Relative Weight Strength ("RWS") and Relative Bulk Strength ("RBS")

The most important drawback of these measures is the straight comparison of chemical energies on a percentage basis without considering how efficiently the energy is used. To illustrate this point, the RWS and RBS of an ordinary fuel such as petrol will be far higher than any explosive because their energy content is higher. However, you will not be able to break much rock with a litre of petrol whereas you will be able to break a few cubic metres of rock with a similar quantity of explosive. The reason why these indicators have become standard is attributable to a number of circumstances. In the old days of nitroglycerin based explosives, strength were measured comparing the relative displacements in a pendulum of equal (very small ~10g ) quantities of the explosives being compared. With the advent of bulk explosives, it became apparent that quantities of around 10g of bulk explosives did not give a representative indication of their strength in the pendulum test. As a result, a different base for comparison was established by using a calculated energy release and referring it to a known standard energy release such as ANFO. The first problem with this comparison arises with the standard itself. ANFO being a generic explosive is highly variable in its characteristics from packing density to energy release. After some controversy, the energy release was established at 890cal/g by the Los Alamos (USA) laboratory. However, some manufacturers do not use this value but instead use a lower 860 cal/g which makes the RWS and RBS look much more impressive.

To complicate things, some manufacturers have introduced the term Relative Effective Energy ("REE") which is calculated using a computer code. This code calculates the energy releases of ANFO and the explosive in question above a cut-off pressure of around 100MPa (thought to be the threshold below which gases do not perform useful work) and compares them on a percentage basis. Under this measure, the result is biased towards the highest pressure explosive because ANFO releases a considerable amount of its energy below the cut-off pressure. By changing the cut-off pressure, the measurements can be biased towards a particular explosive. Again these computer codes are not publicly available and cannot, therefore, be cross-checked.