Particle Size and Distribution Questions and Answers

Answer: A

Since a real particle has no single diameter, every technique reports the size of a sphere that would behave the same way in that measurement. Two methods can therefore give different answers for the same powder without either being wrong.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the cause. A real particle is not a sphere, so every technique reports the diameter of the sphere that would behave identically in whatever it measures - the same scattering pattern, the same settling velocity, the same displaced volume. A plate-shaped particle therefore has one equivalent diameter by sedimentation and quite another by laser diffraction, and neither is wrong.

Answer: C

Correct answer: False

A single mean conceals everything that matters. Two powders with the same mean can differ entirely in width, and a bimodal powder may have a mean at which almost no particles exist. A useful description gives at least the d10, d50 and d90, and preferably the whole curve - which is why instruments report a distribution rather than a number.

Answer: D

It is the median of the distribution and is quoted with d10 and d90 to describe the spread. Whether the basis is volume, mass or number matters greatly and should always be stated. Reports that omit the basis are effectively uninterpretable.

Answer: A

Subtracting 1 from 25 gives 24, and dividing by the d50 of 5 gives 4.8. Span is a dimensionless measure of how broad a distribution is and allows powders of different average size to be compared. It says nothing about whether the distribution is symmetrical.

Answer: B

Span is the normalised width, so a larger value means a broader distribution. A monosized powder would approach a span of zero, though no real powder comes close to that. Even a carefully classified powder retains some spread.

Answer: C

Correct answer: a few large particles dominate the volume but not the count

Volume goes with the cube of the diameter, so one 10 micrometre particle occupies the volume of a thousand 1 micrometre ones. A powder that is overwhelmingly fine by number can therefore be overwhelmingly coarse by volume, and the same measurement reported on the two bases gives means that differ severalfold. Which basis is right depends on the question: contamination counting wants number, and packing or sintering wants volume.

Answer: D

Volume scales with the cube of the diameter, so a tenfold increase gives a thousandfold volume. This is precisely why number and volume distributions of the same powder look so different. Neither is wrong; they answer two different questions.

Answer: A

Both are true and the reason gives the arithmetic: a single ten-micrometre particle balances a thousand of one micrometre, so a handful of coarse grains shifts the volume median considerably while the count barely moves.

Answer: B

Correct answer: the angular pattern of light scattered by the particles

A particle scatters light through an angle that depends inversely on its size - large particles scatter narrowly forward, small ones over a wide angle. A detector array records the whole pattern and the distribution is computed from it by inverting a scattering model. It is fast, needs little sample and covers a wide range, which is why it has displaced most alternatives.