The minimum amount of an odourant perceived (smelled) is known as the detection threshold (DT). This minimum amount represents the odour concentration. This serves as a measurement of 1 odour unit or 1 OU/m³ or 1 OU_E/m³.  We can determine this through a selected panel and usually represents 50% of the population.

According to the EN13725 standard, the detection threshold represents a sample. For instance, the dilution factor at which the sample has of being detected under the conditions of the test.  According to E679-04, the detection threshold represents the lowest concentration of a substance in a medium. Above all, it relates to the lowest physical intensity at which a stimulus is detected.

An odour is presented to a panel in order to judge at which dilution the sample can initially being detected. The odours are presented in a decreasing diluted series of samples (neutral gas and odourant) until the dilution at which detection occurs.

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European Dynamic olfactometry is described in detail by the standard EN 13725. This standard was last released in 2003 and is currently under review for revision. As of 2020, an updated version is still waiting to be finalized in the UK. Key elements of this standard are the focus on quality criteria for precision and repeatability. The EN13725 tries to link these values to standard values of n-butanol in nitrogen. It is assumed in this standard that whenever a laboratory complies with the overall sensory quality criteria for n-butanol, the quality level is transferable to other, environmental, odours.

The Standard describes the different test modes such as Triangular Forced Choice, Binary Forced Choice, and Yes/No method for determining the true odour concentration of a sample bag.

Testing Requirements

Many aspects of testing are defined including:

• Number of panellists (min. 4)
• Acceptance criteria of each panellist
• Airflow (20 litres per minute) design of the olfactometer, presentation airspeed, minimum dilution, maximum dilution, and maximum step size
• Calculation of individual detection thresholds and overall detection threshold
• Design of the testing room including temperature and humidity
• materials used in the olfactometer

One of the main objectives of the EN13725 standard is inter-laboratory testing and panellist screening.

While the EN13725 does cover sampling, it is generally accepted that sampling techniques require much greater attention in the next revision of the standard. The standard does provide guidance on the type of sample bags (nalophan, Tedlar, PTFE, etc) and on the dilution devices required (stack diluting sampler, flux chamber, static hood, etc).

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