> Also, why is everyone measuring helium in volume (litres, cubic feet)? This is highly dependent on temperature and pressure. Why not state quantities in terms

> Also, why is everyone measuring helium in volume (litres, cubic feet)? This is highly dependent on temperature and pressure. Why not state quantities in terms of mass? This problem appears way more often than I'd like. In measuring gas flow through valves there's a quantity called "standard cubic feet per minute". The only problem is that there is no uniformly agreed upon standard reference temperature and pressure! https://en.wikipedia.org/wiki/Standard_cubic_feet_per_minute https://en.wikipedia.org/wiki/Standard_cubic_feet_per_minute It's even worse than described on Wikipedia, as I've found at least one valve manufacturer claims that the reference pressure is the pressure they used upstream of the valve, not something near atmospheric as Wikipedia claims is typical: http://btrettel.nerfers.com/archives/72 http://btrettel.nerfers.com/archives/72 Whenever I get a valve I either have to do my own flow tests or call the manufacturer and hope I get someone who knows a thing or two about their testing procedures. As you said, the entire issue can be avoided by simply specifying mass or moles. I don't quite know why people don't do this, but I suppose it speaks to the power of tradition. Far too few people think about if what they are doing makes sense.