A ratio of diluting neutral gas to odorous sample gas.  According to ASTM E679, this is a ratio of diluting gas to the odour sample. According to 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

EN 13725:  (diluting gas + odourous sample)/odourous sample = 6:1 = 6

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There are a number of ASTM standards related to odour assessment and regulation:

• ASTM E679-04 United States ‘standard practice for determination of odour and taste threshold by a forced-choice ascending concentration series method of limits’
• D1292 test method for odour in the water
• E544 practices for referencing suprathreshold odour intensity
• E1432 practice for defining and calculating individual and group sensory thresholds from forced-choice data sets of intermediate size

The ASTME679-04 is the most commonly used standard in the USA for olfactometry, however, outside of the USA, EN13725 standard is more commonly accepted.

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ASTM E679-04, the United States standard for the “determination of odour and taste threshold by a forced-choice ascending concentration series method of limits”, provides guidance on both olfactometry and sampling.

The ASTM E679-04 is the most commonly used standard in the USA for olfactometry, however, outside of USA standard EN13725 is more commonly accepted.

Differences between ASTM E679-04 and EN13725 is mostly on panellist screening and on minor details of olfactometry. One highly controversial difference is the presentation flow, which is set at only a minimum of 5 litres per minute in ASTM E679-04 and 20 litres per minute in EN13725. It is however technically possible to run at 20 litres per minute and still comply with the ASTM standard.

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