Evaluate parameter value in MATLAB

Hi,

If I have a parameter, e.g.:

>> model.param.set('outer_z', '((volume*formfactor^2)/(2*pi))^(1/3)');

How can I in the easiest way evaluate the value of this parameter (like the 'Value'-column in the GUI)?

I get an error message when trying to use:
>> mphglobal(model,'outer_z')
"com.comsol.util.exceptions.FlException: Unknown dataset feature"

Regards,
Johan