Airspeed Corrections for Optical Array Probe Sample Volumes

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Abstract
The Particle Measuring System’s optical array probes have a sample volume that depends upon the diameter of the particle measured. The sample volume also depends upon the velocity of particles that pass through the probe because of the electronic response time of these instruments. This note discusses an algorithm that has been derived to calculate sample volume as a function of size and velocity, and demonstrates the need for such an algorithm by comparison of measurements from several types of optical array probes and a forward-scattering spectrometer probe. These comparisons show that the optical array probes greatly underestimate droplet concentrations of particles less than 100 μm in diameter at typical aircraft research speeds unless sample volumes are adjusted for electronic response time limitations. Abstract The Particle Measuring System’s optical array probes have a sample volume that depends upon the diameter of the particle measured. The sample volume also depends upon the velocity of particles that pass through the probe because of the electronic response time of these instruments. This note discusses an algorithm that has been derived to calculate sample volume as a function of size and velocity, and demonstrates the need for such an algorithm by comparison of measurements from several types of optical array probes and a forward-scattering spectrometer probe. These comparisons show that the optical array probes greatly underestimate droplet concentrations of particles less than 100 μm in diameter at typical aircraft research speeds unless sample volumes are adjusted for electronic response time limitations.

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