Thermo Scientific DCT6088, User Manual

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Product Overview
Transit Time Accuracy
Once the differential transit time is calculated, several additional variables
must be taken into consideration. The overall velocity of the fluid is
comprised of many individual local velocities that vary according to their
distance from the pipe wall. The velocities in the center of the pipe are
higher than the velocities near the pipe wall. The combination of these
individual velocities for a specific type of fluid within a specific pipe yield a
velocity distribution known as the flow profile, which is a function of the
Reynolds number (see Figure 1–2). By properly configuring the flowmeter,
the effects of the flow profile are taken into consideration when calculating
the mean fluid velocity. The flowmeter then multiplies this velocity by the
pipe’s cross-sectional area to obtain volumetric flow.
Figure 1–2. Flow profiles
Transit Time
Accuracy
Non-invasive 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 the operator to know
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 accumulate deposits that
may reduce the ID and 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 reference 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.
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