What are the critical factors in measurement of odour concentration?
A dynamic olfactometer is a gas diluting apparatus and also an interface between a panel of human observers and an odorous gas sample diluted at various concentrations. Olfactometry requires a very high standard of testing conditions. These include an odour-free testing environment, a highly accurate and repeatable olfactometer and effective panelist management.
Odour-free testing environment: An odour-free testing environment is an important element in the olfactometry testing process. An odour-free room provides not only a relaxed resting environment for the comfort of panelists, but also eliminates background odour that may lead to olfactory adaptation and fatigue, which would otherwise affect the sense of smell of panelists and result in a failure to detect odour at low concentrations. The test room air should be filtered using an activated carbon filter with a minimum air exchange rate of at least 12 times per hour.
Olfactometer calibration: The olfactometer must be calibrated against a tracer gas to check that the dilution setting of the olfactometer meets repeatability and stability criteria. It is not sufficient to check the dilution setting simply against gas flow rate. Firstly, the olfactometer must be able to demonstrate its stability within 5% at each dilution level. This ensures that all panelists receive the same level of odour concentration. Secondly, the olfactometer must be able to repeat each dilution level within 20% of the setting value. This ensures that the dilution steps will be evenly distributed across the testing range. Further details are provided in Dutch standard NVN 2820 and the draft CEN/TC264 standard.
Panelist management: Panelists should be trained and screened using reference air incorporating certified n-butanol at a concentration of 60 ppm using the same procedure as used for environmental samples. Means and standard deviation are calculated for 12 logarithms of individual thresholds (break points). The criterion is that means should be in the range of 3.00 to 4.38 with a standard deviation ofless than 0.916.