It's the same exactly as a pyncnometer. This is a flask of known volume that is measured with dry ore in it, and full to the stopper with water. It is specially constructed to leak water out of its stopper.
Once you know the weight of the ore, in the box, you then know its volume when you add the water, as the water's weight is equal to so many volumes reliably. If BV is box volume, OV is ore volume, WW is water weight, OW is ore weight, Specific Gravity pre unit ore is SGore, and SG of water is SGwater it reduces to..
1. OV=BV-(WW/SGw)
2. SGore=OW/OV
E.G. Box is 3'X3'X3' = 27 cubic feet.
Ore is 2000 pounds.
Total weight with water is 3049.94 lbs
Water to fill is 930.08 lbs
Volume of water is 930.08/62.4 = 14.91 cubic feet of water.
Volume of ore is 27 cubic feet minus 14.91 cubic feet. =12.09 cubic feet.
Specific gravity of ore is 2000/12.09 = 165.43 lbs per cubic foot. divided by 62.4 to get the number of times heavier than water and that is 2.65
The expansion factor of ore is usually described in stopes such that in order to keep a constant volume, 33% of the ore must be removed after blasting. That is 33% of the broken rock must be hauled out of the drawpoints in order to maintain a constant volume of a void within which to work. If 33% of the total-expanded equals the expansion amount of the previous or unbroken rock, then 33-1/3/66-2/3 = the expansion factor - or 50%. Rock volume will expand 50% to include voids, when broken.
As you break the rock more, in part the new grains expand into the voids already made, so the expansion increases less and less as you grind.
On the other hand when you take soil with voids in it, and expand it by digging, its expansion is a bit different.
EC<:-}
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