Compare this with the USCS system in which parametersmay be expressed in several different ‘base’ units, combinedwith a multiplying factor. For example, the equation fordetermining the minimum allowable corroded shell thicknessof storage tanks istmin¼2:6ðH²1ÞDGSEwheretminis in inches, fill height (H) is in feet, tank diameter(D) is in feet,Gis specific gravity,Sis allowable stress in psiandEis joint efficiency.Note how, instead of stating dimensions in a single baseunit (e.g. inches) the dimensions are stated in the mostconvenient dimension for measurement, i.e. shell thickness ininches and tank diameter and fill height in feet. Rememberthat:.This gives the same answer; the difference is simply in themethod of expression..In many cases this can be easier to use than the morerigorous SI system – it avoids awkward exponential (106,10-6,etc.)factorsthathavetobewritteninandsubsequently cancelled out..The written terms tend to be smaller and more convenient.1.3.3Trends in code unitsUntil fairly recently, ASME and API codes were writtenexclusively in USCS units. The trend is increasing to expressall units in dual terms USCS(SI), i.e. the USCS term followedby the SI term in brackets. Note the results of this trend:.Not all codes have been converted at once; there is aninevitable process of progressive change.