Accusonic 8510+ series Reference Manual page 73

A-6 Multiple-Path Transit-Time Flowmeters Principle of Operation
The above analysis assumes that there is no cross flow in the pipe (due to upstream disturbances such as
elbows). This assumption is good for applications where there are at least 10 diameters of upstream straight
pipe. For less available straight pipe, cross paths may maintain accuracy - see Operating Principle.
For cross path installations, the above error is reduced to the theodolite resolution.
Travel Time Measurement
Travel time measurement is dependent on the digital oscillator accuracy, oscillator frequency, and the time
delays in the transducers, cable and system. A precision oscillator, accurate to within ±0.005%, is used as
the conversion trigger to translate the analog received signal into digital RAM for processing by the DSP
firmware. DSP techniques are then used to detect the first negative edge of the received acoustic pulse and
determine the actual travel time. The flow measurement uncertainty from all timing errors is calculated to
be:
E = 0.0001 or .01%
T
Radius Measurement
Radius measurement is typically done from the inside with the pipe dewatered. The radius is measured at
several sections to account for normal pipe out-of-roundness and give an average radius through the meter
section. When done according to Accusonic procedures, the radius measurement can be completed to
within ±0.2% (e.g., for a 6-foot (1.8m)-diameter pipe, the radius is measured to within 1/16 in (1.5mm) or
for a 10-foot (3m)-diameter pipe, the radius is measured to within 1/8 inch (3mm). The flow measurement
uncertainty due to radius measurement error is:
2
E = (1 - (1/1.002) ) = 0.004 or 0.4%
R
Velocity profile uncertainty is estimated by numerical analysis of the ability of a 4-path chordal integration
to fit simulated velocity profiles. Discharge uncertainty including profile integration error is determined to
be less than:
E = 0.002 or 0.2%
I
Therefore, the total flow measurement uncertainty for a 4-path flowmeter, installed according to
specifications is:
2 2 2 2 2 1/2
E = (E + E + E + E + E ) = 0.0049 @ 0.005 or 0.5%
Q L q T R i
For other situations such as open channel systems, 2-path systems, compound meters, etc., the accuracy
would be determined through an error analysis similar to the above with the additional sources of error
considered. For example, for an open channel system, there would be additional uncertainties due to level
measurement and surface velocity determination.
Typical system uncertainties for various meter applications are presented in the following table:
Typical System Uncertainties