2
.....Theory of operation
FIGURE 1.2-B: FLOW PROFILES
1.3 Transit Time Accuracy
Noninvasive ultrasonic measurements are subject to a variety of effects that can influence measurement
accuracy. All ultrasonic instruments are velocity measuring devices and only infer volumetric flow from the
operator-entered parameter of pipe inside diameter (ID). When this value is squared to get cross-sectional
area, a 1% error yields a 2% error in volumetric flow. In practice, commercially fabricated pipe seldom has
ID consistency much tighter than 1%, and unless the pipe to be measured has been accurately measured,
this uncertainty is not reducible through instrument calibration.
The more sophisticated transit time flowmeters incorporate flow profile corrections to compensate for the
pipe’s cross-sectional velocity profile with a changing Reynolds number. However, this requires that the
operator knows the inside roughness of the pipe to be measured. The instrument may infer a roughness if
none is entered by the operator, but that is only an estimate based on the characteristics of new pipe.
Pipes can, of course, accumulate deposits which may not only reduce the ID, but affect the roughness as
well. Errors on the order of 2% as a result of this phenomenon are not uncommon.
While other factors may influence instrument accuracy to a lesser extent, the issues described above are
the major elements of pipe dependency upon absolute instrument accuracy. While calibration on a refer-
ence flow loop under known conditions is a useful exercise to determine the accuracy potential of an
instrument, it is not a guarantee of absolute accuracy on different pipes under field conditions.
To Next Page
Loading...
