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== SI-compliant expressions == SI-compliant units that can be used as alternatives are shown in the chart below. Expressions that the BIPM explicitly does not recognize as being suitable for denoting dimensionless quantities with the SI are marked with <span style="color:darkred;">'''!'''</span>. {|class="wikitable" style="font-size: 95%;" |+ Notations for dimensionless quantities |- ! Measure ! SI<br>units ! Named<br>parts-per ratio<br>(short scale) ! Parts-per<br>abbreviation<br>or symbol ! Value in<br>scientific<br>notation |- |style="background:#f2f2f2" align="right"| A [[Strain (materials science)|strain]] of... |align="center"|2 [[Centimetre|cm]]/[[metre|m]] |align="center"|2 parts per hundred |align="center"| 2%<ref>According to [http://www.bipm.org/en/publications/si-brochure/section5-3-7.html BIPM's SI brochure, section 5.3.7], "When [the percent symbol] is used, a space separates the number and the symbol %." This practice has not been well adopted with regard to the % symbol, is [[WP:%|contrary to Wikipedia's ''Manual of Style'']], and is not observed here.</ref> |align="center"|2 × 10<sup>−2</sup> |- |style="background:#f2f2f2" align="right"| A sensitivity of... |align="center"|2 [[Milli-|m]]V/V |align="center"|2 parts per thousand |align="center"|2 ‰ <span style="color:darkred;">'''!'''</span> |align="center"|2 × 10<sup>−3</sup> |- |style="background:#f2f2f2" align="right"| A sensitivity of... |align="center"|0.2 mV/V |align="center"|2 parts per ten thousand |align="center"|2 ‱ <span style="color:darkred;">'''!'''</span> |align="center"|2 × 10<sup>−4</sup> |- |style="background:#f2f2f2" align="right"| A sensitivity of... |align="center"|2 [[Micro-|μ]]V/V |align="center"|2 parts per million |align="center"|2 ppm |align="center"|2 × 10<sup>−6</sup> |- |style="background:#f2f2f2" align="right"| A sensitivity of... |align="center"|2 [[Nano-|n]]V/V |align="center"|2 parts per billion <span style="color:darkred;">'''!'''</span> |align="center"|2 ppb <span style="color:darkred;">'''!'''</span> |align="center"|2 × 10<sup>−9</sup> |- |style="background:#f2f2f2" align="right"| A sensitivity of... |align="center"|2 [[Pico-|p]]V/V |align="center"|2 parts per trillion <span style="color:darkred;">'''!'''</span> |align="center"|2 ppt <span style="color:darkred;">'''!'''</span> |align="center"|2 × 10<sup>−12</sup> |- |style="background:#f2f2f2" align="right"| A mass fraction of... |align="center"|2 mg/kg |align="center"|2 parts per million |align="center"|2 ppm |align="center"|2 × 10<sup>−6</sup> |- |style="background:#f2f2f2" align="right"| A mass fraction of... |align="center"|2 μg/kg<!--NOTE TO EDITORS: SI convention is to use the base unit kilogram in formulas and equalities. However, whenever an SI prefix, like milli (m) or micro (μ), is used with the unit of mass, the unit gram (symbol g) is used. This results in a mix of units, such as mg (a thousandth of a gram) over a kilogram (a thousand grams). Thus, one microgram divided by one kilogram is one ppb.--> |align="center"|2 parts per billion <span style="color:darkred;">'''!'''</span> |align="center"|2 ppb <span style="color:darkred;">'''!'''</span> |align="center"|2 × 10<sup>−9</sup> |- |style="background:#f2f2f2" align="right"| A mass fraction of... |align="center"|2 ng/kg |align="center"|2 parts per trillion <span style="color:darkred;">'''!'''</span> |align="center"|2 ppt <span style="color:darkred;">'''!'''</span> |align="center"|2 × 10<sup>−12</sup> |- |style="background:#f2f2f2" align="right"| A mass fraction of... |align="center"|2 pg/kg |align="center"|2 parts per quadrillion <span style="color:darkred;">'''!'''</span> |align="center"|2 ppq <span style="color:darkred;">'''!'''</span> |align="center"|2 × 10<sup>−15</sup> |- |style="background:#f2f2f2" align="right"| A volume fraction of... |align="center"|5.2 μL/L |align="center"|5.2 parts per million |align="center"|5.2 ppm |align="center"|5.2 × 10<sup>−6</sup> |- |style="background:#f2f2f2" align="right"| A mole fraction of... |align="center"|5.24 μmol/mol |align="center"|5.24 parts per million |align="center"|5.24 ppm |align="center"|5.24 × 10<sup>−6</sup> |- |style="background:#f2f2f2" align="right"| A mole fraction of... |align="center"|5.24 nmol/mol |align="center"|5.24 parts per billion <span style="color:darkred;">'''!'''</span> |align="center"|5.24 ppb <span style="color:darkred;">'''!'''</span> |align="center"|5.24 × 10<sup>−9</sup> |- |style="background:#f2f2f2" align="right"| A mole fraction of... |align="center"|5.24 pmol/mol |align="center"|5.24 parts per trillion <span style="color:darkred;">'''!'''</span> |align="center"|5.24 ppt <span style="color:darkred;">'''!'''</span> |align="center"|5.24 × 10<sup>−12</sup> |- |style="background:#f2f2f2" align="right"| A stability of... |align="center"|1 (μA/A)/[[minute|min]] |align="center"|1 part per million per minute |align="center"|1 ppm/min |align="center"|1 × 10<sup>−6</sup>/min |- |style="background:#f2f2f2" align="right"| A change of... |align="center"|5 nΩ/Ω |align="center"|5 parts per billion <span style="color:darkred;">'''!'''</span> |align="center"|5 ppb <span style="color:darkred;">'''!'''</span> |align="center"|5 × 10<sup>−9</sup> |- |style="background:#f2f2f2" align="right"| An uncertainty of... |align="center"|9 μg/kg |align="center"|9 parts per billion <span style="color:darkred;">'''!'''</span> |align="center"|9 ppb <span style="color:darkred;">'''!'''</span> |align="center"|9 × 10<sup>−9</sup> |- |style="background:#f2f2f2" align="right"| A shift of... |align="center"|1 nm/m |align="center"|1 part per billion <span style="color:darkred;">'''!'''</span> |align="center"|1 ppb <span style="color:darkred;">'''!'''</span> |align="center"|1 × 10<sup>−9</sup> |- |style="background:#f2f2f2" align="right"| A strain of... |align="center"|1 μm/m |align="center"|1 part per million |align="center"|1 ppm |align="center"|1 × 10<sup>−6</sup> |- |style="background:#f2f2f2" align="right"| A [[temperature coefficient]] of... |align="center"|0.3 (μHz/Hz)/°C |align="center"|0.3 part per million per °C |align="center"|0.3 ppm/°C |align="center"|0.3 × 10<sup>−6</sup>/°C |- |style="background:#f2f2f2" align="right"| A frequency change of... |align="center"|0.35 × 10<sup>−9</sup> ƒ |align="center"|0.35 part per billion <span style="color:darkred;">'''!'''</span> |align="center"|0.35 ppb <span style="color:darkred;">'''!'''</span> |align="center"|0.35 × 10<sup>−9</sup> |} Note that the notations in the "SI units" column above are for the most part [[Dimensionless quantity|dimensionless quantities]]; that is, the units of measurement factor out in expressions like "1 nm/m" (1 n<s>m</s>/<s>m</s> =1 × 10<sup>−9</sup>) so the [[ratio]]s are pure-number [[coefficient]]s with values less than 1.
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