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
You are correct that the more accurate measurement would be by mass. However it can be hard to measure the mass of something. For example the regulator on the gas flask would be much more complex if it had to measure the flow rate by mass and not by volume. The easiest way would have been to measure the flow rate in volume and then use a convertion factor. This is how they are able to list the volume of air in the cylinder. They know the weight of the gas in the cylinder and then use a convertion factor to a standard volume of oxygen (volume in standard atmospheric pressure and temperature). But of course as the gas is pressurized it does not take up as much volume as if it had been at standard atmospheric pressure. Another way to measure gas is by pressure. Of course this does not give you how much gas there is as this depends on the volume of the tank but it is often much easier to measure then the weight of the tank itself.
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.
O2 is stored in liquid form. It has an expansion ratio of 1:861 at STP. So a 1 liter bottle of LO2 will provide 861 liters of O2 gas.
Probably much easier to calculate everything based on the literage than the mass (about 27 kilos) which isn’t very useful. That’s 630 L is at atmospheric pressure meaning they have squeezed all of that volume under pressure into the bottle to store and transport it.
A liter is a liter of gas as normal atmospheric conditions (I believe the exact definition varies depending on where you are).
Volume measurements are nice because they don’t really care about what the gas is.
If I have a size “300” gas cylinder (aka size “T”) I know that stores 300 “standard” cubic feet of gas. That gas could be oxygen, nitrogen, methane, ammonia, air, forming gas, whatever. If I had to label everything based on mass then I would run into issues based on the fact that those gasses are all different densities and so a “full” cylinder of each would be a different mass.
You do see containers that are labeled based on mass, but these tend to be containers that are storing petroleum gasses, which tend to turn into a liquid at low pressures. If you’re American the classic example of this is probably the “20 pound” propane tank that gets used on propane grills.
Once at a dentist, I (while on nitrous) explained they were getting ripped off buying their nitrous by the liter, and not by the lb, they were basically returning 2/3 empty tanks cause pressure dropped…not cause they were out.
They thought I was crazy…but when I filled auto nitrous, I always measured by weight…not volume.
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