Would have been best if you had saved the vi with data in the graph ;), but I think I know what problems you have.
Take a look at the Savitzky-Golay filter, that filter actually does a fit and can do a differentiantion more robust to noise. (Use the advanced FIR SG coeff vi and the help .... more theorie here)
Some of your problems might also come from filterartifacs, so capture some more data and cut the ends to avoid that.
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'
