As a lubricant ages, oxygen slowly attacks its base oil and forms acidic compounds, and left unchecked those acids corrode metal and lay down varnish. Total acid number is the oil-analysis parameter that measures how far that process has gone. Reported as milligrams of potassium hydroxide per gram of oil, it puts a number on the acidity of an in-service lubricant so a reliability team can watch it climb and act before damage starts. This guide explains what TAN measures, why a rising trend signals base-oil breakdown and additive depletion, and how tracking it against a condemning limit schedules an oil change at the right time.
Total acid number (TAN) in one line: Total acid number, or TAN, is an oil-analysis measurement of the concentration of acidic compounds in a lubricant, expressed as the milligrams of potassium hydroxide needed to neutralise the acids in one gram of oil. Because oxidation of the base oil produces acids, a rising TAN indicates that the oil is breaking down and its antioxidant additives are being used up, so tracking TAN against a condemning limit tells a reliability team when to change the oil before corrosion and varnish begin.
Total acid number quantifies acidity by neutralisation. In the laboratory the acids in a measured amount of oil are titrated with a base, potassium hydroxide, and the result is reported as the mass of that base, in milligrams, required to neutralise the acids in one gram of oil, giving units of mg KOH per gram. A higher number means more acid present. The measurement can be done by a colour-indicator method or, more commonly for dark in-service oils, by a potentiometric method that follows the neutralisation electrically, since a dark oil hides a colour change.
It is important to read TAN as a relative, trended quantity rather than as an absolute pass-or-fail figure that means the same for every oil. Different lubricant formulations start life with different baseline acid numbers because some additives are mildly acidic by nature, so a fresh oil is not necessarily at zero. What matters is the change from that baseline over the life of the charge, so a meaningful TAN programme establishes the new-oil value for each product and watches how far and how fast the in-service oil rises above it.
TAN should not be confused with base number, sometimes called total base number, which measures reserve alkalinity and is the more relevant parameter for engine oils that must neutralise combustion acids. For industrial oils such as turbine, hydraulic, compressor, and gear lubricants, TAN is the primary oxidation indicator because those oils do not carry a large alkaline reserve and their degradation shows up directly as rising acidity. Knowing which number applies to which application keeps the interpretation honest.
The reason TAN is such a valued parameter is that it tracks the fundamental ageing mechanism of most industrial oils: oxidation. When oxygen reacts with the base oil, accelerated by heat, water, and metal catalysts, it forms organic acids, and those acids are exactly what TAN detects. So a TAN that climbs over successive samples is a direct readout of the base oil breaking down. The steeper the climb, the faster the degradation, which is why the trend line matters more than any single value.
A rising TAN usually goes hand in hand with the depletion of the oil's antioxidant additives. Fresh oil is protected by antioxidants that sacrifice themselves to interrupt the oxidation chain, and while they last the acid number stays low and stable. Once they are largely consumed, oxidation accelerates and TAN begins to rise more quickly, which is why the shape of the trend often shows a long flat period followed by a sharper upward turn. That inflection is a useful signal that the oil's protective reserve is running out and that its remaining useful life is shortening.
The consequences of ignoring a rising TAN are concrete. The acids themselves corrode bearing metals, especially soft alloys, and the oxidation products they accompany polymerise into sludge and varnish that foul valves, clog fine clearances, and coat heat-exchange surfaces. Catching the rise early, while TAN is still moderate, allows an oil change or a corrective action before this damage begins, whereas letting it run to a high value invites the very failures the monitoring was meant to prevent. This is why TAN is treated as a leading indicator of oil health rather than a post-mortem measurement.
TAN becomes actionable when it is compared to a condemning limit, the acid number at which the oil is considered no longer fit for service. That limit is usually set relative to the new-oil baseline for the specific product and application, often expressed as an increase above baseline or as an absolute ceiling appropriate to the equipment builder's guidance, because different lubricants and machines tolerate different levels of acidity. When a sample crosses the alarm level below the condemning limit, it is a warning to investigate and plan; when it reaches the condemning limit, the oil should be changed and the cause of the accelerated oxidation, such as overheating or water ingress, addressed.
As with other oil-analysis parameters, the power is in the trend, not the point. Watching TAN across successive samples reveals whether acidity is stable, drifting slowly, or accelerating, and it is the acceleration that lets a team predict when the condemning limit will be reached and schedule the oil change in advance rather than reacting after the oil is already spent. TAN is also strongest read alongside its companions: a rising TAN together with a climbing viscosity and depleted additives paints a coherent picture of oxidative breakdown, while a TAN rise with water present suggests moisture is driving the oxidation.
Holding and trending TAN across a fleet is where a cloud SCADA and asset-monitoring platform helps. In distributed operations such as oil and gas production, turbines, compressors, and hydraulic systems are spread across many remote sites, and a platform such as Merobix can store each asset's TAN history, compare it to the baseline and condemning limit for that oil, and raise an approaching limit to the maintenance team the way a process alarm is raised. Because oxidation is driven by heat, correlating a rising TAN trend with the operating temperatures the platform already logs can point straight at the cause, and generating the oil-change work order from the same system keeps the finding from being stranded in a separate lab report.
It is the mass of potassium hydroxide, in milligrams, needed to neutralise the acids in one gram of the oil sample. A higher number means more acidic compounds are present, so it is a direct measure of the oil's acidity. The measurement comes from titrating the acids in the oil with that base until they are neutralised.
Oxidation, the reaction of oxygen with the base oil accelerated by heat, water, and metal catalysts, produces organic acids, and those acids are exactly what TAN measures. While the oil's antioxidant additives last, TAN stays low, but once they are depleted oxidation accelerates and TAN climbs more steeply. So a rising TAN is a direct signal that the base oil is breaking down and its protective reserve is running out.
Each oil and application has a condemning limit, the acid number at which the oil is no longer fit for service, usually set relative to the new-oil baseline. Reliability teams trend TAN across samples, use an alarm level below the condemning limit as an early warning, and change the oil when it approaches or reaches the condemning limit. Trending the rise lets them predict when that point arrives and schedule the change in advance rather than reacting late.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.