def psy_const_of_psychrometer(psychrometer, atmos_pres):
    """
    Calculate the psychrometric constant for different types of
    psychrometer at a given atmospheric pressure.

    Based on FAO equation 16 in Allen et al (1998).

    :param psychrometer: Integer between 1 and 3 which denotes type of
        psychrometer:
        1. ventilated (Asmann or aspirated type) psychrometer with
           an air movement of approximately 5 m/s
        2. natural ventilated psychrometer with an air movement
           of approximately 1 m/s
        3. non ventilated psychrometer installed indoors
    :param atmos_pres: Atmospheric pressure [kPa]. Can be estimated using
        ``atm_pressure()``.
    :return: Psychrometric constant [kPa degC-1].
    :rtype: float
    """
    # Select coefficient based on type of ventilation of the wet bulb
    if psychrometer == 1:
        psy_coeff = 0.000662
    elif psychrometer == 2:
        psy_coeff = 0.000800
    elif psychrometer == 3:
        psy_coeff = 0.001200
    else:
        raise ValueError(
            'psychrometer should be in range 1 to 3: {0!r}'.format(psychrometer))

    return psy_coeff * atmos_pres