New issue 1037: Units system has problems in comparing between cosmological datasets with different scale factors https://bitbucket.org/yt_analysis/yt/issue/1037/units-system-has-problems-in... Cameron Hummels: At present, when you try to compare/combine quantities from two separate datasets, yt defaults to internally converting those quantities to non-comoving physical units before making the comparison/operation. I think this *should* be the default behavior as it covers the bulk of the use cases, but it would be good to allow for comparisons/combinations using comoving units (of different scale factors) as well. I recently ran into this issue in trying to compare two separate datasets at different redshifts (ie scale factors) from a cosmological simulation where the units were enzo's `code_length`, which is in comoving units. In the end, I had to just grab the `.v` values out of the `YTArray` in order manually subtract them because subtracting using the `YTArray` yielded incorrect answers. Here is an example demonstrating this problem. Let's say a galaxy sits at the center of this box at both z=99 and z=0. we can see how far it has moved in comoving units by subtracting off it's position at z=0 from it's position at z=99. http://paste.yt-project.org/show/5655/ and its output: http://paste.yt-project.org/show/5656/ I think this could be addressed by just creating an additional attribute on `YTArrays` for converting to comoving units when making comparisons/operations with other `YTArrays`.