Hausner Ratio

Pour a powder into a cylinder without tapping it and weigh it: that is the apparent or bulk density. Now tap the cylinder a standard number of times and weigh again: that is the tapped density. The ratio of the two is the Hausner ratio, proposed by Henry Hausner in the International Journal of Powder Metallurgy in 1967, and it remains the cheapest useful test a powder receives.

What it measures is how much air the loose powder was holding, which comes down to inter-particle friction and cohesion. A powder whose particles are large, smooth and spherical rolls into a tight packing on its own and barely tightens when tapped — a ratio near 1.0. A powder that is fine, irregular, satellited, damp or electrostatically charged builds arches and bridges as it pours, holds a great deal of void, and collapses substantially when shaken — a ratio of 1.3 or more. Carr's classification, which the trade uses for both indices, runs roughly: under 1.11 excellent, to 1.18 good, to 1.25 fair, to 1.34 passable, to 1.45 poor, and above that very poor.

The Hausner ratio and the Carr compressibility index are the same measurement. They come out of one cylinder, one balance and one tapping run, and they are related exactly by $CI = 100(1 - 1/HR)$ and $HR = 1/(1 - CI)$. If your certificate quotes one and your specification asks for the other, that is a conversion and not a retest. This site builds both pages because both forms are entrenched — powder metallurgy grew up on Hausner's ratio, pharmaceutical and food powder handling on Carr's percentage, and the additive standards inherited both — but there is no second piece of information hiding in the pair.

Two honest limits, and they matter for powder-bed fusion specifically. First, this is a bulk test. It says nothing about how a powder behaves under a moving blade or roller, and two powders with identical Hausner ratios can spread quite differently. The recoater's problem is harder than the cylinder's: lay a 20 to 60 µm layer uniformly across a whole plate in a second or two, with no streaks, no drag marks and no short-fill at the far edge. That is a more demanding flow question than filling a hopper, and it is why gas-atomised spherical powder sieved to a narrow cut costs what it does, and why some shops keep a spread-and-photograph test alongside the tapping cylinder.

Second, the tapping procedure is standardised for a reason. The number of taps, the drop height, the cylinder diameter and the sample mass all change the result, and a Hausner ratio quoted without its method is not comparable to anything. Record the standard alongside the number.