O 10 Q

10 100 1000 PPM BUTANOL (BY VOLUME)

S = kIn on log-log coordinates

S = PERCEIVED INTENSITY k = Y-INTERCEPT I = ODORANT CONCENTRATION n = SLOPE OF PSYCHOPHYSICAL FUNCTION

FIG. 5.26.2 Butanol reference scale.

Ct odorant concentration at threshold

0 No odor

1 Very faint odor

2 Faint odor

3 Easily noticeable odor

4 Strong odor

5 Very strong odor

The limitations of category scaling are that the number of categories to choose from is finite and they are open to bias through subjective number preferences or aversions.

Magnitude Estimation

In magnitude estimation, observers create their own scales based on a specific reference point. The data from several observers is then normalized to the reference point. Magnitude estimation is a form of ratio scaling and has many advantages over category scaling (i.e., an unlimited range and greater sensitivity); however, it does require a more sophisticated observer and statistical analysis.

Reference Scales

The reference scale for measuring odor intensity is standardized as ASTM Standard Practice E 544 (ASTM 1988). The method uses a standard reference odorant, 1-butanol, set in a series of known concentrations. The advantages of this method are that it 1) allows the comparison of subjective odor intensities between laboratories, 2) allows for odor control regulations to be expressed in terms of perceived intensity rather than odor thresholds, and 3) allows cross modality comparisons (i.e., sound and odor). One disadvantage is that some people find it difficult to compare odors that have a different odor character than the reference standard.

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