Odor - The Electronic Nose Artificial Olfaction Technology - page 103

• Static dilution olfactometry
• Dynamic dilution Olfactometry
- free choice (Yes/No Olfactometer)
- forced Choice Olfactometer
- Triangular Forced Choice Olfactometer.
(a) Static Dilution Olfactometry
In Static dilution a sample of odorous gas is diluted in 100 ml glass syringe at
various dilution levels. The diluted samples are expelled into the nostrils of the
panelists. The odor detection threshold is determined graphically from the dilution
levels and panelist response data. A panel of eight panelists is preferred.
(The American Society for Testing and Materials (ASTM D-1391) static
dilution/syringe method was developed in 1978 and was withdrawn by the ASTM
E-18 Committee on March 29, 1986, however, the procedure is still in use,
principally in the USA).
(b) Dynamic Dilution Olfactometry
A dynamic olfactometer provides a continuous and constant-diluted odor
stimulus by mixing controlled flows of sample and odorless air. With dynamic
methods, larger samples are used and dilutions are presented at more reproducible
flow rates and for longer duration for panelists to evaluate. The presentation of
odorous sample dilutions to panelists and their responses depend on three sensory
effects: Judgment criterion, anticipation and adaptation. The judgment criterion
determines how the panelist is to response when asked whether or not an odor is
sensed. This is the case particularly when a single stimulus is presented and a yes
or no answer is requested as to the sensation of odor. The anticipation effect is a
tendency to expect an odor to occur when odor less or weak samples are con-
secutively presented. The adaptation effect is a temporary loss of sensitivity after
smelling an odor. When a weak odor is detected initially, the same odor may not
be detected again after smelling a stronger odor unless the panelist has had suf-
ficient time to recover his or her olfactory sense.
When a forced choice method is used, a panelist, typically trained to conduct
these evaluations, must identify the presentation that is different from the others at
each level, even if it is a guess. This permits use of all the data. The threshold of
detection is the dilution level at which the panelist can determine a difference
between the diluted and the odorless samples. After the detection threshold is
reached, the panelist continues the evaluation at the next level or two to be certain
the identification was not made by chance. Examples of the dilution to threshold
methods include use of scentometery and Olfactometry. Figure 4.10 illustrates the
forced choice Olfactometer and Fig. 4.11 illustrates the dynamic olfactory sche-
matic diagram.