A measure of how strong an odour may be based on an initial perception. Odour intensity and odour concentration have a non-linear relationship. For instance, if two distinct odours are both at a concentration of 3 OU/m3, the resultant odour intensity measurement may result in one being more offensive than the other. For example, a low odour concentration of hydrogen sulfide (rotten egg smell) may have a higher odour intensity due to the nature of the smell.
Unlike measurements of odour concentration that take place at the detection threshold (1 OU/m3), odour intensity measurements are taken at and beyond the detection threshold. Note that the odour concentration is known for each odour intensity measurement, since the detection threshold is known a prior and can be adjusted by changing the dilution.
Once the odour concentration and intensity is determined, a relationship exists between odour concentration and intensity and can be expressed in a nonlinear mathematical relationship. Intensity increases linearly with the logarithm of the odour concentration. Some formulae include Weber Fechner Law (exponential function) and Stevens Law (power function). Webner Fechner Law is shown below:
I = kw log(C/Co) + const
I: Odour Intensity
kw: Weber-Fechner constant
Co: Concentration of odorant at the detection threshold (by definition equals 1OU when
using odour units);
const: a constant which relates to the use of mean intensity levels that can be calculated from the line of best fit for each odorant.
An example calculation can be found in the German Standard VDI 3882 for the Webner Fechner Law.
Stevens Law is shown below:
I = k(C)n
I represents odour intensity
This function can be plotted as a straight line in logarithmic coordinates as:
log I = log K + nlog (C)