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Partsper notation is used, especially in science and engineering, to denote relative proportions in measured quantities; particularly in lowvalue (highratio) proportions at the partspermillion (ppm) 10^{–6}, partsperbillion (ppb) 10^{–9}, and partspertrillion (ppt) 10^{–12} level. Since partsper notations are quantityperquantity measures, they are known as dimensionless quantities; that is, they are pure numbers with no associated units of measurement. In regular prose, partsper notations generally take the literal “parts per” meaning of a comparative ratio. However, in mathematical expressions, partsper notations function as coefficients with values less than 1.
Contents
Overview
Partsper notation is often used in the measure of dilutions (concentrations) in chemistry; for instance, for measuring the relative abundance of dissolved minerals or pollutants in water. The expression “1 ppm” means a given property exists at a relative proportion of one part per million parts examined, as would occur if a waterborne pollutant was present at a concentration of onemillionth of a gram per gram of sample solution.
Similarly, partsper notation is used also in physics and engineering to express the value of various proportional phenomena. For instance, a special metal alloy might expand 1.2 micrometers per meter of length for every degree Celsius and this would be expressed as “α = 1.2 ppm/°C.” Partsper notation is also employed to denote the change, stability, or uncertainty in measurements. For instance, the accuracy of landsurvey distance measurements when using a laser rangefinder might be 1 millimeter per kilometer of distance; this could be expressed as “Accuracy = 1 ppm.”^{[1]}
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