Large Scale Wind Tunnel Humidity Sensor Using Laser Diode Absorption Spectroscopy

Abstract

Over the last several decades, laser diode absorption (LDA) spectroscopy has emerged as a useful technique to non-intrusively interrogate fluid flows in numerous aerospace applications. Combined with recent advances in diode laser technology and fiber optic signal delivery, this technique has become more attractive for operational applications such as measuring humidity in AEDC’s 4T, 16T and 16S wind tunnels. In order to determine if LDA spectroscopy is a capable companion or replacement for the current Chilled Mirror Hygrometer (CMH) system, a spectral model and data analysis codes were developed along with verification, calibration and validation of the LDA based system called the Laser Hygrometer (LH). Using the spectral model, the 1394.7 nm and 1395.0 nm water vapor lines were selected for this specific application based on the expected flow properties. Next, a laboratory verification of the system was performed to ensure the LDA system was operating correctly. Once verified, a calibration of the system was performed in AEDC’s Precision Measurement Equipment Lab using a NIST traceable humidity generator. The system was then tested in AEDC’s 4T wind tunnel in comparison to the existing CMH system which resulted in a measured humidity difference between the two systems of approximately 2%. These results demonstrate that the LH is an attractive candidate to be used as an operational humidity measurement system in AEDC wind tunnels due not only to its acceptable accuracy, but also its simplicity. However, further system testing will be required to prove its long term reliability in an operational environment.