To those of you familiar with pH meters, I would like to pose an odd question. For some reason, I have recently had some difficulty in calibrating my HM Digital pH-200 meter with any of the testing solutions. When I attempted it, I could not manually calibrate the numbers by a long shot -- 4.01; 7.0; 10 -- even though I have tried different batches of the solutions, as well as different brands; have also figured in the temperature differences in terms of pH; and even the auto-calibration function is miles off.
The probe itself is undamaged and the tip had been replaced within the last year. To further confuse things, when I placed the probe in distilled water, it registered an an ideal pH 7.0 at 20˚ C -- exactly what to expect from distilled or deionized water . . .