TIG Welder Oxygen Depletion risk

To gauge the risk of oxygen depletion due to release of the compressed argon, I've followed the British Compressed Gases Association's guidance node 11 - Reduced oxygen atmospheres. In it, they lay out a calculation for resulting oxygen concentration after a gas release in section 3.2 and a worked example in Appendix 1.

The following is the calculation for G8 given a 20L argon cylinder:

Total room volume: 4.5m * 6.1m * 2.6m = 71.37m3

Cautious estimate that 20% of that volume is taken by cupboards/tools/stuff, so actual volume of air in the room: Vr = 71.37 * 0.8 = 57.096m3

Volume of oxygen after gas release: Vo = 0.21 * (Vr - Volume of gas in cylinder) = 0.21 * (57.096m3 - (20L * 200bar / 1000)) = 0.21 * (57.096m3 - 4m3) = 11.15016m3

Resulting oxygen concentration: Cox = 100 * Vo / Vr = 100 * 11.15016 / 57.096 = 19.53%

This oxygen concentration is well above the 18% minimum recommended by the HSE.