code: def _len_tube(Flow, Diam, HeadLoss, conc_chem, temp, en_chem, KMinor):
      
      num1 = pc.gravity.magnitude * HeadLoss * np.pi * (Diam**4)
      denom1 = 128 * viscosity_kinematic_chem(conc_chem, temp, en_chem) * Flow
      num2 = Flow * KMinor
      denom2 = 16 * np.pi * viscosity_kinematic_chem(conc_chem, temp, en_chem)
      len = ((num1/denom1) - (num2/denom2))
      return len.magnitude
docs: Length of tube required to get desired head loss at maximum flow based on
      the Hagen-Poiseuille equation.