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Technical Guide
DAN-LIQ-Turbine Meter-TG-0807
August 2007
Page 5
Surge And Pressure Relief Valves
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 eld 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 ow 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 ow information is transmitted by frequency and
by counting (totalizing) the pulses.
The permanent magnet produces a magnetic eld 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 eld to deect to accommodate its presence. This deection
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
eld to deect in the opposite direction. So as each blade passes the pickoff, it produces a separate and distinct
voltage pulse. Since the uid 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.
MAGNETIC
SENSORS
(PICKOFFS)
CLAMP
O-RING
BLADES
PICKOFF #1 PICKOFF #2
INSULATOR
UNIVERSAL
MOUNTING
BOX
(UMB)
UMB
MOUNTING
BOX PAD
THIS 1/2 PULSE
IS NOT USED
BY READOUTS
A
B
C
A
ONE
PULSE
B
C
ONE
UNIT
VOLUME
MAGNETIC
SENSORS
(PICKOFFS)
CLAMP
O-RING
BLADES
PICKOFF #1 PICKOFF #2
INSULATOR
UNIVERSAL
MOUNTING
BOX
(UMB)
UMB
MOUNTING
BOX PAD
THIS 1/2 PULSE
IS NOT USED
BY READOUTS
A
B
C
A
ONE
PULSE
B
C
ONE
UNIT
VOLUME
Figure 5 - Assembly of Daniel
®
UMB showing
dual pickoff conguration
Figure 6 - Voltage Output, Peak to Peak