The manufacturer of the main propulsion diesel engines on your uninspected fishing trawler specifies the requirements for total hardness of freshwater makeup for the closed, recirculating freshwater cooling system in milligrams per liter (mg/L). If the freshwater test reported a total hardness of 5.7 grains per gallon (gpg), using the conversion chart of the illustration, what would be the total hardness in mg/L? Illustration MO-0208
• Use the Hardness Unit Conversion Table to go from gpg (grains per gallon) to ppm/mg/L. • Notice that mg/L and ppm are in the same box in the table – they are treated as equivalent here. • Pay close attention to whether the table converts from columns to rows or from rows to columns before you multiply.
• Which column corresponds to the unit you are starting with (gpg), and which row corresponds to the unit you are ending with (ppm, mg/L)? • If 1 gpg equals a certain number of mg/L according to the table, how do you scale that value for 5.7 gpg? • After multiplying, which answer choice is closest to your calculated value, and does it make sense in terms of order of magnitude (very small, small, or moderate hardness)?
• Identify the correct intersection value in the table where the gpg column meets the ppm, mg/L row. • Confirm that you are multiplying, not dividing, when converting from a smaller unit count (gpg) to a larger numerical expression (mg/L). • Remember that in this context 1 ppm = 1 mg/L, so no further conversion is needed once you have ppm.
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