The HI98128 autodetects the buffer being used. A pH 7 buffer will generate a voltage close to 0 mV and the meter accepts a point withing approx. +/- 60 mv. A pH 10 buffer generates appox -180 mv (+/- mv) and pH 4 generates + 180 mV (+/- mV). The +/- mV represents some range of voltage based on the mV generated in pH 7 and the slope of 85% (150 mV) or greater. So there is a range.
Based on this information either:
1) pH 7 is not pH but some other value greater. It would to be around pH 9.
2) the pH electrode is faulty
To determine which you can do some quick checks.
1) What is the pH value when placed in pH 4 buffer. If it shows pH 7 then it is a faulty electrode
2) If it shows pH 4 then the pH 7 buffer is no good.
If you do not have pH 4 then some common items that can be used to check pH. Orange juice is around pH 4. Baking soda in water will be around pH 7.8. If the readings are in the range of +/- 0.5 pH then the buffers the buffers are the suspect. Do not use these solutions to calibrate. They are not standards of a known value that we base all of our measurements on. They are only being used to check the mV response. Using two points will confirm there is a deflection voltage (slope). If you have any questions feel free to e-mail. From the forum sign up I have allowed the e-mail feature from members.
The pHep 5 has 1 year warranty on the meter and 6 months on the electrode. Contact
tech@hannainst.com and they will take care of you.