TECHNICAL DESCRIPTION MULTICAL
®
401
5512-091 GB/02.2008/Rev. J1
37
In principle, flow is obtained by measuring the flow velocity and multiplying it by the diameter of the measuring
pipe:
AFQ
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-known factors. The length with which the
signal travels can be expressed as
VTL
, and can also be stated as:
V
L
T
where:
L is the measuring distance
V is the sound radiation velocity
is the time
21
11
VV
LT
In connection with ultrasonic flow sensors the velocities
1
V og
2
V can be stated as:
FCV
1
and FCV
2
respectively
where:
C is the velocity of sound in water
By using above formula following is obtained:
FCFC
LT
11
that can also be written as:
22
2
)()(
)()(
FC
F
LT
FCFC
FCFC
LT
As FC , it is possible to omit
2
F and write the formula as:
2
2
L
CT
F
To counteract 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.
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