ASTM D664 is a laboratory method that measures the acidity of an insulating liquid by titrating a sample with a standard base (potassium hydroxide) while a pH electrode monitors the progress of the reaction, then records the volume of base consumed to reach a specific endpoint. The result is expressed as acid number in milligrams of potassium hydroxide (KOH) per gram of sample. This test applies to mineral oils, silicones, and less-flammable hydrocarbon liquids drawn from transformers, voltage regulators, oil circuit breakers, reclosers, and other liquid-filled apparatus. Unlike the visual or electrophoretic acid-number methods ASTM D974 and others employ, the potentiometric approach automatically detects the endpoint electronically, reducing operator interpretation and improving reproducibility across labs.
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 representative sample is drawn per ASTM D923 from a live sampling valve, or from a bottom drain if deliberate sediment inspection is the goal, and placed in a clean, dry, light-protected container to prevent further oxidation and moisture ingress during shipment. At the lab, the sample is prepared per ASTM D664 (typically dissolving a measured mass in a solvent), then titrated in a potentiometric titrator: the sample solution is placed in the titration cell with a standard KOH titrant and a combined pH electrode; the titrant is added incrementally or continuously while the electrode monitors pH, and the instrument automatically identifies the endpoint (or endpoints, if the sample contains both weak and strong acids) and records the volume of titrant consumed. The instrument then calculates and reports the total acid number.
The acid number result in milligrams KOH per gram of sample, the sample temperature at test, the liquid type (mineral oil, silicone, or less-flammable hydrocarbon), and the test date and location. The sampling site (top, middle, or bottom drain), the tester's name or initials, any customer comments or prior history of the liquid, and the lab's instrument identification and calibration status are also captured so results can be audited and trended over time.
NETA ATS-2025 Table 100.4.1 sets an upper limit on acid number for new mineral oil; silicone and less-flammable hydrocarbon liquids are evaluated against their own respective limits in Tables 100.4.2 and 100.4.3. A result at or below the applicable table limit is acceptable at acceptance; in maintenance, trending is as important as any single number - a gradual year-over-year rise may not trigger rejection immediately, but it signals that the liquid is aging and reconditioning or replacement should be scheduled before the limit is breached. A result sharply above the limit, or a sharp upward jump from the prior baseline, calls for investigation and typically leads to reconditioning or replacement before the unit returns to service. See the purchased NETA standard for the complete table and any equipment-type or voltage-class exceptions.
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