IEEE Optical feedback interferometry (OFI) based flowmetry enables simple, robust, self-aligned and low cost systems to measure the fluid flow velocity with reasonable accuracy. The particle concentration in the fluid causes significant changes in the signal of OFI sensors. While the spectral analysis of the particle induced Doppler shift remains as the most usual approach to determine the flow properties, different processing algorithms have been proposed in order to evaluate the average flow velocity within the measurement volume. In this paper, the validity of the commonly used methods with regards to particle concentrations and flow rates is verified.