# Bulk density

As bulk density ρ Sch ( English bulk density or poured density ), colloquially also "bulk density", one describes the density , i. H. the mass per volume , a mixture of a granular solid (" bulk material ") and a continuous fluid that fills the voids between the particles . The fluid can also be air. The individual components must not become detached from one another .

The bulk density is similar, but not identical, to the specific weight , which indicates the weight force per volume.

On the bulk density, a distinction tapped density (English tapped density ) and density (English also bulk density ).

## definition

The bulk density is defined analogously to the density of gases, liquids and solids as the ratio of the mass of the bulk to the bulk volume taken up : ${\ displaystyle \ rho _ {\ mathrm {Sch}}}$${\ displaystyle m}$${\ displaystyle V _ {\ mathrm {Sch}}}$

${\ displaystyle \ rho _ {\ mathrm {Sch}} = {\ frac {m} {V _ {\ mathrm {Sch}}}}}$

The derived SI unit of bulk density is kilograms per cubic meter :

${\ displaystyle \ left [\ rho _ {\ mathrm {Sch}} \ right] = \ mathrm {\ frac {kg} {m ^ {3}}}}$

For a mixture consisting of components : ${\ displaystyle n}$${\ displaystyle \ rho _ {\ mathrm {Sch}}}$

${\ displaystyle \ rho _ {\ mathrm {Sch}} = {\ frac {m_ {1} + m_ {2} + \ ldots + m_ {n}} {V_ {1} + V_ {2} + \ ldots + V_ {n}}}}$

In the case of mixing a solid ( ) with a gas ( ), the bulk density can also be determined by taking the porosity into account : ${\ displaystyle m _ {\ mathrm {s}}, V _ {\ mathrm {s}}, \ rho _ {\ mathrm {s}}}$${\ displaystyle m _ {\ mathrm {g}}, V _ {\ mathrm {g}}, \ rho _ {\ mathrm {g}}}$ ${\ displaystyle \ varepsilon}$

${\ displaystyle \ varepsilon = {\ frac {V _ {\ mathrm {g}}} {V _ {\ mathrm {s}} + V _ {\ mathrm {g}}}}}$
${\ displaystyle \ Rightarrow \ rho _ {\ mathrm {Sch}} = {\ frac {m _ {\ mathrm {ges}}} {V _ {\ mathrm {ges}}}} = {\ frac {m _ {\ mathrm { s}} + m _ {\ mathrm {g}}} {V _ {\ mathrm {s}} + V _ {\ mathrm {g}}}} = (1- \ varepsilon) \ cdot \ rho _ {\ mathrm {s }} + \ varepsilon \ cdot \ rho _ {\ mathrm {g}}}$

If the porosity is less than 0.98 and the gas is present at moderate pressures ( ), the summand can be neglected: ${\ displaystyle \ varepsilon}$${\ displaystyle \ rho _ {\ mathrm {g}} \ ll \ rho _ {\ mathrm {s}}}$${\ displaystyle \ varepsilon \ cdot \ rho _ {\ mathrm {g}}}$

${\ displaystyle \ varepsilon \ cdot \ rho _ {\ mathrm {g}} \ ll (1- \ varepsilon) \ cdot \ rho _ {\ mathrm {s}} \ quad \ Leftrightarrow \ quad m _ {\ mathrm {s} } \ gg m _ {\ mathrm {g}}}$
${\ displaystyle \ Rightarrow \ rho _ {\ mathrm {Sch}} \ approx (1- \ varepsilon) \ cdot \ rho _ {\ mathrm {s}} = {\ frac {m _ {\ mathrm {s}}} { V _ {\ mathrm {s}} + V _ {\ mathrm {g}}}}}$

## Grain density

In DIN EN ISO 17892-3 (formerly DIN  18124) the grain density is defined as a soil mechanical parameter . It is determined by drying the substance at 105 ° C., weighing it and then measuring the displacement volume in a measuring liquid; the substance must be insoluble in the measuring liquid. The grain density is - as shown above - the ratio of dry matter to displacement volume: ${\ displaystyle \ rho _ {s}}$ ${\ displaystyle V_ {k}}$ ${\ displaystyle m_ {d}}$

${\ displaystyle \ rho _ {s} = {\ frac {m_ {d}} {V_ {k}}}}$

and, like the true density, is given in kg / m³. As a test device z. B. used a capillary pycnometer.

## Hectolitre mass

typical hectolitre masses of grain
(100 kg / hl = 1000 kg / m³ = 1 t / m³)
Type Grain Hectolitre mass
Heavy grain Bread wheat 72-82 kg / hl
rye 70-75 kg / hl
Feed wheat 65-71 kg / hl
Light grain barley 60-65 kg / hl
oats 45-50 kg / hl
Cereal product wheat flour 50-55 kg / hl
Rye meal approx. 50 kg / hl
Grain sampler
Bars for barley

In warehouses and mills for grain as well as in all food processing plants in which grain, grain products or flour-like products are processed, the terms hectolitre mass (formerly hectolitre weight ) or natural weight (physically correct would be natural mass ) are used instead of the bulk density , in each case in the unit of measurement kg / hl .

The hectolitre mass of grain is determined with grain samples that have a volume of ¼ l, 1 l or 20 l. These are filled and the mass is determined using a scale.

The values ​​of the 20 l probes are used as a reference , they only have to be extrapolated with a factor of 5 to 1 hl = 100 l:

${\ displaystyle {\ frac {m} {20 \; {\ text {l}}}} = {\ frac {5 \, m} {100 \; {\ text {l}}}} = {\ frac { 5 \, m} {\ text {hl}}} \ left (= {\ frac {50 \, m} {\ mathrm {m ^ {3}}}} \ right)}$

However, if the bulk density is determined with a 1/4 l or a 1 l prober, the values ​​must then be corrected using "official tables", since the bulk density also depends on the volume of the prober (!):

${\ displaystyle \ mathrm {\ rho _ {hl}} = f (V) \ neq {\ text {const.}}}$
${\ displaystyle \ Rightarrow {\ frac {m} {1 \; {\ text {l}}}} \ neq {\ frac {100 \, m} {100 \; {\ text {l}}}} = { \ frac {100 \, m} {\ text {hl}}}}$

The hectolitre mass is determined in order to know the need for storage space in the silo , if z. B. Grain batches are to be stored. A reliable statement about the grain quality based on the hectolitre mass can not be made, since the hectolitre mass depends on many different factors (grain shape, moisture , pollution , etc.). In terms of the tendency, a higher hectolitre mass promises better quality.

Since grain is a natural product , the values ​​can fluctuate more than indicated above depending on the harvesting conditions .