Base Number (BN, formerly TBN) – On-Site and Off-Site

BN additives are used to neutralise acidic contamination resulting from combustion.  As such they are only found in diesel engine oils. High BN numbers are associated with larger, mainly marine engines using high sulphur residual fuel. Lube oil base number is evaluated as one might assess remaining fuel in a tank, i.e., it is a negatively evolving number. As the lube oil base number gets lower it is time to change the oil.


 

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There are two popular Off-Site BN test methods:

• ASTM D2896. The titrant utilized is perchloric acid, a very strong acid that titrates virtually all alkalinity in the lube
• ASTM D4739. The titrant utilized is hydrochloric acid, a strong acid but one that only titrates ‘strong’ base. Proponents of this system (most commercial labs) feel the result is more emblematic of remaining, accessible BN for purposes of combustion acid neutralization

The on-site BN test from Kittiwake uses a reagent method that accurately replicates the laboratory results without the complication or risk of an acid titration.   

 

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Maintenance philosophies vary as to when BN should trigger a lube change, 50% of the starting BN, e.g., BN starts at 10, tests at 5. There is confusion over use of high BN cylinder oils in marine applications where the vessel has reason to switch fuels as a result of environmental legislation.  General advice is that this is not a real problem unless operated on an inappropriate BN for the fuel sulphur level for many days.  Lube oil base number is not generally a reason to change oil in applications where fuel sulphur levels are very low, e.g. on highway truck engines operated in Europe or USA.



Above: Failure related to Total Base Number

AN rises and lube oil base number decreases to the point where they’re near equal, e.g., AN is 4.9, BN is 4.8 or vice versa.  In automotive applications, nearly everyone drains lube at 2.0 BN or lower. If one drains lube, one should change the filter(s). You should always know the New Lube’s starting BN in order to monitor decreasing levels as to percentage or limit. In extremely large sumps, those containing high hundreds or thousands of gallons of lube, it is not uncommon to ‘sweeten’ the sump by draining off no more than half the fluid, usually less, then topping up with fresh lube. This is purely an economic consideration meant to forestall or minimize lube consumption expense.