def density_standard(components):
    """
    Natural gas density at standard temperature, kg/m3
    :param components: (list)  List of gas components. Each item is an object of class GasComponent
    :return: (float) The density of natural gas an standard parameters, kg/m3
    """
    return sum([component.density_standard * component.volume_percentage * 0.01 for component in components])