I am trying to calculate the total HI mass within a sphere. I have a field called 'HI_Density'. And I tried to use the script 

HI_Mass = sphere['enzo','HI_Density'].in_units('Msun').sum()

But I got an error that says the unit code_mass/code_length**3 cannot be converted to code_mass.

So I thought I might need to create a derived_field called 'HI_Mass' which mutiplies the HI_Density with code_length**3. But I don't see that there is any existing field of cell length/volume. So how should I do this?  Or is there an easier way to compute the total HI mass without deriving a field?

Thanks,

Jingjing

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