Flexim FLUXUS G801 - Page 12

3 General principles
3.1 Measurement principle FLUXUS G801
2020-06-26, UMFLUXUS_G801V1-0EN
12
Standard volumetric flow rate
N
(or
S
)
Volumetric flow rate of a gas under specified standards conditions. During the measurement of a gas, the temperature and
the pressure have a strong influence on the measured operating volumetric flow rate. The measured operating volumetric
flow rate can be converted into the standard volumetric flow rate
N
by the transmitter:
N
=
where
The values for standard pressure p
N
and standard temperature T
N
can be entered. The compressibility coefficient of the
gas K is stored in the data set of the fluid or can be entered by the user. The operating temperature T and the operating
pressure p can be fed into the transmitter via the inputs or entered as constant values.
Mass flow rate
= · ρ
The mass of the fluid that passes through the pipe per unit time. The mass flow rate is calculated from the product of the
volumetric flow rate and the density ρ.
3.1.2 Measurement of the flow velocity
The signals are emitted and received by two transducers alternatively in and against the flow direction. If the fluid is
flowing, the signals propagating in the fluid are displaced with the flow.
Caused by this displacement, the sound path of the signal is reduced in flow direction and increased in the opposite
direction.
This causes a change in the transit times. The transit time of the signal in flow direction is shorter than the transit time
against the flow direction. The transit time difference is proportional to the average flow velocity.
The average flow velocity of the fluid is calculated as follows:
v = k
Re
· k
a
·
where
N
– standard volumetric flow rate
– operating volumetric flow rate
p
N
– standard pressure (absolute value)
p – operating pressure (absolute value)
T
N
– standard temperature in K
T – operating temperature in K
K – compressibility coefficient of the gas: ratio of the compressibility factors of the gas at operating conditions
and at standard conditions (Z/Z
N
)
v – average flow velocity of the fluid
k
Re
– fluid mechanics calibration factor
k
a
– acoustic calibration factor
Δt – transit time difference
t
γ
– transit time in the fluid
V
·
V
·
V
·
V
·
V
·
p
p
N
-----
T
N
T
-----
1
K
---
V
·
V
·
m
·
m
·
V
·
V
·
t
2 t


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