Surge And Pressure Relief Valves

Technical Guide

DAN-LIQ-Turbine Meter-TG-0807

August 2007

MAGNETIC PICKOFF OF ROTOR VELOCITY

The angular velocity of the turbine rotor is taken through the turbine meter wall by means of a magnetic pickoff. The stainless steel meter body is non-magnetic and offers negligible effect on a magnetic field set up by a permanent magnet in the pickoff coil.

Turbine blades, made of a paramagnetic material (which properties cause it to be attracted by a magnet), rotate past the pickoff coil, generating irregular shaped voltage pulses. The frequency of these pulses is linearly proportional to the angular velocity of the rotor and thus to the flow rate. Additionally, each pulse is incrementally proportional to a small unit of volume. The amplitude of the pulses will vary in proportion to blade velocity but is not considered in the measurement process. Flow rate and total flow information is transmitted by frequency and by counting (totalizing) the pulses.

The permanent magnet produces a magnetic field which passes through the coil and is concentrated to a small point at the pickoffs. In Figures 5 and 6 below, as a turbine blade (A) moves into close proximity to the pickoff point, its magnetic properties cause the magnetic field to deflect to accommodate its presence. This deflection causes a voltage to be generated in the coil. As the blade passes under the pickoff point (B), this voltage decays, only to build back in the opposite polarity as the leaving blade - now in position (C). This causes the magnetic field to deflect in the opposite direction. So as each blade passes the pickoff, it produces a separate and distinct voltage pulse. Since the fluid surrounding each blade represents a discrete unit of volume, each electrical pulse also represents a discrete unit of volume. Turbine meter output is rated in pulses per gallon, pulses per liter, or other standard engineering units.