def freePathCalc(P, T, lambdar = 6.730*10**-8, Pr = 101.3 , Tr = 296.15, S = 110.4):
    """
    Calculates gas mean free path in m
    
    Parameters
    ----------
    P : float
        pressure
        
    T : float
        Temperature (K)
        
    lambdar : float
        reference mean free path (m)
        
    Pr : float
        Reference pressure (kPa)
    
    Tr : float
        Reference Temperature (K)
    
    S : float
        Sutherland constant (K)
        
    Returns
    -------
    lambdao : float
        Gas mean free path in m
    """
    
    lambdao = lambdar * (Pr/P)*(T/Tr)*((1+S/Tr)/(1+S/T))
    
    return lambdao