Principle

The odour sample is collected in a bag, which is sealed in a pressurised sample holder. The sample is mixed with odourfree air in custom designed venturi gas jets without passing through a flow measurement device. The dilution ratios are adjusted by a series of needle valves controlled by the digital precision motion controller with ±0.0001 revolution accuracy. The diluted odour sample is released via one of two sniffing cups (another cup contains only odourfree air). In succession, each member of a panel of six human observers compares the odour of the two cups without knowing which cup has the diluted odour sample. The panellists are required to select the cup from which the odour is presented and to nominate the confidence level of their selection (guessing, inkling, or certainty). The olfactometer prepares the next dilution step in ascending concentration level (or descending dilution ratios). The above procedures are then repeated until all panellists detect an odour with the confident level of certainty. The odour concentration is then calculated using the panellists’ responses over the range of dilution settings. There are four operational modes:

Odour concentrations for environmental samples

Odour concentration is the odour strength expressed in terms of the number of times the sample is diluted to reach its odour threshold. At the start of the measurement, determining the presentation series is of paramount importance to exclude the possibility of a bias due to the operator’s preference in choosing the concentration range. The intelligent auto-range feature allows the optimum starting step to be selected based on the panellists’ responses. If more than 50% of panellists record certainty and correct (C&C) responses at the first dilution step, the current responses are discharged and the first dilution step is restarted at a new starting step. If necessary, the process is repeated. Otherwise, the next dilution step is prepared. The first series is completed when all panellists record C&C responses. A fresh starting step for the next series is then calculated according to the result of the first series. The testing is repeated until all panellists record C&C responses. Furthermore, a statistical analysis of the two series is performed to see if two means have a significant difference. If so, a third series will be added and the results will only be based on the two series which show no significant difference.

Odour concentrations for reference gases

The European CEN standard requires a reference gas (such as n-butanol at 60 ppm) to be presented at each panel session. The results are used to monitor the performance of the laboratory and individual panellists’ performances. In this mode, many other reference gases can also be added and their odour thresholds are determined.

Odour intensity

Odour intensity is the perceived psychophysical strength of an odour above its odour threshold. Typically, odour intensity doubles if chemical concentration triples.

The objective of the odour intensity measurement is to experimentally determine the relationship (Weber-Fechner). Normally, five dilution steps and a blank are presented randomly. The minimum dilution step is the level just above its odour concentration. The panellists sniff the diluted odour from the left cup and select one of seven odour intensity levels: no odour, very weak, weak, distinct, strong, very strong and extremely strong. As a result, the regression between odour intensity levels and the dilution ratios can be determined.

Ambient odour Intensity index

Using the odour intensity levels mentioned above, the ambient odours are presented to the panellists. Should the averaged response to the ambient odour be greater than the distinct level, it is regarded that the sample has an odour. The results can be used to solve odour complaints and determine the odour dispersion patterns.