Odor - The Electronic Nose Artificial Olfaction Technology - page 89

stimulus. The detection threshold occurs at a lower concentration than the rec-
ognition threshold.
Odor concentration is measured as dilution ratios and reported as Dilution
Threshold and Recognition Threshold or Dilution to Threshold (D/T) and some-
times assign the pseudo-dimension of odor units per cubic meter. Dilution to
Threshold (D/T) ratio is a measure of the number of dilutions needed to make the
odorous air nondetectable. Odor unit is the concentration divided by the threshold.

4.5.2 Odor Intensity

It is the perceived strength of an odor above its threshold. It is related to the
odorant concentration. It is determined by an odor panel and is described in
categories which progress from ''not perceptible'', then ''very weak'', through to
''extremely strong'' [ 7 ].
Generally odor intensity increases with the odorant concentration. The rela-
tionship between intensity and concentration can be expressed as:

I ¼ k ðÞ n
or Log I ¼ Log K þ n Log

ðÞ

where,
I Intensity;

C Concentration;

K Constant; and

n Exponent.

This is known as Stevens' law or the power law [ 8 ]. Odors are ranges (n) from
about 0.2-0.8, depending on the odorant. For an odorant n = 0.2, a tenfold
reduction in concentration decreases the perceived intensity by a factor of only 1.6,
whereas for an odorant with n = 0.8, a tenfold reduction in concentration lowers
the perceived intensity by a factor of 6.3. This is an important concept that is
related to the basic problem of reducing the odor intensity of a substance by air
dilution or other means. Odor Intensity is expressed in ppm of butanol.
The German standard Olfactometry Determination of Odor Intensity VDI 3882
Part 1 (VDI, 1992) provides qualitative descriptions of odor intensity with a
numerical scale that may be used in back-calculating the corresponding odor
concentration. These descriptions are represented in Table 4.2 .
Odor intensity is a useful dimension to compute because some odors are per-
ceived as being stronger than others. In other words, all odors will be just
detectable at a concentration of 1 OU/m 3 , however, at twice the concentration, or
2 OU/m 3 , some odors may be perceived as very weak while others may be per-
ceived as distinct. At ten times the concentration, or 10 OU/m 3 , one odor may be
perceived as distinct while another odor at 10 OU/m 3 concentrations may be very