Turbine flow meter:

• The flow of the liquid caused the rotor to spin at an angular velocity which is proportional to the velocity of the liquid. The speed of the rotor is detected by a pickup on the outside of the tube.
• The liquid may be swirling when its enter the meter which may speed up or slow down the meter. This can be corrected by using flow straightner (bundle of tubes element or radial vane elements). Straightening elements consist of a cluster of vanes or tubes inside a piece of straight pipe. This is placed at upstream of the meter. If a plain straight piece of pipe is before the meter, it should be about 20*diameters equivalent in length minimum.
• The vane is made of ferromagnetic material.
• A high frequency Ac voltage (10KHz) is applied to the coil.
• Generally turbine meters perform well, if the Reynolds number is greater than 4000 and less than or equal to 20000. Because viscosity, temperature variation affects accuracy.
• The turbine meter operating temperature ranges from -200 to 450 0C.
• Density changes do not greatly affect turbine meters.
• The number of pulses per unit volume is called the meters K-factor (pulse/gallon)
• Available in sizes from 5 to 600 mm.

Four main types of pickups:-

• Magnetic reed switch: magnets in the rotor cause the reed switch to on and off giving a digital output this device needs a power supply.
• Optical pickup (photo electric): A beam of light (usually infra red) is interrupted. ( the problem with this method is that the windows tend to become fouled, and so light transmission through the liquid is not to be recommended in many application).
• Inductive pickup: work on the principle of a moving magnet producing a flux which produces a voltage in an inductive pick up.
• Reluctive pickup: it works similar to inductive but the magnet is in the pickup ant rotor in magnetic.
• Radio frequency (RF): an oscillator applies a high frequency carrier signal to the coil in the pickup assembly, an passing of the rotor blades modulates the carrier.