The gas odour level of natural gas needs to be monitored at regular intervals to secure the required level of the organosulphur compounds tetra-hydro-thiophene (THT) and tert-butyl-mercaptane (TBM) or the sulfur-free (Gasodor®-S-Free®) markers which are used worldwide to odorize natural gas. The safety concern is to assure a minimum concentration of the alarming smell while not overdosing the high-priced odour markers.