Abstrict A method of calibrating an ultrasonic flow meter (10) first determines
a static flow offset (52). Next, a dynamic flow offset (54) and
a flow speed (56) are determined. An adjusted flow speed (58) is
determined by subtracting the static flow offset and one half the
dynamic flow offset from the flow speed to form an adjusted flow
speed.
Claims What is claimed is:
1. A method of calibrating an ultrasonic flow meter, comprising
the steps of:
(a) measuring a flow speed in a forward flow situation under a
flowing condition, to form a forward flow speed;
(b) measuring a flow speed in a reverse flow situation under the
flowing condition, to form a reverse flow speed;
(c) adding the forward flow speed to the reverse flow speed to
form the dynamic flow offset.
2. The method of claim 1 further including the steps of:
(d) subtracting one half the dynamic flow offset from a measured
flow speed to form an adjusted flow rate;
(e) determining a volume flow rate using the adjusted flow rate.
3. The method of claim 1 wherein step (a) comprises the steps
of:
(a1) measuring a plurality of flow speeds;
(a2) averaging the plurality of flow speeds to form the forward
flow speed.
4. The method of claim 1 wherein step (b) comprises the steps
of:
(b1) measuring a plurality of flow speeds;
(b2) averaging the plurality of flow speeds to form the reverse
flow speed.
5. The method of claim 1 wherein step (a) further includes the
step of determining a static flow offset and subtracting the static
flow offset from the flow speed to determine the forward flow speed.
Description FIELD OF THE INVENTION
The present invention relates generally to the field of ultrasonic
flow meters and more particularly to a method of calibrating an
ultrasonic flow meter.
BACKGROUND OF THE INVENTION
Ultrasonic flow meters have many advantages over other methods
of determining flow rates. Ultrasonic flow meters can continuously
measure the flow rate, while other methods generally measure average
flow rates. In addition, ultrasonic flow meters are obstructionless
and work with non-conductive fluids.
Ultrasonic flow meters have a pair of transducers that are placed
on either side of the flow path of a fluid flowing through a conduit.
The transducers are pointed at each other and placed on either side
of the flow path of a fluid flowing through a pipe. The line between
the transducers has a component in the direction of the fluid flow.
The principle used to detect flow rates is that the transit time
of an ultrasonic packet will increase in the upstream and decrease
in the downstream path. The amount by which the transit time changes
is directly proportional to the flow rate. It is well known that
ultrasonic flow meters have fixed offsets. Normally the offset is
measured by having the ultrasonic flow meter measure the flow rate
with no fluid flow through the meter. The meter should read zero
so any reading is an offset that is subtracted from all future readings.
This process is performed manually by adjusting a potentiometer.
Unfortunately, the offset can drift over time, especially in the
case of clamp-on transducers. In addition, this technique only compensates
for static (no-flow) offsets and does not take into account offset
that occur when a fluid is flowing through the meter.
Thus there exists a need for a calibration method that can automatically
detect both static and dynamic offsets in an ultrasonic flow meter
and compensate for both offsets.
SUMMARY OF THE INVENTION
A method of calibrating an ultrasonic flow meter that overcomes
these and other problems first determines a static flow offset.
Next, a dynamic flow offset and a flow speed are determined. An
adjusted flow speed is determined by subtracting the static flow
offset and one half the dynamic flow offset from the flow speed
to form an adjusted flow speed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an ultrasonic flow meter attached
to a conduit;
FIG. 2 is a flow chart of a process of calibrating an ultrasonic
flow meter;
FIG. 3 is a flow chart of a process of determining a static offset
in an ultrasonic flow meter; and
FIG. 4 is a flow chart of a process of determining a dynamic offset
in an ultrasonic flow meter.
