AutoCAD Inventor :: Negative Mass Calculation In 2013?

My company recently installed Inventor 2013. In previous versions we used a part with negative density similar to seawater to calculate submerged weight and CoG.

In IV 2013, it seems that the ability to calculate negative mass is not longer available.

This is how it worked in pre 2013 IV, when I could specify a negative density.

What I have is a spool made of steel. This spool is modelled as an iAssembly with four configurations:

-In air, empty

-In air, filled with glycol

-Submerged, empty

-Submerged, filled with Glycol

For this example, I now need three parts:

-Steel Pipe

-Glycol derived from the inside volume the pipe

-Seawater buoyancy derived from the outside volume of the pipe.

Each part has its own density, and the assembly weight and CoG correspond to the assembly configuration (with or without glycol) and environment (in air or submerged).

The seawater buoyancy represents the water displaced by the total spool assembly, regardless of the contents. To achieve correct weight in the submerged state, the buoyancy material has to have a negative density.

I do not want to make any of the mass parameters static, and I also fear that the CoG might not be updated correctly when using that method.

As I no longer can define a negative density, any optional method to achieve this without overriding the mass value.

Autodesk Product Design Suite 2013 SP1
Win 7 Enterprise 64bit
Intel Xeon CPU E3-1240 V2 @ 3.40GHz
16Gb Ram
Nvidia Quadro 2000