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Laser optical volume flow standard

The in-situ calibration of volume flow rate measuring devices allows an increased efficiency of thermal power plants

PTB-News 3.2016
Especially interesting for

power plant operators

manufacturers of volume flow rate measuring devices

The decisive factor for increasing the energy efficiency of thermal power plants is to reduce the measurement uncertainty of the hot water volume flow rate measurement. The current uncertainty of approx. 2 % is not sufficient to optimize the control of power plants, which limits their efficiency. For this reason, PTB has developed a laser optical volume flow measurement standard (LVN) which allows the calibration of measuring instruments with an uncertainty of 0.15 %.

In the case of the LDA method, two laser beams are made to overlap. At the point where the beams cross, the measuring volume forms which allows fluid velocities to be measured. The velocity profile inside the pipe is measured on a measuring grid (yellow in the top picture) through a window; by integration, this provides the volume flow rate.

At 2 %, the current uncertainty of volume flow rate measurements in power plants is too high. This is, in part, due to the fact that there is no test facility in the world which enables the calibration of volume flow measuring instruments under conditions that are similar to those encountered in power plants (i.e. at water temperatures of 400 °C and pressures of 300 bar). On the other hand, internal fittings such as valves or bends have an influence on the velocity profile inside a pipe, and thus on the measurement. For this reason, PTB has developed a compact laser optical volume flow rate measurement standard (LFS) which allows measuring instruments to be calibrated on site (i.e. while they are mounted and in operation) with an uncertainty of only 0.15 %.

This procedure is based on laser Doppler anemometry (LDA) which is, itself, based on the scattering of light on small water impurities. Hereby, two laser beams are made to overlap at a certain angle. At the point where the two beams cross, which is the measuring volume, an interference fringe pattern forms. An impurity particle moving through the measuring volume with the flow generates a scattered light signal whose frequency is proportional to the particle velocity. The fluid velocity is measured by means of LDA at several positions which are distributed across the section of the pipe. From this data, the velocity profile is reconstructed and integrated in order to calculate the volume flow rate.

The main challenge in developing the LFS was to considerably reduce the current measurement uncertainty of 4.5 % of the LDA volume flow measurement technique. The highest uncertainty contribution hereby came from the local resolution of the measurement procedure, which corresponds to a measuring volume of approx. 2000 μm in length. Due to the extended measurement procedure, the local resolution has already been improved to reach 6 μm. For this purpose, two measuring volumes with variable interference fringe intervals are superimposed, which allows the position at which the particles cross the measuring volume to be determined more accurately. Superimposing the two measuring volumes places high requirements on the positioning of the laser beams. At each measurement point within the pipe cross section, four laser beams with a diameter of 150 μm each must be made to overlap. Measurement procedures have therefore been developed which allow the position of the laser producbeams to be determined for the first time with high metrological accuracy.

This method provides a measurement uncertainty of 0.15 %, i.e. improved by more than a factor of 10. A comparison measurement carried out with the heat meter test section – a gravimetric standard measurement facility used to realize the volume flow up to 90 °C with an uncertainty of 0.04 % – showed excellent agreement.


Markus Juling
Department 7.5
Heat and Vacuum
Phone: +49 (0)30 3481-7815

Scientific publication

M. Juling: Rückgeführte Volumenstrommessung mittels ortsaufgelöster Laser-Doppler-Anemometrie. Dissertation, TU Berlin (2016), doi:10.14279/depositonce-5170