A ratio of diluting neutral gas to odourous sample gas. According to ASTM E679, this is a ratio of diluting gas to the odour sample. According to the EN13725, this is a ratio of the sum of the diluting gas and odour sample, to the odour sample. It is calculated based on the flow rate.
Example: 5 parts of diluting air to 1 part of sample
ASTM E679: Diluting gas/odourous sample = 5:1 = 5