The only reliable way to determine whether a pH tester is accurate or not is to test it in standard solutions. Just like the only way you can tell if a scale is accurate is to test the standard weights. Comparing one pH tester to another or judging the test results based on past experience or your expected results are scientifically incorrect and can be misleading.


To test the accuracy of your tester, follow the 3 simple steps:

  1. Calibrate it in the pH 7.00 standard buffer first,
  2. Then measure the pH value of pH 4.00 standard buffer. 
  3. A tester is considered accurate and in good condition if the reading difference between the measurement and standard value is ≤ 0.3 pH (in this case ≥3.7 or ≤4.3). Of course, the smaller the difference, the better the condition of the pH tester is.

*Please make sure the standard buffers are in good quality (fresh and non-contaminated, and made by legitimate companies producing scientific instruments).


If the difference is greater than 0.3 pH, in most cases it’s the pH probe’s problem –– either it gets contaminated, damaged, or it’s just aged and passed its service life (usually 1 to 2 years depending on frequency of use, test samples, and how well it's maintained). If this occurs, firstly make sure the glass sensor is intact (not broken or damaged); then try cleaning the probe thoroughly in soap water with a soft brush and then soak it in the storage solution for 12-24 hours. If that still doesn’t work, it’s time to replace your pH probe (if it’s replaceable) or send in the tester to us for repair or replacement just so you can always get reliable results.


*If you have Apera’s portable or benchtop meters, the electrode’s slope data will be shown upon finishing every two points of calibrations. The electrodes’ slope is typically between 90% to 105%. The closer it is to 100%, the better the electrode’s condition is. If the slope is less than 85%, an error code will show up and it’s time to replace the aged or damaged electrode.