I am trying to get pH of DI suspension, but prior to the suspension I am measuring the pH of DI water for comparison. My measurement is with SPER scientific benchtop pH meter. Before my measurements, I calibrate with standard buffer solutions of pH 7, 4, and 10. I then measure pH of DI water (18.2 Mohm-cm). The probe reads the pH at around 8.5

Should I reconsider the DI water, or is it the probe that is reading wrong (for buffer solutions, it reads precise pH). Is there a way to read correct pH of the DI water.