TECHNICAL DESCRIPTION
MULTICAL
®
41
5512-313 GB/01.2010/Rev. C1
24
In principle flow is obtained by measuring the flow velocity and multiplying it by the area of the measuring pipe:
A
F
Q
×
=
where:
Q
is the flow
F
is the flow velocity
A
is the area of the measuring pipe
The area and the length that the signal travels in the sensor are well-know factors. The length with which the
signal travels can be expressed as
V
T
L
×
=
,
and can also be stated as:
V
L
T
=
where:
L
is the measuring distance
V
is the sound radiation velocity
T
is the time
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
×
=
Δ
2
1
1
1
V
V
L
T
In connection with ultrasonic flow sensors the velocities
1
V
and
2
V
can be stated as:
F
C
V
−
=
1
and
F
C
V
+
=
2
respectively
where:
C
is the velocity of sound in water
By using above formula following is obtained:
F
C
F
C
L
T
+
−
−
×
=
Δ
1
1
that can also be written as:
2
2
2
)
(
)
(
)
(
)
(
F
C
F
L
T
F
C
F
C
F
C
F
C
L
T
−
×
=
Δ
⇓
+
⋅
−
−
−
+
×
=
Δ
As
F
C
〉〉
it is possible to omit
2
F
and write the formula as:
2
2
×
×
Δ
=
L
C
T
F
To minimise the influence from variations in the velocity of sound in the water, this is measured. Velocity of
sound in water is measured by means of the built-in ASIC. For this purpose a number of absolute time
measurements are made between the two transducers. These absolute time measurements are then converted
into the current velocity of sound used in connection with the flow calculations.