Why "Do" function is not returning a list?


Here is a snippet of my code:

miuD = -22880.5;

M = 107.87;

k = 1.3807*10^-23;

GGvarT[x_] = 3/x^3NIntegrate[y^2Log[1 – Exp[-y]], {y, 0, x}];

Pvar[T_] =
Exp[miuD/T + 3GGvarT[thetaD/T] + Log[1.8810^20kM^1.5*T^2.5]];

Ptest = {}; Do[Ptest = Pvar[T], {T, 830, 1224}];Ptest

Mathematica output -> Ptest = 0.00190348

I was hoping to get a list of Ptest for T ranging from 830 to 1224. Instead, I am only getting the value of Ptest at T=1224. Help!