The Sing Around technology of
a flow meter
D-Flow uses a refined version of the sing-around method. One
or more pairs of ultrasonic transducers are attached to a meter
body. In the figure, one pair of transducers is shown in a diagonal
type of flow meter with one upstream and one downstream transducer.
A sing-around loop is established by the UFO ASIC by sending
an ultrasound pulse from Transducer 1 towards Transducer 2.
The sound pulse is received and fed back into the UFO ASIC,
which then emits a new sound pulse from Transducer 1. This loop
is maintained for N number of sing-around loops. The UFO ASIC
measures the total time it takes to complete the N sing-around
loops. The time it takes for the sound to travel between the
transducers once in the downstream direction is determined by
dividing the measured total time by N. The time required for
the sound propagation in the upstream direction is determined
in the same manner.
Descriptions of variables
t1
t2
vm
c
L
A
k
V
=
=
=
=
=
=
=
=
Sound propagation time downstream
Sound propagation time upstream
Velocity of media
Speed of sound in the media
Distance between transducers
Cross-sectional area of the meter pipe
Calibration curve
Volume flow
The measured downstream time t1
that represents the time it takes for sound to travel between
Transducer 1 and Transducer 2 can be expressed as: t1
= L / (c + vm)
L is the known distance
between the transducers. The speed of sound c and the fluid
velocity vm are both unknown.
In order to be able to solve for the fluid velocity vm
independently from the speed of sound c
the upstream time t2 is
also measured. Synonymously with t1,
t2 can be expressed as: t2
= L / (c - vm)
By combining the two equations the speed of sound c can be eliminated
and the fluid velocity vm
can be solved as: vm
= k L/2 (1/ t1 - 1/ t2)
By also using the calibration factor k,
compensation is provided for flow profile effects among other
things. The calibration factor is experimentally determined.
From the fluid velocity the volume flow V
can be formed as: V
= A k L/2 (1/ t1 - 1/ t2)
A is the cross-sectional
area of the flow tube.
Most of the companies manufacturing ultrasonic flow meters employ
the transit-time method, which is a less accurate method to
measure the time of the sound propagation. The basic physics
are the same for a transit-time flow meter and a sing-around
flow meter, but the transit-time flow meter performs only a
single sound transmission in each direction. As the sing-around
method uses multiple loops, the sing-around flow meter benefits
from a much better time resolution. This is of great importance
when measuring small flow rates, since the time difference is
then very small between the upstream and downstream measurements.
The extreme time resolution of an ultrasonic flow meter based
on the sing-around method will allow the flow range to be expanded
down to lower flow velocities, while retaining good accuracy.
D-Flow’s UFO ASIC, the core of D-Flow technology, contains
all the major functionality required in an ultrasonic flow meter
based on the sing-around method. The time resolution of the
D-Flow UFO ASIC can be set to just a few picoseconds. The UFO
ASIC is software-adaptable to fit a wide range of flow measurement
applications. The ASIC supports two pairs of ultrasonic transducers.
This means that one ASIC can support two single-path flow meters
or one dual-path flow meter.
