At KEM, we market density reference solutions with known density values.
https://www.kem.kyoto/en/products/density/den_sol/

KEM density reference solutions are JCSS accredited, and are considered traceable reagents under the Japanese national standards.
If the accuracy in the instrument specifications is obtained using these density reference solutions, then instrument integrity has been demonstrated.
To purchase a density reference solution, contact your KEM sales representative.