Figure 3: Measured profiles (profile class “slightly disturbed”) and results diagram (profile class “fully developed”)
The OSC method is also often used as a recalibration tool for flow sensors which were subsequently added as instrumentation to the grid lines. Because those flow sensors are often ultrasonic clamp-on or other comparatively easy-to- install flow meters, they often lack the needed measurement precision. An additional calibration after mounting can reduce the measurement uncertainty of these flow sensors by several percentage points. Of the 241 flow sensors calibrated to date, 45 showed measurement deviations of more than 4%, after accounting for the inherent uncertainty of the OSC method. This means approximately 19% of all inspected flow sensors show
Figure 5 shows the achievable total measurement uncertainties for each profile class. The minimum uncertainty for the “fully developed” profile class consists only of non-profile-related contributions. Practical experience More than 240 large flow meters have been calibrated in Germany, Austria, Switzerland and Australia on pipelines ranging from DN 100 to DN 1200 (Figure 6). The operating conditions of the measurements ranged from 5°C to 150°C in fluid temperature, fluid pressures ranging from 2 bar to 18 bar, and pipe sizes between DN 100 and DN 1200.
Figure 4: Main contributors to the uncertainty budget.
Figure 5: Profile classes and associated realistic expanded uncertainties (k=2).
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