I work at a hospital and I noticed that the oxygen cylinders are labelled as containing 630 litres of oxygen. (Yes yes, suuuper busy day at work) Similarly the flow rate out of the cylinder is set in L/min. This piqued my interest as I know a litre is a set volume, yet clearly the cylinder was not that large. So what is a litre in this case? …and why is it this way?
In: Physics
The liters are a normalized measurement
Gas cylinders are often marked with how many liters of the gas you could get out of it if you expand that gas to standard pressure (1 atm) at standard temperature (25C). This helps to get around the different pressures that can be in the cylinder, an 11 liter (actual volume) tank pressurized to 300 bar has 50% more gas in it than an 11 liter tank pressurized to 200 bar so marking it with the nominal volume of gas once depressurized helps avoid people needing to do math
Most things that are done with gasses are measured with volume rather than weight which again avoids having to do math. A person breaths about 10 liters/minute, you could convert that into kg but you’d have to convert back to liters a lot because all the papers and whatnot report that type of figure in liters. A regulator on the cylinder would give you 10 L/min regardless of if you’re working with Helium or Sulfur Hexaflouride, but if you measure the outflow mass it would be very very different and you’d have to know all your gas masses and that’s a pain to work with.
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