Solver of the Colebrook equation in VB

' Useful implementation of Colebrook-White equation solver
Function Colebrook2(Rr As Double, Re As Double) As Double
  Dim f_min As Double, f_max As Double, f As Double
  Dim g_lower As Double, g_middle As Double
  Dim n As Integer

  f_min = (2.51 / Re) ^ 2 * (1 - Rr / 3.7) ^ (-2)
  f_max = ((2.51 / Re + Log(10) / 2) / (1 - Rr / 3.7)) ^ 2
  n = 0

  Do
    n = n + 1
    f = (f_min + f_max) / 2
    g_middle = -2 / Log(10) * Log(Rr / 3.7 + 2.51 / Re * f ^ (-1 / 2)) - f ^ (-1 / 2)
    g_lower = -2 / Log(10) * Log(Rr / 3.7 + 2.51 / Re * f_min ^ (-1 / 2)) - f_min ^ (-1 / 2)
    If (g_middle = 0 Or (f_max - f_min) / 2 < 0.000001) Then
      Exit Do
    Else
      If (g_middle * g_lower > 0) Then
        f_min = f
      Else
        f_max = f
      End If
    End If
    Colebrook2 = f
    If (n > 100) Then
      Colebrook2 = CVErr(xlErrNA)
      Exit Do
    End If
  Loop
End Function