Pipes And Conduits That Range From Partially Full To Surcharged; System Accuracy And Measurement Uncertainty - Accusonic 8510+ series Reference Manual

Pipes and Conduits That Range From Partially Full to Surcharged

For compound applications, Accusonic meters are designed to automatically change mode of operation
from open channel to full pipe as the conduit surcharges. The method of flow calculation used is based on
depth, number of paths submerged, and path locations. From the above, it can be seen that to calibrate an
acoustic flowmeter, all that is required is to measure the distance between the transducers, the angle of the
transducers with respect to the centerline of the pipe or channel, and the physical dimensions of the pipe or
channel. The multiple chordal path acoustic method is an absolute flow measurement method that does not
require calibration by comparison to another flow measurement method.

System Accuracy and Measurement Uncertainty

For pipeline flow measurement using a 4-path flowmeter, the be ± 0.5 percent of actual flow. This is for all
flows with velocity above is installed according to Accusonic specifications in a section of pipe with a
minimum of ten diameters of upstream straight pipe. For installations having between four and ten
diameters of straight pipe upstream of the meter section, four crossed paths (eight paths total) are required
to maintain an accuracy of ± 0.5 percent of flowrate.
To assure the specified accuracy, the flowmeter integrates the four velocities for each measurement plane
(one for four path, two for four crossed paths) to calculate flowrate. Where crossed paths are used, the
flowmeter software is designed to utilize velocity information from each plane of transducers to quantify
and correct for crossflow. System accuracy is determined by assigning an expected error to each
component of flow measurement and then defining the total system uncertainty (accuracy) as the square
root of the sum of the squared values of the individual errors. See the sections below for additional details
on the following sources of error for pipeline flow measurement:
• Path Length Measurement
• Path Angle Measurement
• Travel Time Measurement
• Radius Measurement (or area for non-circular conduits)
• Velocity Profile Integration Error
Path Length Measurement
Path length measurement is typically done with the pipe dewatered. Using steel tape measures in larger
pipes and calipers or micrometers in smaller pipes, individual path length uncertainty is less than 0.15%
(e.g., a 1/16-in (1.5mm) error in a 4-ft (1.2m) path length would result in a 0.13% error in velocity
calculation). However, since there are 4 paths and the error is random, overall flow measurement
uncertainty due to path length measurement error would be:
Path Angle Measurement
Path angle measurement is typically done with the pipe dewatered using a theodolite. The theodolite is
capable of measuring angles to within ±20"; however, the primary source of error is the ability to set the
theodolite up on the pipe centerline. Careful set-up, according to Accusonic procedures, will assure that the
theodolite is within ±0.1
As for paths nominally at 45
would be:
E =1/4 (4 x 0.0015
L
o
(±6') of the true centerline.
o
, the flow measurement uncertainty due to path angle measurement error
E = (1-(cos 45.10/cos 45.00)) = 0.0017 or 0.17%
Ø
2 1/2
) = 0.00075 or 0.075%
A-5