def etaCalc(T, Tr = 296.15, S = 110.4, nr = 1.83245*10**-5):
    """
    Calculates dynamic gas viscosity in kg*m-1*s-1
    
    Parameters
    ----------
    T : float
        Temperature (K)
        
    Tr : float
        Reference Temperature (K)
    
    S : float
        Sutherland constant (K)
        
    nr : float
        Reference dynamic viscosity
        
    Returns
    -------
    eta : float
        Dynamic gas viscosity in kg*m-1*s-1
        
    """
    eta = nr * ( (Tr + S) / (T+S) )*(T/Tr)**(3/2)
    return eta