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

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

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.

Standard reference conditions in current use
Temperature Absolute pressure Relative humidity Publishing or establishing entity
°C kPa % RH
0 100.000   IUPAC (present definition)
0 101.325   IUPAC (former definition), NIST, ISO 10780
15 101.325 0 ISA, ISO 13443, EEA, EGIA
20 101.325   EPA, NIST
25 101.325   EPA
25 100.000   SATP
20 100.000 0 CAGI
15 100.000   SPE
°F psia % RH
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.

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