def check_valid_area(pressure, temperature):
    """Checks valid area of viscosity functions"""
    if (temperature > 900.0 + 273.15 or temperature > 600.0 + 273.15 and 
        pressure > 300.0 or temperature > 150.0 + 273.15 and pressure > 
        350.0 or pressure > 500.0):
        return False
    return True
