Siemens SIPART DR22 Manual page 31

Manual
Pressure p
Kp/cm
Figure 1-12 Active pressure measuring method, Principle
Physical notes
The active pressure measuring method is based on the law of continuity and Bernoulli's energy
equation.
According to the law of continuity the flow of a flowing liquid in a pipe is the same at all places.
If the cross-section is reduced at one point, the flow speed at this point should increase. Ac-
cording to Bernoulli's energy equation the energy content of a flowing material is made up of the
sum of the kinetic energy (due to the speed) and the potential energy (of the pressure).
An increase in speed therefore causes a reduction in pressure.
This drop in pressure, the so-called "active pressure" Δp is a measure of the flow q.
The following applies:
with c as a factor which depends on the dimensions of the pipe, the shape of the constriction,
the density of the flowing medium and some other influences.
The equation states that the active pressure generated by the constriction is in the same ratio
as the square of the flow.
SIPART DR22 6DR2210
C79000-G7476-C154-03
1.5.2
Temperature t
2

q = c ·
Δp
1.5 Functional description of the structure switches
Analog input signal processing freely connected (S4 = 1)
Active pressure Δp
Measured value
transmission
Δp
t
Calculation
e
q
1 Technical Description
q
Function block rE
29