Unlike positive displacement (PD) meters, turbine meters are inferential-type meters that infer volumetric flow rate from the measurement of rotational movement (angular velocity) of a bladed rotor or impeller suspended in the flow stream. Two basic assumptions are necessary to obtain volumetric flow rate from a turbine meter:
1. Volumetric flow rate is proportional to the average stream velocity.
2. Average stream velocity is proportional to the rotor (blade) angular velocity.
As fluid passes smoothly through the meter, it causes the rotor to revolve with an angular velocity proportional to flow. The rotor blades, passing through the magnetic field of the pick-up, generate a pulsing voltage in the coil of the pick-up assembly. Each voltage represents a discrete volume. The frequency of the voltage generated is proportional to the rotor speed or the rate of flow. The number of pulses per unit of flow is termed the K factor. The actual K factor for each meter is determined by factory calibration and is provided with each meter. The total number of pulses, integrated over a period of time, represents the total volume metered.
Turbine meters can be either in-line or insertion type. In-line turbine meters are commonly used in both liquid and gas service. Insertion turbine meters are used in gas, steam, air, water, or oil. In liquid service, they may be used to pace automatic line samplers. A bi-directional flow option is often available on each of these meters.