This is a follow-up to our earlier article on Using Common Sense and Science in Expressing Gas Concentrations, inspired by a real-life adventure of one of our sales engineers.
As our hero was slogging through some e-mail inquiries, he came upon two units of measurement he had not seen before:
ppmv and µg/Nm3
ppmv is simply parts-per-million by volume. This notation would distinguish it from parts-per-million based on weight. As it happens, when one is working with solids or liquids—i.e. when one leaves the world of gas measurements—parts-per-million is understood to be by weight (strictly speaking by mass).
As discussed in the article referenced above, the most proper way to think of parts-per-million in the gas world is as molar concentration: µmoles of minor component / total moles in mixture. By tradition, and because ppmv would be the same as the molar concentration for ideal gases, many people still speak of gas measurements as being in parts-per-million by volume.
µg/Nm3 means micrograms per normal cubic meter (Nm3). The “normal” cubic meter is defined as being at 0°C (273.15°K) and 101.325 kPa or 760 mmHg (i.e. 1 atmosphere of absolute pressure). However, this notation is no longer appropriate unless the specific reference conditions are explicitly stated, since there are currently many different definitions of what constitutes standard reference conditions.
|Temperature||Absolute pressure||Relative humidity||Publishing or establishing entity|
|0||100.000||IUPAC (present definition)|
|0||101.325||IUPAC (former definition), NIST, ISO 10780|
|15||101.325||0||ISA, ISO 13443, EEA, EGIA|
|60||14.696||SPE, OSHA, SCAQMD|
|60||14.73||EGIA, OPEC, EIA|
|59||14.503||78||Army Standard Metro|
|59||14.696||60||ISO 2314, ISO 3977-2|
Here are the full names of the entities listed in the above table–
|IUPAC||International Union of Pure and Applied Chemistry|
|NIST||National Institute of Standards and Technology|
|ISA||ICAO’s International Standard Atmosphere|
|ISO||International Organization for Standardization|
|EEA||European Environment Agency|
|EGIA||Electricity and Gas Inspection Act (of Canada)|
|EPA||U.S. Environmental Protection Agency|
|SATP||Standard Ambient Pressure and Temperature|
|CAGI||Compressed Air and Gas Institute|
|SPE||Society of Petroleum Engineers|
|OSHA||U.S. Occupational Safety and Health Administration|
|SCAQMD||California’s South Coast Air Quality Management District|
|OPEC||Organization of Petroleum Exporting Countries|
|EIA||U.S. Energy Information Administration|
|Std. Metro||U.S. Army’s Standard Metro (used in ballistics)|
As you can see, by using the unit µg/Nm3, you are bound to be misunderstood—if not in the definition of normal or standard conditions, then by the difficulties inherent in using mass/volume units rather than parts-per-million.