def __zedwalther(kin):
    """

    Calculate the z-parameter for the Walther equation (ASTM D341).

    Parameters
    ----------
    kin: scalar
        The kinematic viscosity of the lubricant.

    Returns
    -------
    zed: scalar
        The z-parameter.

    """
    zed = kin + 0.7 + 10 ** (-1.47 - 1.84 * kin - 0.51 * kin ** 2)
    return zed