More on Expressing Gas Concentrations

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.

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.