Material file for argon gas

I'm doing some CFD work that involves argon. The material files for argon include "Argon [gas]," which has has the density given by the function 522.2077 * T^-1.

I'm not sure where that expression is from, and when I use something that makes sense (to me), i.e. 0.039946 * pA/Rconst/T, I obtain different results.

So I have two questions:

1) Where does the 522.2077 * T^-1 expression come from?

2) Am I making a mistake by using my own expression for density?

Thanks!