why when calculating rise time we use 2.2 × tau? (RC Low Pass Circuit)
For charging: 

$$V(t) = V_0(1 - e^{-t/\tau})$$

Lets say rise time 10% to 90% of V0. So in this equation put: $$V(t) = 0.1 \cdot V_0$$ 
and find \$t_1\$. Similarly put: $$V(t) = 0.9 \cdot V_0$$ and find \$t_2\$. You'll get: $$rise time = t_2-t_1 = 2.2 \tau$$