Reproduction of results

I have permanently problems with the reproduction of titration curves, using 848 Titrino plus, even for the simplest titration, 0.02 N HNO3 with 0.02 N NaOH. For the titrations, made in the same day, the equipment displays different allure (form) of curve just in the vicinity of the equivalence point. When I calculate the first derivative, using the Origin 6.1 program), I obtain different values of V(NaOH) and pH at the equivalence point (EP), while the apparatus shows other values.

The method parameters are following:

1) MET – monotone titration at the EP;

2) The titrant is a 0.02 N NaOH. Titration solvent is a solution of 5 mL of 0.02 N HNO3 and 9 mL 1 N NaNO3 as a support electrolyte, diluted to 27 mL with distilled water. Method parameters:

Start conditions:

Activation pulse= off

start delay time= 300s

start volume = 0.00000 mL

titration rate = 0.1 mL/min

Titration parameters; titration rate= user, temp. = 25 degrees, sensor=pH electrode, stirrer=on, stirring rate=8, titration rate = 0.1 mL/min, volume increment = 0.1 mL.

Stop conditions: pH = 11.5

Evaluation; EP recognition= greatest.

Please, could you point out the possible sources of obtained experimental errors? First of all, why the results indicated by the apparatus differ from those calculated by me. For example, at EP apparatus shows pH = 7.646 and V(NaOH) = 2.9912 mL, while my calculated values (using first derivative obtained by Origin 6.1) are: pH = 6.91 and V(NaOH) = 3.05 mL.

Thanks for your time