Laser optical flow measurement
Large heat and water meters are often used continuously for decades after their first calibration and installation without any recalibration. On-site calibration of flow sensors is rather limited until today and often-times lackluster in precision. Therefore in most cases, the flow sensors are demounted and recalibrated on external test benches. The drawbacks of this procedure are an unavoidable interruption of supply and the physical pipe system, with all its implications, which add up to the already high costs of the demounting and recalibration procedure.
Even if the flow sensor is externally recalibrated, the ideal flow conditions in test benches often times differ significantly from the operation conditions of the on-site flow meter installation. These variations range from temperature shifts, different flow conditions or even electromagnetic differences. The resulting measurement error due to stated reasons remains unknown and could generate major impacts – ranging from technical difficulties in control systems to accounting fairness between involved parties. Accurate measurement results allow examining saving potentials in heat grids and more precise calculations of key performance indicators leading to increased energy efficiency.
The patented measurement technology (co-owner ILA R&D GmbH) is based on the optical measurement principle of Laser Doppler Velocimetry (LDV). With this principle and our measurement technology, local flow velocities are measured across one or multiple pipe diameter paths. By integration of this measured cross section velocity profile you can create a referencing volume flow to test the flow meter against.
The necessary optical access to the flowing fluid is realised by hot-tapping without the need to interrupt pipe operations. The safety of this process is reviewed and certified by TÜV Rheinland. During calibration process, measurement values of the flow sensor are logged with the highest possible temporal resolution.
The measurement error is calculated through a comparison between the calculated LDV volume flow and an time averaged volume flow of the test device on various test points. Based on the on-site specific measurement uncertainty budget and calibrated measurement-tools, the calibration results are traceable to national measurement standards of the Physikalisch Technische Bundesanstalt PTB (German National Metrology Institute).