Specific Gravity and DensityHow Do They Differ?Confused about specific gravity (SG) and density? SG is the ratio of the absolute density of the specified material at a specified temperature over the absolute density of water at a specified reference temperature (usually 15oC). The density values should of course be in the same units. Because it's only a ratio, SG has no units. However, because the density of water at 15oC happens to be very close to 1.00 grams per mL, the units of SG are also close to grams per mL. Why use SG instead of density? Firstly, it's universal - whether you're thinking in pounds and cubic inches or kilograms and millilitres, the value doesn't change. Secondly, when you're measuring and reporting density at a high level of precision, you have to consider the difference between absolute density (which is the density of the specified material in a vacuum), and the density in air, which is slightly lower and varies to a small degree with air pressure. Air exerts a buoyancy effect of roughly 0.0012 grams per millilitre at 20oC, so the weight of a material in air is slightly lower than what would be measured in a vacuum. Using SG eliminates any ambiguities between the absolute and measured density. Is there really much difference between SG and density? If you're using units of g/mL, or kg/L, then unless you're working at very high precisions the difference is tiny. At densities close to 1.0 g/mL, SG exceeds "density in air" by roughly 0.002 SG units. The relative difference diminishes at higher densities. For a solution with a "density in air" (at a normal air pressure) of 1.000 g/mL, the absolute density would be 1.0011 g/mL (if you used stainless steel weights to calibrate your balance), and the specific gravity relative to water at 15oC would be 1.00205. Conclusion: unless you're concerned with the third decimal place of SG, you can safely assume that SG has units of grams per millilitre. Total Aluminium As Alumina What Do We Mean?We're frequently asked why the concentration of aluminium-based coagulants is expressed as aluminium oxide (Al2O3). Aluminium oxide is after all something that really can't exist in solution. The answer is that it's a long-standing convention in the industry to do so. By standardising the concentration units for aluminium-based coagulants, simpler comparisons between different products can be made. For example, if the concentration of alum sulphate solution was expressed as "% Al2(SO4)3", then it would be more difficult to directly compare its concentration with a solution of polyaluminium chloride with units expressed as "%Al2(OH)3Cl3". Although this method of expressing the total aluminium concentration has persisted, it would probably be more convenient still if units of "total percent aluminium" were used. To convert "total aluminium as alumina" to "total aluminium", simply divide by 1.889. For example, a solution concentration of "10.0 weight % total aluminium as alumina" is the same as "5.29 weight % total aluminium". |
