquid passing through the meter in a given period of time, Q = Volume of samples × Number of samples
Almost in all design, the moving part has a tight tolerance; it prevents the flow of fluid through PD flow meter without being measured (slippage). In some system, the output signal is used by secondary electronic devices to determine the flow rate. A sensor system gives the additional capability to the Positive displacement flowmeter; it can generate a pulse output which may be read on integral display or give user interface. Each pulse represents a discrete volume of fluid, so they are ideally suited for the automatic batching system.
- Difference between Differential Pressure Flow Meter and Positive Displacement Flow Meter
Specification of Positive displacement flowmeterPerformance Specifications of PD flow meter
- Minimum and maximum measurable flow rate
- Operating temperature range
- Operating pressure range
- Maximum allowable fluid viscosity
- Type of fluid (liquid or gas)
- Size of connection
- Percentage of accuracy
Types of Positive displacement flowmeterThe common types are
Gear: The internal gears in the chamber rotates as the fluid passes through them. According to the shape, there are different type of Gear meter.
1. Oval gear
2. Helical gear
Reciprocating and Oscillating piston type: The piston is mechanically operated to fill the cylinder then to discharge. The number of strokes represents the volume flow.
Read: Oscillating piston flow meter working, its advantages and disadvantages
Nutating Disc type: A disk mounted in spear move side by side about an axis as a finite amount of fluid flow through them.
Rotary Vane type: Rotating impeller contain two or more vanes, it passes the finite amount of fluid in every rotation.
Diaphragm type: Finite amount of fluid drawn into inlet side of the oscillating diaphragm, then discharge into outlet.