ASTM D974 is a laboratory test that measures the cumulative concentration of acidic compounds dissolved in a sample of insulating liquid - such as mineral oil, silicone, or a less-flammable hydrocarbon - drawn from a transformer, voltage regulator, oil circuit breaker, recloser, or other liquid-filled apparatus. The test involves titration with a standardized potassium hydroxide solution to neutralize the acids present in the sample, and the result is reported in milligrams of potassium hydroxide per gram of liquid (mg KOH/g). The method can also determine a strong acid number if the sample contains strong acids that behave distinctly from the weaker organic acids, reported in the same units. Acid number is one item in the broader insulating-liquid screen NETA specifications call out alongside dielectric breakdown voltage, color, visual condition, water content, and power factor.
Acidic compounds form inside transformer oil and other insulating liquids as the fluid oxidizes over time - a process that accelerates with exposure to heat, air, and water. Acids corrode copper windings and degrade paper insulation, setting up an internal failure that can progress to an unplanned outage or catastrophic event. A steadily rising acid number in successive maintenance samples drawn from the same unit over years is an early signal that oxidation is accelerating faster than the unit's natural degradation curve, pointing to a gasket leak, prolonged overtemperature operation, or another condition that demands investigation before the unit fails. A high acid number on a newly received transformer or one returning from repair can reject the unit or demand reconditioning before energization.
Acceptance: the liquid is screened before a new or reconditioned unit is first energized, confirming the fill meets new-liquid limits and establishing a baseline for the unit's useful life. Maintenance: the same test is repeated on a periodic interval - typically every one to three years for units in continuous service - or triggered by an event such as a through-fault, a rise in dissolved-gas levels, or a visible darkening of the fluid, and is ordinarily drawn alongside the rest of the insulating-liquid battery and a dissolved-gas analysis sample so the lab results tell a complete story rather than one number in isolation.
A representative sample is drawn per ASTM D923 from a live sampling valve after flushing, or from the bottom drain if monitoring for settled sediment or free water is the goal; the live sample is treated as representative of the bulk liquid, while bottom samples are not. The sample is placed in a clean, dry, light-protected container and shipped and stored to keep moisture and light exposure to a minimum before it reaches the lab. There, the analyst uses a calibrated potentiometric or visual titration apparatus to add potassium hydroxide solution drop by drop until the sample changes color (or the electrode potential shifts, in potentiometric mode), marking the point where all acids in the sample have been neutralized. The volume of titrant used is converted to acid number in mg KOH/g using the sample mass and the solution's known concentration.
The acid number in mg KOH/g, and if determined, the strong acid number in the same units, captured alongside the companion insulating-liquid screen results drawn from the same sample: dielectric breakdown voltage, specific gravity, interfacial tension, color, visual condition, water content, and power factor at 25 degrees Celsius. The sample origin, temperature when drawn, and test-instrument identification (make, model, serial, calibration due date) are recorded so the result can be reproduced and trends tracked over time.
The maximum acceptable acid number is set by NETA ATS-2025 for new liquid and NETA MTS-2023 for in-service liquid, with different limits depending on liquid type (mineral oil, silicone, less-flammable hydrocarbon) and voltage class. For mineral oil in a new transformer, the acceptable maximum is a single value that applies across all voltage classes; for liquid that has been in service, the acceptable maximum is ordinarily higher to reflect the fluid's normal aging, and may include an advisory that a sharp upward trend between consecutive samples, even if both fall below the limit, warrants reconditioning or replacement. A result above the applicable limit calls for investigation and usually liquid reconditioning or replacement before the unit is energized or returned to service. See the purchased standard for the complete acceptance table and any liquid-type or voltage-class exceptions.
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