def feldspar(piezometer=None):
    """ Data base for calcite piezometers. It returns the material parameter, the exponent
    parameter and a warn with the "average" grain size measure to be use.

    Parameter
    ---------
    piezometer : string or None
        the piezometric relation

    References
    ----------
    | Post and Tullis (1999) https://doi.org/10.1016/S0040-1951(98)00260-1

    Assumptions
    -----------
    - The piezometer of Post and Tullis (1999) requires entering the median
    apparent grain size calculated from equivalent circular diameters (ECD) with
    no stereological correction. The function will convert this value to the mean
    linear intercept (LI) grain size using the De Hoff and Rhines (1968) empirical
    relation LI = ECD / sqrt(4/pi)

    Returns
    -------
    The differential stress in MPa, a floating point number
    """

    if piezometer is None:
        print('Available piezometers:')
        print("'Post_Tullis_BLG'")
        return None

    elif piezometer == 'Post_Tullis_BLG':
        B, m = 433.4, 1.52
        warn = 'Ensure that you entered the apparent grain size as the median in linear scale'
        linear_interceps = True
        correction_factor = 1.0

    else:
        feldspar()
        raise ValueError('Piezometer name misspelled. Please choose between valid piezometers')

    return B, m, warn, linear_interceps, correction_factor