http://www.who.int/medicines/publications/pharmacopoeia/Bulk-tapped-densityQAS11_450FINAL_MODIFIEDMarch2012.pdf
http://www.usp.org/sites/default/files/usp_pdf/EN/USPNF/revisions/m99375-bulk_density_and_tapped_density_of_powders.pdf
https://www.ecn.nl/docs/society/horizontal/CEN-Determination%20of%20bulk%20density,%20Version%20240805.pdf

True density is constant value for a matter. True density is the density of the solid material. As a geologist, calcite mineral has true density of 2.72 gr/cm3. 
Apparent density is similar to the true density except the volume of closed pores is also included so it is less than true density. for example a crabonate rock which is composited of pure calcite, show apparent density less than 2.72 gr/cm3
Bulk Density: Bulk density is a characteristic of a volume of divided material such as powders, grains, and granules. It includes the volume of the solid material, open and closed pores, and the interparticle void. so its less than Apparent density for same sample.Bulk and true are different. Bulk including the pores within particles whereas true excludes the same. Hence bulk density is smaller than true onehttp : / / www.who.int/medicines/publications/pharmacopoeia/Bulk-tapped-densityQAS11_450FINAL_MODIFIEDMarch2012.pdfhttp://www.usp.org/sites/default/files/usp_pdf/EN/USPNF/revisions/m99375-bulk_density_and_tapped_density_of_powders.pdfhttps://www.ecn.nl/docs/society/horizontal/CEN-Determination%20of%20bulk%20density,%20Version%20240805.pdfTrue density is constant value for a matter. True density is the density of the solid material. As a geologist, calcite mineral has true density of 2.72 gr / cm3. Apparent density is similar to the true density except the volume of closed pores is also included so it is less than true density. for example a crabonate rock which is composited of pure calcite, show apparent density less than 2.72 gr / cm3Bulk Density : Bulk density is a characteristic of a volume of divided material such as powders, grains, and granules. It includes the volume of the solid material, open and closed pores, and the interparticle void. so its less than Apparent density for same sample .

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Envelope Density: Like bulk density, envelope density is determined from the volume of the solid material, open pores, and closed pores. Envelope density is determined for a single, consolidate quantity of material, therefore there are no interparticle voids between packed particles.

The following discussion addresses both bulk density and apparent density: https://www.tienhieptruyenky.com/post/what_is_the_difference_between_Bulk_density_and_Apparent_density
I have discussed the determination of bulk density by the Archimedes method for porous samples elsewhere at this forum: https://www.tienhieptruyenky.com/post/How_do_i_calculate_for_percentage_porosity_in_a_ceramic_material/1
The bulk density of a powder is the ratio of the mass of an untapped powder sample and its volume including the contribution of the inter particulate void volume. Hence, the bulk density depends on both the density of powder particles and the spatial arrangement of particles in the powder bed. The bulk density is expressed in grams per millilitre (g/ml) although the international unit is kilogram per cubic metre (1 g/ml = 1000 kg/m3 ) because the measurements are made using cylinders.

Envelope Density: Like bulk density, envelope density is determined from the volume of the solid material, open pores, and closed pores. Envelope density is determined for a single, consolidate quantity of material, therefore there are no interparticle voids between packed particles.The following discussion addresses both bulk density and apparent density: https://www.tienhieptruyenky.com/post/what_is_the_difference_between_Bulk_density_and_Apparent_densityI have discussed the determination of bulk density by the Archimedes method for porous samples elsewhere at this forum: https://www.tienhieptruyenky.com/post/How_do_i_calculate_for_percentage_porosity_in_a_ceramic_material/1The bulk density of a powder is the ratio of the mass of an untapped powder sample and its volume including the contribution of the inter particulate void volume. Hence, the bulk density depends on both the density of powder particles and the spatial arrangement of particles in the powder bed. The bulk density is expressed in grams per millilitre (g/ml) although the international unit is kilogram per cubic metre (1 g/ml = 1000 kg/m3 ) because the measurements are made using cylinders.

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The bulk density of a powder is determined by measuring the volume of a known mass of powder sample, that may have been passed through a sieve, into a graduated cylinder (Method A), or by measuring the mass of a known volume of powder that has been passed through a volumeter into a cup (Method B) or a measuring vessel (Method C).
volume determined by the use of graduated cylinder, the cylinder can be tapped giving a measure of the so called Tapped Density.
The tapped density is an increased bulk density attained after mechanically tapping a container containing the powder sample. The tapped density is obtained by mechanically tapping a graduated measuring cylinder or vessel containing the powder sample. After observing the initial powder volume or mass, the measuring cylinder or vessel is mechanically tapped, and volume or mass readings are taken until little further volume or mass change is observed. The mechanical tapping is achieved by raising the cylinder or vessel and allowing it to drop, under its own mass, a specified distance by either of three methods as described below. Devices that rotate the cylinder or vessel during tapping may be preferred to minimize any possible separation of the mass during tapping down.
True density: The mass of a particle divided by its volume, excluding open and closed pores. as true/skeletal volume for true density measurement- includes volume of solid phase only and any closed pores within that are not open to the outside surface. For regular shapes this is easy to measure using an accurate balance and a ruler, for irregular or fractured shapes we employ the Archimedean principle of fluid/gas displacement to measure the volume. The gas pycnometry operating principle is Gas Law: PV = nRT Where P is the gas pressure, V is the volume containing the gas, n is the number of moles of the gas, R is the universal gas constant and T is the temperature of the gas. Introducing a sample into an analysis chamber of a known volume we determine the volume of a gas displaced by the sample and true volume of the analysed material. Mo
The bulk density of a powder is determined by measuring the volume of a known mass of powder sample, that may have been passed through a sieve, into a graduated cylinder ( Method A ), or by measuring the mass of a known volume of powder that has been passed through a volumeter into a cup ( Method B ) or a measuring vessel ( Method C ). volume determined by the use of graduated cylinder, the cylinder can be tapped giving a measure of the so called Tapped Density. The tapped density is an increased bulk density attained after mechanically tapping a container containing the powder sample. The tapped density is obtained by mechanically tapping a graduated measuring cylinder or vessel containing the powder sample. After observing the initial powder volume or mass, the measuring cylinder or vessel is mechanically tapped, and volume or mass readings are taken until little further volume or mass change is observed. The mechanical tapping is achieved by raising the cylinder or vessel and allowing it to drop, under its own mass, a specified distance by either of three methods as described below. Devices that rotate the cylinder or vessel during tapping may be preferred to minimize any possible separation of the mass during tapping down. The mass of a particle divided by its volume, excluding open and closed pores. as true / skeletal volume for true density measurement – includes volume of solid phase only and any closed pores within that are not open to the outside surface. For regular shapes this is easy to measure using an accurate balance and a ruler, for irregular or fractured shapes we employ the Archimedean principle of fluid / gas displacement to measure the volume. The gas pycnometry operating principle is Gas Law : PV = nRT Where P is the gas pressure, V is the volume containing the gas, n is the number of moles of the gas, R is the universal gas constant and T is the temperature of the gas. Introducing a sample into an analysis chamber of a known volume we determine the volume of a gas displaced by the sample and true volume of the analysed material. Mo

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