The outlet concentration $C_A$ of a plug flow reactor $\text{(PFR)}$ is controlled by manipulating the inlet concentration $C_{A 0}$. The following transfer function describes the dynamics of this $\text{PFR.}$
$$\frac{C_A(s)}{C_{A 0}(s)}=\exp \left[-\left(\frac{V}{F}\right)(k+s)\right]$$
In the above equation, $V=1 \mathrm{~m}^3, F=0.1 \mathrm{~m}^3 \mathrm{~min}^{-1}$ and $k=0.5 \mathrm{~min}^{-1}$. The measurement and valve transfer functions are both equal to $1.$ The ultimate gain, defined as the proportional controller gain that produces sustained oscillations, for this system is______________ (rounded off to one decimal place).