Determining Net Quantity of Gases for Shipping by Air

How much gas can I ship by air per cylinder?

In theory this is an easy question. In practice, it can be easy too, or of course, we can make life difficult on ourselves and do some math.

We’ll start with the easy stuff. Look up the commodity in IATA Section 4.2 (a.k.a. The Blue Pages). We’ll use UN1978, PROPANE, CLASS 2.1 as an example. It is forbidden on passenger aircraft and we find a value of 150 kg as a maximum net per package on Cargo Aircraft Only.

Appendix A defines “Net Quantity” as “the weight or volume of the dangerous goods contained in a package excluding the weight or volume of any packaging material.” This means we are only concerned with the weight of the gas, not including the weight of the cylinder.

So that should be easy right? We just have to weigh the full cylinder and subtract the tare weight of the cylinder (what it weighs without the gas). Yes, that’s pretty much it! Using our example, we weigh our propane tank, subtract the tare weight of the tank and so long as that difference doesn’t exceed 150 kg, we’re in good shape.

One final thing to confirm is that the cylinder doesn’t exceed any maximum quantity that may be limited by the country of manufacture. In Canada, this can be referenced using the CSA Standard B339-08.

Now, any chemist will tell you tell you how difficult it becomes to figure out how much chemical was put into a vessel if the tare weight of the vessel isn’t known. For gases though, we can make some calculations if we know a little bit about pressures and volumes. Scientists have written various formulae to describe gas behavior in its ideal state (you may have heard of the ideal gas law). A variation on the gas law that can help determine the quantity of gas is Boyle’s Law. If we don’t know the volume of gas (what it would be in the atmosphere), but instead only the volume of the cylinder (water-carrying capacity) and the pressure of the gas within the cylinder, we could determine what the volume of the gas would be at atmospheric pressure. This makes many assumptions about other variables being constant.

Volume_atmospheric (L) =
[Pressure_cylinder x Volume_cylinder (L)] / Pressure_atmospheric

Whether we use the pressure in kPa or atmospheres will not matter as these units cancel out. Essentially we’re using the pressures as a ratio. Once we have the volume, we can reference that the standard volume of 1 mole of an ideal gas at standard temperature and pressure equals 22.4 L and with some formula rearrangement, we can end up with:

Net Quantity of gas (kg) =
[Volume_atmospheric (L) / 22.4 L] x [molecular weight of gas (g/mol) / 1000]

Note: The factor of 1000 that we divided by has some units that help to cancel out everything else.

If we had a propane tank that held 20 L (water-carrying capacity) and it was at a pressure 5x that of atmosphere, using the molecular weight of propane as 44.1 g/mol, we arrive at 0.197 kg propane, well below the 150 kg we’re allowed to ship. Keep in mind though that this whole calculation is based on the assumption that whatever gas we are considering acts “ideally”. At the end of the day, the best thing to do is weigh the cylinder empty and full, take the difference and use that as your net quantity. It’s just the most simple.

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