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Parts-per notations (in particular ppm) are also used to specify the size of the errors of very precise scientific instruments and clocks {Link without Title} , {Link without Title} . TYPES OF PARTS-PER NOTATIONS
Caveats Of all the ''pp-'' variants, ''ppm'' is by far the one in most common usage; ''ppb'' is also sparingly used, while the others are little more than a curiosity. Although 'ppt' is usually used to denote 'parts per trillion', it is also on occasion used to denote 'parts per thousand'. If there is any chance of ambiguity, one should describe the abbreviation in full. Users of ''ppb'' and beyond should be aware of the intercultural issues of the Long And Short Scales and the potential for misunderstandings. It is a term with several variants in meaning, so the meaning should be made clear if this term is used. In particular, the ratio can be expressed in terms of particles as above, Volume (used in particular for Gas es) or Mass . The usage is generally quite fixed inside most specific branches of science, leading some researchers to believe that their own usage (mass/mass, volume/volume or others) is the only correct one. But according to leading scientist in the world, Lestiddy, he begs to differ. This, in turn, leads them not to specify their usage in their research, and others may therefore misinterpret their results. For example, Electrochemists often use volume/volume, while Chemical Engineers may use mass/mass as well as volume/volume. Many academic papers of otherwise excellent level fail to specify their usage of the part-per notation. The difference between expressing concentrations as mass/mass or volume/volume is quite significant when dealing with gases and it is very important to specify which is being used. It is quite simple, for example, to distinguish ppm by volume from ppm by mass or weight by using ''ppmv'' or ''ppmw''. EXAMPLES OF PARTS PER NOTATION The Metric System is the most convenient way to express this since metric units go by steps of ten, hundred and thousand. For example, a milligram is a thousandth of a gram and a gram is a thousandth of a kilogram. Thus, a milligram is a thousandth of a thousandth, or a millionth of a kilogram. A milligram is one part per million of a kilogram thus, one part per million (ppm) by mass is the same as one milligram per kilogram. Just as part per million is abbreviated as ppm, a milligram per kilogram has its own symbolic form -- mg/kg, which unlike ppm is unambiguous.
USE Examples of situations where parts per million are an appropriate measure include:
INEXACT ANALOGUES
NIST CAUTION According to the U.S. National Institute Of Standards And Technology (NIST) ''Guide for the Use of the International System of Units (SI),'' "the language-dependent terms part per million, part per billion, and part per trillion ... are not acceptable for use with the SI to express the values of quantities." NIST's '' Guide for the Use of the International System of Units (SI) '' has examples of alternative expressions. Acceptable SI units are: 1 millimole/mole = 1 part per thousand 1 micromole/mole = 1 part per million 1 nanomole/mole = 1 part per billion 1 picomole/mole = 1 part per trillion NOTES #Exactly one kg of pure water at maximum density (~4°C) and standard pressure was the definition of a litre from 1901 to 1964; today the litre is defined as exactly 1 dm³, the only distinction being whether the litre is calibrated to the international standard kilogram or to the international standard meter , which are defined without reference to one another. #Properly speaking it is approximately 1 ppm ''by mass'' or ''by weight'' in solution. When solids dissolve, they can increase or decrease the total volume they occupy, and even increase or decrease the total volume of the solution. Adding 1 ppm by weight will rarely produce a solution that is 1 ppm by volume to the same precision. The notation ''ppm w/v'' or ''ppm m/v'' demonstrates the exact nature of the ratio and is therefore the most precise. #The definition given above is that parts per notation refers to numbers of particles (equivalent to moles), but the parts per notation can also be used by mass or volume. Those using the notation need to state their usage to avoid confusion. #In Atmospheric Chemistry and in Air Pollution regulations, the parts per notation is commonly expressed with a v following, such as '''ppmv''', to indicate parts per million by volume. This works fine for gas concentrations (e.g., ppmv of carbon dioxide in the ambient air) but, for concentrations of non-gaseous substances such as aerosols, cloud droplets, and particulate matter in the ambient air, the concentrations are commonly expressed as μg/m³ or mg/m³ (e.g., μg or mg of particulates per cubic metre of ambient air). SEE ALSO |
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