In practice, transformer oil should remain neutral to slightly acidic, and pH is not usually the primary control parameter, but it is still indicative of oil aging. For fresh mineral oil, pH is typically in the range of about 6.5 to 7.5. During service, slight acidification is acceptable, but a pH dropping below roughly 5.5-6.0 usually indicates significant oxidation and formation of acidic by-products. At that point, further testing of acid number (TAN) and sludge formation becomes more important than pH alone.