Which of the following parts per unit is commonly used to report SOAP results?

The correct choice is commonly expressed using parts per million (ppm) because this unit offers a practical way to quantify the concentration of contaminants detected in engine oil analyses through the Solid Oil Analysis Program (SOAP). Using parts per million allows for easier interpretation of results, especially since it corresponds to commonly encountered contamination levels in aviation engines.

For instance, when analyzing oil for wear metals or other contaminants, many findings will typically fall within the range of a few to several thousand parts per million, making ppm a suitable scale for reporting. Understanding results in terms of parts per million can help maintenance personnel and engineers assess the health of the engine and determine necessary actions based on industry thresholds and standards.