Volatile functions across books

Hello, I have a problem with volatile functions.

I created my own volatile function that I use in some cells and recalculates itself when something is changed in any sheet of my opened workbook.That function is stored in a .xla module that I use in several workbooks. The problem is that when I open other workbook the values of one spreadsheet are updated with the calculations of the other one. How can I avoid that?

This is a simplified example:

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Saludos
Juan Carlos del Río