def _P23_T(T):
    """Define the boundary between Region 2 and 3, P=f(T)

    Parameters
    ----------
    T : float
        Temperature, [K]

    Returns
    -------
    P : float
        Pressure, [MPa]

    References
    ----------
    IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the
    Thermodynamic Properties of Water and Steam August 2007,
    http://www.iapws.org/relguide/IF97-Rev.html, Eq 5

    Examples
    --------
    >>> _P23_T(623.15)
    16.52916425
    """
    n = [348.05185628969, -1.1671859879975, 0.0010192970039326]
    return n[0] + n[1] * T + n[2] * T ** 2
