Odor - The Electronic Nose Artificial Olfaction Technology - page 106

During an odor test, the odor panelist (assessor) sniffs a diluted sample of the
odor as it is discharged from the olfactometer as one of three sample presentations
(one presentation with the diluted odor and two with odor free air). The assessor
sniffs all three of the presentations and must select the one of the three that is
different from the other two, even if they must guess. This statistical approach is
called ''triangular forced-choice.'' The assessor continues to additional levels of
higher concentration (lower dilution) presentations following this procedure.
Therefore, ''odor concentration'' or odor strength is a number derived from the
laboratory dilution of collected odors. The dilution ratio (total presentation volume
divided by odor sample volume) at each sample presentation level is used to
calculate the concentration of the evaluated sample.
The individual thresholds of eight to ten assessor responses are averaged to
determine the detection threshold for which 50 % of individuals will observe the
presence of an odor. The ''detection threshold'' value that is obtained from odor
testing is derived from the dilution ratios, and is therefore dimensionless. How-
ever, the pseudo-dimensions of ''Odor Units'' (O.U.) or ''Odor Units per Unit
Volume'' are commonly applied. For example: ''Odor Units per cubic meter.''
It should be noted that the dilution of the actual odor emission sample by the
olfactometer is the physical process that occurs in the atmosphere downwind of
the odor source. The ''receptor'' (citizen in the community) receives and sniffs the
diluted odor. The dilution ratio is an estimate of the number of dilutions needed to
make the actual odor emission ''non-detectable'' (Detection Threshold). If the
receptor detects the odor, then the odor in the atmosphere is above the receptor's
detection threshold level.

Appendix I: Odor Descriptors for Commonly Encountered

Compounds [ 24 - 26 ]

Substance Odor Substance Odor

Acetaldehyde Apple, stimulant Dimethyl sulfide Rotten vegetable

Acetic acid sour vinegar Diphenylamine Floral

Acetone chemical/sweetish/solvent Diphenyl sulfide Burnt rubber

Acetonitrile Ethereal Ethanol Pleasant, sweet

Acrylaldehyde Burning fat Ethyl acetate Fragrant

Acrolein Burnt sweet, pungent Ethyl acrylate Hot plastic, earthy

Acrylonitrile Onion, garlic, pungent Ethylbenzene Aromatic

Aldehydes C9 Floral, waxy Ethyl mercaptan Garlic/onion, sewer,

decayed cabbage, earthy

Aldehydes C10 Orange peel Formaldehyde Disinfectant, hay/straw-

like, pungent

Allyl alcohol Pungent, mustard-like Furfuryl alcohol Ethereal

(continued)