JonnyC
Basically during use, antioxidants in oil deplete, when antioxidants reach around 10 - 20% of original concentration, oxidation accelerates and carboxylic acids (AN)accumulate and oil basestock starts to polymerize (viscosity increase/varnish formation).
In combustion engines, the oils contain additives (BN)to neutralize combustion gases. Some high temperature hydraulic fluids also have additives to neutralize strong acids from additive decomposition - basically independent of oxidation. So BN is only useful in systems that create strong acids.
RULER is an on-ste instrument that primarily measures antioxidants - FTIR, LC and other laboratory methods also available.
AN/BN are usually done by titration on-site or in lab although RULER and FTIR non-titration methods also for AN/BN.
The time from when the antioxidants deplete until AN increases is application dependent. The higher the operating temperature, the shorter the time until AN exceeds limits. So for low temperature applications with big oil reservoirs, the time between antioxidant depletion and AN increase can be months/years. For higher temperature systems, the time between depletion and AN increase can be days.
As with any oil condition trending program, you have to develop some history to understand the degradation mechanisms so that you can determine which technique is of most value. RULER is predictive/can detect abnormal conditons early on (once antioxidant depletion rate established can extrapolate to predict future readings such as when the oil will break)while AN is reactive (can't predict when oil will break)has been around for a very long time and most applications have well established limits for safe usage although recent changes in basestocks and antioxidants are requiring that some established limits and capabilities of other techniques be revisited.