Why bother? TBN is a dead test, chemical titrations date back to the late 17th century...where is the source of acid (other than weak organic acids)...do you have high sulphur fuel? Here in SE Asia we now have almost suplhur free fuel...I could agree with TBN testing 20 years ago when a certain engine company scared the world with tales of high sulphur fuel resulting from changing sources of crude oil supplies to the refineries of the world, but almost all refineries today are multi-crude capable. Again, I have to ask, where is the strong acid coming from that requires the alkaline additive that is only going to cause more sulfated ash deposits in the piston ring area?
If it is the weak organic acids you are worried about, can you really keep your oil clean of dirt and soot (in todays EGR era) or other contaminants (wtaer glycol etc) to stay in line with the relative amount of weak organic acids? I don't think so...but if you must, try the Ruler instrument instead of TBN....it's easier than trying to make a chemometric FTIR model.