def is_critical_flow(P1, P2, k):
    r'''Determines if a flow of a fluid driven by pressure gradient
    P1 - P2 is critical, for a fluid with the given isentropic coefficient.
    This function calculates critical flow pressure, and checks if this is
    larger than P2. If so, the flow is critical and choked.

    Parameters
    ----------
    P1 : float
        Higher, source pressure [Pa]
    P2 : float
        Lower, downstream pressure [Pa]
    k : float
        Isentropic coefficient []

    Returns
    -------
    flowtype : bool
        True if the flow is choked; otherwise False

    Notes
    -----
    Assumes isentropic flow. Uses P_critical_flow function.

    Examples
    --------
    Examples 1-2 from API 520.

    >>> is_critical_flow(670E3, 532E3, 1.11)
    False
    >>> is_critical_flow(670E3, 101E3, 1.11)
    True

    References
    ----------
    .. [1] API. 2014. API 520 - Part 1 Sizing, Selection, and Installation of
       Pressure-relieving Devices, Part I - Sizing and Selection, 9E.
    '''
    Pcf = P_critical_flow(P1, k)
    return Pcf > P2
r'''Determines if a flow of a fluid driven by pressure gradient
    P1 - P2 is critical, for a fluid with the given isentropic coefficient.
    This function calculates critical flow pressure, and checks if this is
    larger than P2. If so, the flow is critical and choked.

    Parameters
    ----------
    P1 : float
        Higher, source pressure [Pa]
    P2 : float
        Lower, downstream pressure [Pa]
    k : float
        Isentropic coefficient []

    Returns
    -------
    flowtype : bool
        True if the flow is choked; otherwise False

    Notes
    -----
    Assumes isentropic flow. Uses P_critical_flow function.

    Examples
    --------
    Examples 1-2 from API 520.

    >>> is_critical_flow(670E3, 532E3, 1.11)
    False
    >>> is_critical_flow(670E3, 101E3, 1.11)
    True

    References
    ----------
    .. [1] API. 2014. API 520 - Part 1 Sizing, Selection, and Installation of
       Pressure-relieving Devices, Part I - Sizing and Selection, 9E.