def uch_psri(x, dp, d_pipe, gs, rhog, rhos, ut):
    """
    Choking velocity from PSRI 2016 notebook. Uses MKS units for input
    parameters but returns Uch in ft/s. Function requires SciPy fsolve to solve
    for Uch.

    Parameters
    ----------
    x : float
        Solve for Uch with SciPy fsolve
    dp : float
        Particle size [m]
    d_pipe : float
        Pipe diameter [m]
    gs : float
        Solids mass flux [kg/(s m^2)]
    rhog : float
        Density of gas [kg/m^3]
    rhos : float
        Density of particle [kg/m^3]
    ut : float
        Terminal velocity [m/s]

    Returns
    -------
    uch : float
        Choking velocity [ft/s]

    References
    ----------
    Notebook from PSRI 2016 workshop, pages J-20 and J-72.
    """
    uch = x
    g = 32.2
    dp = dp * 3.28084
    d_pipe = d_pipe * 3.28084
    gs = gs * 0.204817303
    rhog = rhog * 0.062428
    rhos = rhos * 0.062428
    ut = ut * 3.28084
    f1 = (uch - ut) / (g * dp) ** 0.5 - (gs / (uch * rhog)) ** 0.35 * (d_pipe /
        dp) ** 0.35 * (rhos / rhog) ** 0.1
    return f1
