I proceeded as follows (thanks @Batch123 by the way):
Batch123 offered me to create a field in my personal properties where I would get my SW_masse but the problem was that the value displayed did not allow me to know in which unit it was given (g or kg)
So I did the same thing but by creating a field RECUP_VOLUME where I go to look for my SW_volume and there I get a value of which I am sure of the unity (mm3). All I have to do now is refresh my personal properties and display my RECUP_VOLUME column. This value will then be processed in excel to determine the weight and that's it.
However, this does not explain why I couldn't directly integrate a SW_volume column without going through the custom properties.
In short, the ways of the S.............. Olidworks are impregnable!!
the value of the SW volume is like that of the mass, the unit can change depending on the parts since it is managed at the level of each file (tools/option/document properties/units)
If no one has tinkered with the volume units in the rooms, that's OK, but there may be problems (especially if there are already differences between the mass units).
A solution in excel is to recalculate a ratio between mass and volume: if there are problems with units we will find ourselves with a density greater than 10 or less than 1 (not very common if we work with base metals) -> a simple sorting on the column in question will allow us to find any problem parts.
Indeed, even if a priori the unit of volume is common to all parts unlike the weight, it is a good idea to add a test by checking the weight/volume ratio.
Of course, I will have to be careful, because if ever by bad luck I come across a piece whose unit of weight AND volume are different, I can very well end up with a "correct ratio" when my two units are not the ones I want.