PDA Limitations

Measuring denser dispersed flows the solid- and fluid particles may block the required transparent light paths and the light may also be reflected on fluid particle surfaces which prevent valid signals. The application of the PDA technique is thus limited to dilute dispersions even with forward scattering LDA. 

For gas-solid suspensions, as occurring in the CFB risers, the PDA technique is limited to a mean solid volume fraction of the order of 1-2 %. For transparent fluid particles the PDA technique is limited to a dispersed phase fraction of the same order. 

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Photodiode arrays (PDAs) A spectrally segmented PDA spectrophotometer has also been developed. Radiation from the plasma is predispersed and transmitted through an optical mask prior to dispersion by an echelle grating onto a PDA. Limits of detection are similar to those obtained by conventional detection systems, except at low wavelengths where degradation has been observed. 

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