NOTES ON THE DIFFERENCE BETWEEN SPECIFIC GRAVITY and BULK DENSITY OF PRODUCTS? …. TIPS FROM E4A!
Some folk find it a little difficult getting their minds around SG and BD of products. If you Google these product terms you may get caught up with Wikipedia with quite fancy descriptions. We find that some of the detailed scientific definitions can be quite confusing to the layman, so we will share with you some easier ways of looking at these good old product parameters.
>SG or specific gravity is simply the weight in kilograms of a litre of any product. Water is SG = 1 and most paint products are SG = 1.5 or similar. So, if you took a litre of paint and placed it on a scale (taring or setting the zero first – on the empty tin) you would likely get a reading of 1.5 kgs. Okay so far? Weigh one litre of sea water – you will get 1.03 kgs. Weigh one litre of benzene – you will get 0.876 kgs. Weigh one litre of mercury – you will get 13.63 Kgs!
BUT …. remember these SGs are all at 25 degrees Celsius, as SG does vary against increasing or decreasing temperatures.
Substances with a specific gravity of 1 are neutrally buoyant in water. Those with SG greater than 1 are denser than water and will, disregarding surface tension effects, sink in water. Those with an SG less than 1 are less dense than water and will float on it.
>Bulk Density is a property of powders, particles, or flakes and fibres, or other ‘divided’ solids and is generally measured as dry product. If you take a one litre container and fill it with the powder or other dry product, you can measure the weight, again after taring the scale, and you will again have a reading of kilograms per litre volume. Often this is shown as gms/cm3 or grams per cubic centimetre. So, what are some examples? If you weigh one litre of clay you will get around 0.8 kgs but weigh one litre of calcined clay and you will get 0.48 kgs. Calcium carbonate powder will weigh 0.705 kgs, while gravel would weigh 1.76 kgs and cobalt powder fines 4.10!
AGAIN …. product will vary in dryness and may even be damp; you may spray some sort of liquid over the powder in a mixer for example and the dampened or wetted powder is now a different product – in the processing sense.
Bulk density is not a set property of a material; it can change depending on how the material is handled. For example, a powder poured into a cylinder will have a particular bulk density; if the cylinder is disturbed, the powder particles will move and usually settle closer together, resulting in a higher bulk density. For this reason, the bulk density of powders is usually reported as “freely settled” or “poured” density.
Take care to specify properly detailed SGs or BDs when sending out enquiries or scope of supply requests for any and all equipment enquiries. Of course, other parameters such as viscosities, composition of products in each stream and other details will also be required.
Engineers for Africa
April 2020
