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, and since that is always the way parts-per-million is figured for gas measurement, it is just a more pedantic (or self-consciously complete) way of rendering "ppm."

To be rigorous, the correct textbook definition of parts per million would have it--

mass of one component in milligrams/total mass of the solution in kilograms

Thus, strictly speaking, ppm should be figured as mass, not volume.

In the practical sense, though, this definition is most often applied to liquid solutions, even if two or more gases (and we are usually referring to a pollutant gas in air) also comprise a solution.

The reason that gas parts-per-million is always parts-per-million by volume is that traditionally, gases have been handled by volume or pressure, but usually not by mass. While the "high-loading" balance technique (whereby a cylinder is weighed to milligram resolution, and the target gas is added by mass or weight) is sometimes employed to make calibration gas blends, the gold standard is still a glass flask. Here, a volume of target gas is injected into a flask of known volume, containing the balance gas. The operation must be done at a known and constant temperature and pressure.

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.

Here are the full names of the entities listed in the above table--

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.