A Positive Displacement (PD) Flow Meter works on the principle of trapping and transferring a fixed volume of fluid repeatedly and counting how many times this process occurs.

Step-by-Step Operation :

1. Fluid Entry

The liquid enters the meter and flows into a chamber containing moving elements

(gears, rotors, pistons, or oval gears).

2. Fixed Volume Trapping

The meter traps a known and fixed quantity of fluid between these moving parts and the meter body.

3. Mechanical Movement

The trapped fluid causes the internal elements (e.g., gears or rotors) to rotate.

4. Displacement Cycle

Each rotation or cycle moves a precise volume of fluid from inlet to outlet.

5. Counting Mechanism

The number of rotations is counted using:

• Mechanical counter, or
• Electronic sensor (pulse output)

6. Flow Calculation

Flow rate is calculated as:

Flow Rate = Number of cycles × Volume per cycle

Simple Formula

Q=N×VQ = N \times VQ=N×V Where:

• Q = Flow rate
• N = Number of rotations (or cycles)
• V = Volume per cycle

Key Concept

• The meter physically measures volume, unlike other meters that infer flow from velocity or pressure.
• That’s why PD meters are highly accurate and reliable, especially for viscous fluids.

Common Types of PD Mechanisms

• Oval Gear Type
• Gear Flow Meter
• Piston (Reciprocating) Type
• Nutating Disc Type
• Rotary Vane Type

Important Note

• Accuracy depends on tight internal clearances and proper lubrication from the
• Works best with clean and viscous liquids.