What is the effect of the sample flow velocity on the measurement accuracy?
An incorrect sample flow velocity will affect the particle counting and sizing results. If the flow velocity is too low, particles may remain in the measurement zone, leading to repeated identification of the same particle, and hence a counting error will ensue. At high flow velocity, there will be issues with the frequency response of the detector. When the particle leaves the illuminated area, the corresponding response signal has not reached its maximum value due to the short response time within the cell. In this situation, the pulse signal used for the particle size calculation cannot reflect the real particle size because of the low signal height. The flow velocity limit of an optical particle counter can be determined when the measurement error between the results of particle counting at normal velocity and at varied velocity equals ±5%. The larger the error is allowed, the better the performance of the instrument will be.