DETAILED DESCRIPTION OF THE DRAWINGS
An ultrasonic flow meter 10 attached to a conduit 12 having a fluid
flowing through it is shown in FIG. 1. The ultrasonic flow meter
has a pair of transducers 14 16. The pair of transducers 14 16
are coupled to the decoding electronics 18 that generates and detects
an ultrasonic pulse. A microprocessor 20 controls the decoding electronics
18 and performs all the necessary calculations to calibrate the
ultrasonic flow meter. A series of computer instruction and data
are stored in a memory 22 that is coupled to the microprocessor
20. A display 24 is also coupled to the microprocessor 20.
The process of calibrating an ultrasonic flow meter involves detecting
any fixed offsets in the meter and removing these offsets. FIG.
2 is an embodiment of a process for detecting and eliminating any
fixed offsets from an ultrasonic flow meter. This process can be
stored as computer instruction in the memory 22 and executed by
the microprocessor 20. The process starts, step 50 by determining
the static flow offset at step 52. The dynamic flow offset is determined
at step 54. The dynamic flow offset is determined by measuring the
offset when fluid is flowing through the ultrasonic flow meter.
A flow speed (measured flow speed) is measured at step 56. An adjusted
flow speed is determined by subtracting the static flow offset and
one half the dynamic flow offset from the flow speed at step 58
which ends the process at step 60. In another embodiment the adjusted
flow speed is then used to determine a volume flow rate. The volume
flow rate is the measurement that interests the customer. The adjusted
flow speed is just the line integral of the speed of the fluid at
each point along the line between the two transducers. As a result
this has to be converted to the volume flow rate. A number of different
methods exist to determine the volume flow rate from the adjusted
flow rate. The invention contemplates using any of these different
methods.
FIG. 3 is a flow chart of a process for determining the static
offset. The process starts, step 70 by stopping a flow of a fluid
through the ultrasonic flow meter at step 72. Next, a number of
static offsets are measured at step 74. The static offset is measured
by having the ultrasonic flow meter determine a flow speed in a
no-flow situation. Since the fluid is not flowing, any measured
flow rate is due to the static offset. In one embodiment the ultrasonic
flow meter continuously measures a plurality of static offsets (plurality
of flow speeds) for a static averaging period. The plurality of
static offsets are averaged at step 76 which ends the process at
step 78. By averaging the static offsets, the variable errors (noise)
in the measurement tend to go to zero, since the variable errors
should be just as likely to be positive as negative.
FIG. 4 is a process for measuring a dynamic offset. The process
starts, step 90 by measuring a forward flow speed at step 92 under
a flowing condition. A forward flow speed is measured by making
the transducer 16 (see FIG. 1) a transmit transducer when measuring
the downstream transit time and making the transducer 14 the transmit
transducer when measuring the upstream transit time. Using these
two transit times a flow speed is calculated for a forward flow
situation. Next, the reverse flow speed is measured at step 94.
The reverse flow speed is measured by making the transducer 14 the
transmit transducer when measuring the downstream transit time and
making the transducer 16 the transmit transducer when measuring
the upstream transit time. The dynamic flow offset is then calculated
as the sum of the forward flow speed and the reverse flow speed
at step 96 when ends the process, step 98. When there is no dynamic
flow offset, the sum of the forward flow speed and the reverse flow
speed is zero. This because the reverse flow speed has the same
magnitude but the opposite sign as the forward flow speed. The dynamic
offset is divided by two, because the offset in the forward flow
speed adds to the offset of the reverse flow speed. As in the static
case a plurality of flow speeds can be measured for both the forward
flow situation and the reverse flow situation. Then if the plurality
of flow speeds are averaged the random variations in the measurements
tends toward zero. Since the dynamic offset can be calculated with
the fluid flowing, the dynamic offset can be periodically recalculated
while the ultrasonic flow meter is in use.
Thus there has been described a process for calibrating an ultrasonic
flow meter that compensates for both dynamic and static offsets.
While the invention has been described in conjunction with specific
embodiments thereof, it is evident that many alterations, modifications,
and variations will be apparent to those skilled in the art in light
of the foregoing description. Accordingly, it is intended to embrace
all such alterations, modifications, and variations in the appended
claims. |