Hi Jingjing,

There may be a prettier way to do this, but I would go about this like so:

1) calculate the HI mass in each cell as HI_density * cell_volume:

HI_mass = sp['enzo','HI_Density'] * sp['dx']**3

2) then summing:

total_mass = sum(HI_mass)



Andrew E.

On Fri, Apr 10, 2015 at 4:20 PM, Chen Jingjing <chenjj235@gmail.com> wrote:
Dear yt-users,
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

--
Jingjing Chen [ chenjj235@gmail.com ]
Department of Astronomy
Columbia University
550 West 120th Street
New York, New York 10027
Cell Phone: +1(347)5746709

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--
Graduate Student 
Columbia University
Department of Astronomy