Transformer, Liquid-Filled
ASTM D664 measures the acid number of a petroleum product by potentiometric titration: the sample is dissolved in a toluene and propan-2-ol solvent and titrated with alcoholic potassium hydroxide while a glass electrode tracks the potential, and the result is reported in mg KOH per gram of sample. Method A covers petroleum products and lubricants soluble in toluene and propan-2-ol (the fluid specifications, not D664, name it for insulating oil); Method B is written for biodiesel and biodiesel blends. Within Method A the end point is either a clear inflection in the titration curve (IEP) or the reading of a pH 10 buffer (BEP), which used oils usually need. It is a companion to the color-indicator method ASTM D974 and need not agree with it numerically.
Acid in insulating liquid is produced by oxidation - a degradation process that begins as soon as the oil breathes air in the conservator and accelerates with heat, moisture, and contamination. Acid attacks copper windings and accelerates the formation of sludge and varnish on core and coil surfaces, reducing insulation strength and heat transfer. A rising acid number over time signals approaching depletion of the liquid's antioxidant reserve and is often an early sign that reconditioning or replacement is needed before the liquid becomes so degraded that it loses its insulating or cooling ability. Screening acid number at acceptance also catches poor-quality fill or fill that was damaged during transport.
Acceptance: the acid number is measured before a new or reconditioned transformer or oil-filled device is energized, typically as one item in a multi-test insulating-liquid battery run on the same sample alongside dielectric breakdown, water content, and power factor. Maintenance: repeated on a periodic interval (often annually or every few years depending on the asset criticality and age) or triggered by a through-fault, dissolved-gas alarm, or visual sign of degradation such as darkening or sludge, and compared against the previous baseline and NETA acceptance limits to detect trending.
A sample is drawn per ASTM D923; a used-oil sample is shaken to suspend any sediment and tested promptly. In the laboratory the electrodes are checked against pH 4 and pH 7 buffers, the pH 4 and pH 10 buffer readings are taken daily, and a solvent blank is run. A weighed specimen is dissolved in 60 or 125 mL of solvent and titrated in small increments, by hand or with an automatic titrator, until the potential passes the basic buffer reading. The acid number is calculated from the titrant volume at the end point less the blank. For expected acid numbers from 0.05 to under 1.0 the specimen is about 20 g in 125 mL or 10 g in 60 mL; chloroform in the solvent and other departures are reported because they move the result outside the precision statement.
The acid number in mg KOH/g, the end point type (IEP or BEP), the solvent volume (60 or 125 mL), the specimen mass, whether chloroform was used, the strong acid number when a pH 4 end point is found, and the acid number as reported under the method's rounding, with the liquid type and the companion insulating-liquid results from the same sample.
ASTM D664 sets no limit. The entered two-decimal value is graded against ASTM D3487 for new mineral oil, ASTM D6871 for new natural ester (written for the color-indicator method), ASTM D4652 for new silicone and the NETA ATS and MTS tables for oil received in new equipment and in service. The method reports to the nearest 0.05 mg KOH/g below 2.00, so the report shows that rounded figure beside the graded one. Its precision statement starts at 0.5 mg KOH/g, while transformer oil acid numbers sit well below that, so results there are indicative, and a D664 result should not be compared with a D974 result without naming the method.
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