Strictly, low potential means that e | ψ ψ | ≪ ≪ k B T ; however, the results that the equations yields are valid for a wider range of potentials, from 50–80mV. Nevertheless, at room temperature, ψ ψ ≤ ≤ 25 m V and that is generally the standard. Some boundary conditions that apply in low potential cases are that: at the surface, the potential must be equal to the surface potential and at large distances from the surface the potential approaches a zero value. This distance decay length is yielded by the Debye length λ λ D equation.