Mass or Volume: Important question! In my first blog post on gas mixing and blending, we saw that combustion is an important gas mix application. Combustion or burning is a chemical process, a reaction between molecules. To quantify the gas used we should count these gas molecules and not the space between these gas molecules. We do that by measuring the gas mass flow, the weight of the gas (molecules) and not the volume (molecules + space between molecules). The nice thing about Mass Flow Controllers is that they do just that: they measure and/or control the gas mass flow, they are molecule counters!  This automatically leads to the question of  how we define a ratio? In mass or volumetric ratio units. Let me give a fun example.  If I have two 1 litre bottles and I fill one with lead and the other with helium, the volume ratio is still 1 to 1. (They are both 1 litre in volume).  However, 1 litre if helium weighs 0.1786  grams (oC, atm), one litre of solid lead weights 11340 grams, we have a mass ratio here of 1 to 63494! Some gasses have a much higher density than others, so if somebody wants to mix Helium with Air in a 60%/40% ratio, the first questions should be: is that % of mass or % of volume.