The α values is defined by the multipass test according to ISO 4572. It is a measure of the total quantity of ACFTD supplied to the filter element until the permissible final pressure difference is reached at the rated flow rate QN:
γ = contamination with ACFTD in the flow rate,
Q = flow rate through the filter,
τ = test time until the permissible final pressure difference is reached.
The α value corresponds to the term "apparent dirt holding capacity". It is used to compare the service life of filter elements of the same size with the same β value. The α values is a calculated variable that does not take into account if the filter element actually absorbs the quantity of dirt supplied. If an element has poor filter performance the numerical value for α is very high. It must therefore always be evaluated in conjunction with the filter cumulative efficiency, otherwise it will cause confusion. See also dirt holding capacity of a filter