If I dont get the tbn result, how can I comment this situation. Oil is diesel engine 15w40. I tried 5 times with different weights but everytime I got minus result.

Mr.Hilker,

Please try to run a standard sample to confirm the accuracy of the TBN instrument.

I did, I also changed the electrode's solvent. I think that diesel ongine oils have high tbn values against oxidation formed when an engine works. minus value means tbn was exhausted and zero. am I wrong?

How are you doing your TBN? Type of acid, direct titration versus back titration, etc? Not sure how you get a negative number. The organic acids produced from oxidation during engine use can have a buffering effect on some TBN tests.