All Direct Pressure Blast Machines are filled by "volume", not "pounds" of abrasive.
The weight of each type of abrasive is different per cubic foot of "volume".
The weight is called the "bulk density" of that abrasive.
It just so happens that the most common of all Blast Cleaning Abrasives, Silica Sand, weighs 100 pounds per Cubic Foot, and a 6.5 Cubic Foot Machine could "Theoretically" hold 650 Pounds of Sand, if it could be totally filled. The Chart below shows the "Bulk Density" (Weight per Cubic Foot) of several common Blast Cleaning Abrasives.
ABRASIVE TYPE
BULK DENSITY
Silica Sand
100
Mineral Sands
127
Flint
80
Garnet
147
Coal Slag
85
Copper Slag
112
Nickel Slag
85
Sodium Bicarbonate
61
ABRASIVE TYPE
BULK DENSITY
Nut Shells
45
Corn Cobs
35 to 42
Aluminum Oxide
120
Silicon Carbide
106
Steel Shot/Grit
250
Glass Shot
100
Plastic Grit
45 to 48
Ferric Oxide
172
As a general rule, it can safely be figured that a typical Blast Machine can only hold Abrasive in about 75% of its inside space. So, a typical 650 Pound (6 Cubic Foot) Machine will only freely hold 488 Pounds of Silica Sand which happens to weigh 100 Pounds per Cubic Foot (Bulk Density). But, that same Machine can hold 1219 Pounds of Steel Grit, but only 219 Pounds of Nut Shells. But it is still as full as it possibly can be due to its filling angle of repose.
