The concept of water activity (aw) in transformer oil is mainly addressed in IEC 60814 and more explicitly in IEC 60422, which describes interpretation of dissolved water and moisture equilibrium between oil and paper. These standards explain the relationship between aw, relative saturation, and ppm at a given temperature. IEEE guides (such as IEEE C57.106) also reference relative saturation, but IEC documents are the primary source for formal use of water activity in diagnostics.