3 General principles
FLUXUS H721 3.1 Measurement principle
13
UMFLUXUS_H721V1-5EN, 2022-05-15
Transit time measurement
The average of the transit times of the ultrasonic signals in the fluid and the length of the sound paths in the fluid are used
for the calculation of the sound speed. By using the average, the sound speed is independent of the flow velocity of the
fluid.
Calculation of the sound speed
c
γ
=
where
Calculation of the volumetric flow rate
= k
Re
· A · k
a
·
where
The Reynolds number that depends on the kinematic viscosity is used in the calculation of the fluid mechanic calibration
factor. Since the kinematic viscosity of hydrocarbons, unlike that of other fluids, depends not only on the temperature but
also on the density and type of hydrocarbon, it is determined from the sound speed and the temperature.
Calculation of the standard volumetric flow rate
= · VCF
VCF = CTL · CPL =
where
Fig. 3.5: Average of the transit times in the flowing fluid t
γ
c
γ
– sound speed in the fluid
l
γ
– sound path in the fluid
t
γ
– average of the transit times in the fluid
– volumetric flow rate
k
Re
– fluid mechanic calibration factor
A – cross-sectional pipe area
k
a
– acoustic calibration factor
Δt – transit time difference
t
γ
– average of the transit times in the fluid
– standard volumetric flow rate
– volumetric flow rate
ρ
N
– standardized density, calculated from the measured sound speed and temperature
ρ – density, selected from ASTM 1250 as a function of ρ
N
CPL – correction for the effect of pressure on liquid, selected from ASTM 1250 as a function of ρ
N
CTL – correction for the effect of temperature on liquid, calculated from ρ and ρ
N
VCF – volume correction factor
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