Tau is the total residence time: it is n times the single residence time of each vessel.
It can be derived theoretically (e.g. in context with the derivation of the Bo-Method for describing the axial dispersion), but the proportionality can also be understood, when regarding a RTD-Spectrum (the narrower the peak, the greater the number of vessels. A big number of vessels produces a long total residence time). The standard deviation can be approximated by the following formula (from general statistics, valid for gaussian symmetry):
with this information the number of vessels can be estimated.
