User:Milton Beychok/Sandbox: Difference between revisions

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'''"Parts-per" notation'''  is used in [[science]] and [[engineering]], to denote proportionalities in measured quantities such as proportions at the parts-per-million ('''ppm'''), parts-per-billion ('''ppb'''), and parts-per-trillion ('''ppt''') level. Since parts-per notations are quantity-per-quantity measures, they are known as ''dimensionless quantities''; that is, they are pure numbers with no associated [[measurement|units of measurement]].


==Parts per million (ppm)==
In ordinary prose, 1 ppm is "one part per million parts" and has a numerical value of 1×10<sup>-6</sup> in most (if not all countries).
Parts-per notation is often used in the measure of dilutions ([[concentration]]s) 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 proportion of one part per million parts examined, as would occur if a water-borne pollutant was present at a concentration of one-millionth of a [[gram]] per gram of sample solution.
Similarly, parts-per notation is used also in [[physics]] and [[engineering]] to express the value of various proportions. For example, a metal might expand 1.2 [[micrometre]] per [[metre]] of length for every [[Celsius|degree Celsius]] and this would be expressed as a [[coefficient of thermal expansion]] of 1.2 ppm/°C. As another example, the accuracy of land-survey distance measurements when using a [[laser rangefinder]] might be 1 millimetre per kilometre of distance and this could be expressed as an accuracy of 1 ppm.
==Parts per billion (ppb)==
In ordinary prose, 1 ppb is "one part per billion parts" and has a value of 1×10<sup>-9</sup> in the [[United States]] only. This terminology should be used with great caution because:
*In [[France]] and frequently in continental [[Europe]], 1×10<sup>-9</sup> is ''1 milliard''
*In the [[United Kingdom]] and in other nations using [[British English]], 1×10<sup>-9</sup> is ''1 thousand million'' and 1 billion is 1×10<sup>-12</sup>
==Parts per trillion (ppt)==
In ordinary prose, 1 ppt is "one part per trillion parts" and has a value of 1×10<sup>-12</sup> again in the United States only. This terminology should also be used with great caution because:
*In the United Kingdom and other nations using British English, France and continental Europe, 1×10<sup>-12</sup> is ''1 billion'' and 1 trillion is 1×10<sup>-18</sup>
*Concentrations are sometimes expressed as ppt meaning ''parts per thousand'' which conflicts with ppt meaning ''parts per trillion''.
==Summary==
The parts-per notation is not formally part of the [[International System of Units]] (SI).
The [[International Bureau of Weights and Measures]] (an international standards organization known also by its [[France|French]]-language initials BIPM) recognizes the use of parts-per notation.<ref name="BIPM">BIPM: [http://www.bipm.org/en/si/si_brochure/chapter5/5-3-7.html 5.3.7 ''Stating values of dimensionless quantities, or quantities of dimension one]''</ref> However, the BIPM suggests avoiding the use of ppb and ppt to avoid misunderstandings.
Perhaps the best practice is to use the [[scientific notation]]s 1×10<sup>-6</sup>, 1×10<sup>-9</sup> and 1×10<sup>-12</sup> to avoid misunderstandings.
==References==
{{reflist}}

Revision as of 13:45, 10 August 2008

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