Figure 2: LDV measuring principle and automated profile scanning
Reliable reference measurements under fluctuating conditions
measurements per profile (Figure 2). During this velocity scan, the meter-under-test values are logged at the highest possible temporal resolution. The flow meter measurement error is calculated by comparing the calculated LDV volume flow with the time-averaged volume flow of the meter. Instead of relying on a single measuring point, the OSC method captures the complete velocity distribution across the pipe cross-section. The resulting profile is used to calculate the reference volume flow while simultaneously revealing asymmetry and other hydraulic characteristics. Figure 3 shows typical measured profiles and the results graph from a calibration. The issued calibration certificate also includes information about the meter under test and the measurement conditions. Probe positioning, data acquisition and measurement sequences are fully automated after parameterisation. An optical revolver enables automatic adjustment of the measuring optics for different operating conditions, while remote monitoring allows long measurement campaigns to continue overnight without permanent operator attendance. Calibrations are typically performed at several operating points to assess meter performance across its working range. Because LDV measures velocity directly, it is independent of the measuring principle of the installed flow meter and can therefore be applied to electromagnetic, ultrasonic, orifice flow meters and other meter types.
Unlike laboratory calibrations, field measurements are performed under continuously changing operating conditions. Flow rate fluctuations caused by consumer demand or pump operation are compensated by normalising the LDV measurements with data from an independently installed ultrasonic clamp-on flow meter. This approach keeps the reference measurement independent of the flow meter under test [1]. It relies only on the proven linearity of the clamp-on instrument, which is specifically qualified for this task. Accredited measurement quality Accurate field calibration requires not only precise instrumentation but also a transparent uncertainty evaluation. The OSC method is accredited according to EN ISO/IEC 17025 [2]. To achieve the credibility and traceability required for accreditation, a wide range of uncertainties from multiple sources (see Figure 4) needed to be quantified and put together in an extensive uncertainty budget. The dominant uncertainty contribution originates from the velocity profile at the measuring location. To quantify this influence, CFD simulations were combined with high-precision LDV measurements in a publicly funded research project [3]. Mathematical analysis of the profile allows every measurement to be assigned to one of several characteristic flow classes, each associated with a realistic measurement uncertainty [4].
16 HOTCOOL no.5 2026
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