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
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
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.
If all of the world's current electric power production were fueled by fusion, the helium produced would be a drop in the bucket compared to the total existing helium market.
17.59 MeV / (helium-4 atomic mass) = 0.117 TWh/kg
in thermal energy per kg of helium produced. The U.S. electricity production in 2019 was 4,118 TWh [1] and 4118 TWh / 0.117 (TWh/kg) = 35,196 kg.
If we assume some inefficency in the thermal->electricity conversion, I guess it should produce at least 70,000 kg/year.
Probably because they are talking about liquefied helium.
A convention to reserve mass as the unit of sale for elements in gaseous form seems reasonable, you would want to receive a Very Large Bladder of gas when ordering supplies for an experiment that needs to be chilled with helium.