Calibrating laser Doppler anemometers utilizing an optical chopper.

Area of Science:

• Fluid dynamics and metrology
• Optical measurement techniques

Background:

• Laser Doppler Anemometers (LDAs) are optical instruments used to measure fluid velocity.
• Directly tracing LDA measurements to the International System of Units (SI) is challenging due to complex, proprietary subsystems.
• Existing calibration methods for LDAs are often indirect or lack SI traceability.

Purpose of the Study:

• To develop and validate a method for SI-traceable calibration of complete Laser Doppler Anemometer systems.
• To establish a reliable velocity standard for LDA calibration.
• To assess the accuracy and consistency of LDA measurements across different system configurations.

Main Methods:

• Utilized a complete Laser Doppler Anemometer system calibration approach.
• Employed an optical chopper blade as an accurate, SI-traceable velocity standard.
• Compared calibrations of two LDAs with differing specifications (manufacturer, focal length, sensing volume, orientation) using NIST's wind tunnel.

Main Results:

• Achieved an expanded velocity uncertainty of 0.094% at a 95% confidence level.
• The calibrated LDAs showed excellent agreement, with differences of at most 0.2% across an airspeed range of 0.5–30 m s^-1.
• Demonstrated the effectiveness of the optical chopper blade method for calibrating diverse LDA systems.

Conclusions:

• The optical chopper blade method provides a practical and accurate means for SI-traceable calibration of complete LDA systems.
• This calibration approach ensures high confidence in fluid velocity measurements obtained from LDAs.
• The findings support the use of this method for ensuring metrological traceability in fluid flow measurements.