Measurement uncertainty is an increasingly important issue in relation to scientific papers, contract measurements and quality control.
At many scientific conferences, uncertainty analysis of the presented results is demanded. This requires an uncertainty analysis of all elements in the measurement chain.
It is also common for contract measurements that the uncertainty of the measurement results is documented and traceable.
For measurement equipment used for calibration purposes, traceable uncertainty is a must, and in many cases a legal obligation. Low measurement uncertainty is a competitive parameter for companies offering calibration services.
In the fluid dynamics community, LDA is considered an absolute flow measurement method. Knowing the laser wavelength and the intersection angle between the laser beams, the calibration constant or fringe spacing can be easily calculated. It might thus seem unnecessary to calibrate an LDA system.
In the real world, tolerances on the lasers, optical fibres, lenses and alignment introduce small deviations from the nominal fringe spacing. This leads to systematic errors on the measured velocities, which can be corrected for by calibration of the fringe spacing.
All components in the calibration rig have been certified by an independent accredited company, and are therefore traceable to international standards. The calibration is done by measuring a known reference velocity with the LDA system. The fringe spacing is found by comparing the measured Doppler frequency with the reference velocity.
ISO 9000 certification
There is a growing demand for quality certification of laboratories. An internal reason for this could be a strategic decision to raise the quality of the services offered by the laboratory. The laboratory's customers may demand ISO 9000 certification from it.
In a laboratory using LDA for contract measurements, an important part of the certification procedure is calibration of the LDA system, which is necessary in order to assess the measurement uncertainty. A calibrated LDA probe from Dantec Dynamics makes it easier to obtain the certification.
Velocity reference in wind tunnels
LDA is used in wind tunnels applied for calibration of e.g. vane, cup and thermal anemometers. Documented calibration uncertainty is an important competitive parameter for companies offering such services.
A permanently installed LDA system can measure the reference velocity against which the anemometers are calibrated. An LDA system can also be used to measure the free stream velocity at different pressure drops, and thus calibrate the pressure drop measurement across the contraction of the wind tunnel.
Commissioning of wind tunnels
A standard issue in connection with commissioning of wind tunnels is to measure the velocity and turbulence profiles across the test section.
A calibrated LDA system is the ideal tool for this due to its non-intrusive nature, low uncertainty and high velocity resolution. If the flow rig is to be used for LDA measurements after commissioning, the uniformity of seeding in the test section can also be investigated in the commissioning phase.
Commissioning of water channels
Commissioning of water channels used for cavitation studies includes measurement of the velocity and turbulence profiles in the test section. An LDA system is well suited for this.
Some channels include bubble generators, and full control of the bubble size and spatial distribution is essential to understand cavitation phenomena. An LDA system extended to PDA can offer measurements of the water velocity distribution as well as the bubble size, velocity and spatial distribution.
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