Siemens SITRANS FC620 Operating Instructions Manual page 23

Measuring principle
The small deformation overlying the fundamental vibration is recorded by means of pick-
offs (S1, S2) attached at suitable measuring tube locations. The resulting phase shift Δ φ
between the output signals of pick-offs S1 and S2 is proportional to the mass flow. The
output signals generated are further processed in a transmitter.
Fig. 4: Phase shift between output signals of S1 and S2 pick-offs
Δφ ~ F
Δ φ
m
t
dm/dt
F
c
Density
Using a driver and an electronic regulator, the measuring tubes are operated in their
measurement resonance frequency ƒ . This resonance frequency is a function of measuring tube
geometry, material properties and the mass of the fluid covibrating in the measuring
tubes. Altering the density and the attendant mass will alter the resonance frequency.
The transmitter measures the resonance frequency and calculates density from it
according to the formula below. Device-dependent constants are determined individually
during calibration.
A
Fig. 5: Resonance frequency of measuring tubes
A
ƒ
1
ƒ
2
ρ =
ρ
ƒ
α, β
y
Δφ
dm
~
C
dt
Phase shift
Dynamic mass
Time
Mass flow
Coriolis force
Measuring tube displacement
Resonance frequency with fluid 1
Resonance frequency with fluid 2
α
+ ß
2
ƒ
Fluid density
Resonance frequency of measuring tubes
Device-dependent constants
Measuring principle and flow meter design
ƒ ƒ
1
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FC6x0
S2
S1
t
2
t
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