Sensitivity and Range of PDA

2 Particle Sphericity. The two-detector-pair arrangement has another useful feature, and that is to give information with regard to the curvature over a certain arc of the particle surface. If the curvature measured at two different locations on the surface (phase difference) is identical, the particle is said to be spherical. If the two local curvatures differ, d i2 and I i3 will point at diameter values differing by AD. Consequently, a measure of the deviation from sphericity is available, and if AD exceeds a certain limit set by the user, the particle is said to be invalid. The underlying equations of size determination using the PDA technique assume that the particle is spherical, and hence any deviation from this assumption will introduce errors in the absolute determination of the particle size. 

Ideally, the sum of the phase differences, d i2, d 23. and 4 3i, is zero. However, due to the uncertainty of the phase measurements, this measured sum will deviate from zero. The absolute value of this deviation is the closed-loop phase error, which must not be greater than a certain value (to be set by the user in the software) for the particle to be accepted. This value should be typically set in the range 10 to 15°. 

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