3 General Principles
12
UMFLUXUS_G8V4-6-1EN, 2018-10-10
Sound speed c
Speed of the propagating sound. The sound speed depends on the mechanical properties of the fluid or the pipe material.
In pipe materials and other solid materials, a distinction is made between the longitudinal and transversal sound speed.
For the sound speed of some fluids and materials see annex C.1.
Flow velocity v
Average value of all flow velocities of the fluid over the cross-sectional pipe area.
Acoustic calibration factor k
a
k
a
=
The acoustic calibration factor k
a
is a parameter of the transducer which results from the sound speed c within the trans-
ducer and the angle of incidence (see Fig. 3.2). According to Snell's law of refraction, the angle of propagation in the ad-
joining fluid or pipe material is:
k
a
=
=
=
Fluid mechanics correction factor k
Re
With the fluid mechanics correction factor k
Re
, the measured value of the flow velocity in the area of the sound beam is
converted into the value of the flow velocity across the whole cross-sectional pipe area. In case of a fully developed flow
profile, the fluid mechanics correction factor only depends on the Reynolds number and the roughness of the inner pipe
wall. The fluid mechanics correction factor is recalculated by the transmitter for each new measurement.
Operating volumetric flow rate
= v · A
The volume of the fluid that passes through the pipe per unit time. The operating volumetric flow rate is calculated from the
product of the flow velocity v and the cross-sectional pipe area A.
Standard volumetric flow rate
N
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
=
with
For the setting of the standard pressure p
N
and the standard temperature T
N
see section 16.4.
The compressibility coefficient of the gas K is stored in the data set of the fluid or can be entered by the user (see section
11.2.4).
The operating temperature T and the operating pressure p can be entered as constant values (see section 11.2.5 and
11.2.6).
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
.
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
c
sin
------------
c
sin
------------
c
sin
------------
c
sin
-----------
V
·
V
·
V
·
V
·
V
·
V
·
p
p
N
------
T
N
T
-------
1
K
----
V
·
V
·
m
·
m
·
V
·
V
·
