Odor - The Electronic Nose Artificial Olfaction Technology - page 90

Table 4.2 Odor intensity

Odor strength Intensity level

categories

Extremely strong 6

Very strong 5

Strong 4

Distinct 3

Weak 2

Very weak 1

Not perceptible 0

strong. This means that defining an odor criterion based on odor concentration, as
has historically been done for the purposes of managing odor impact on the
community will result in different perceived odor strengths. The only time this will
not occur is when the odor criterion is equal to the detection threshold (i.e., 1 odor
unit) which effectively becomes a ''no impact'' criterion.
The odor intensity result is expressed in ppm (PPM) of butanol (n-butanol) [ 8 ].
A larger value of butanol means a stronger odor, however, not in a simple
numerical proportion. The butanol concentrations provide a referencing scale for
documentation and communication in a reproducible format [ 9 ]. Common butanol
intensity referencing scales include:

• 12-point static scale starting at 10 ppm butanol with a geometric progression of
two,
• 10-point static scale starting at 12 ppm with a geometric progression of two,
• 8-point dynamic scale starting at 12 ppm with a geometric progression of two,
and
• 5-point static scale starting at 25 ppm with a geometric progression of three.

4.5.3 Scaling Methods

There are various methods to scale perceived magnitude, a category scale, which
can be either number- or word-categorized, is commonly used. Numerical values
on this scale do not reflect ratio relations among odor magnitudes (e.g., a value of 2
does not represent a perceived magnitude twice as great as a value of 1).
Table 4.3 gives examples of category scales. Although category scaling pro-
cedures can be advantageous in field situations, ratio scaling techniques are used
frequently in the laboratory [ 10 ]. Ratio scaling procedures require observers to
assign numbers proportional to perceived magnitude. For example, if the observer
is instructed to assign the number 10 to one concentration and a subsequently
presented concentration seems three times as strong, the observer calls it 30; if
another seems one-half as strong, the observer assigns it 5. This ratio scaling
procedure, called magnitude estimation, was used to derive the power function for
butanol (Fig. 4.3 ). Ratio scaling techniques are allowed for such relationships
because they require subjects to produce numbers to match perceived sensations in
which the numbers emitted reflect the ratio relations among the sensations.