'W' in EIS (equivalent circuit) represents the Warburg impedance which accounts the diffusion controll charge transfer process at the electrode/electrolyte interface resulting impedance Phase angle 45^o. It has unit of ohm (\Omega) or ohm cm^2 (\Omega cm^2). Some time W is also used for Warburd admittance having unit Siemens (\Omega^-1) or Siemens cm^-2. In low frequency or at high time limit , when diffusion layer thickness is grown sufficiently that it can accomodate the pseudocapacitive volumetric charges due to faradaic charge transfer, W approches unity. This can be obtained mathematically applying Tanh(x)=x for low x in finite length Warburg impedance expression.