The only reliable way to determine whether a pH meter 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:
- Set the meter back to factory default status (refer to the meter's user manual for operation)
- Calibrate it in the pH 7.00 standard buffer first,
- Then measure the pH value of pH 4.00 standard buffer (you can also use pH 10.01 buffer or other standard buffers).
A pH meter is considered accurate and in a good condition if the reading difference between the measurement and standard value is ≤ 0.30 pH. In this case, suppose the temperature of the standard solutions is at room temperature (around 77˚F/25˚C), if the test result is ≥3.70 or ≤4.30, then the meter is considered in good condition. Of course, the smaller the difference, the better the condition of the pH meter is.
*Please make sure the standard buffers are in good quality (fresh and non-contaminated, and made by legitimate companies producing scientific products).
P.S. Sometimes even if your meter has passed this simple accuracy test, but it is still giving you inconsistent or fluctuating readings. It doesn't mean that the pH meter per se is not accurate. It could be that the pH electrode of the meter is not suitable for testing your samples, since most pH electrodes are only designed for general water solutions. To solve this problem, please refer to this article.
*The standard value of calibration solutions can change at different temperatures. For example, at 77˚F/25˚C, the standard value of pH 7.00 is 7.00 pH, but if the temperature of the solution is at 104˚F/40˚C, then its standard value becomes 6.97. Please refer to the following table for the relationship between pH and temperature for the most commonly used pH standards in U.S.A. This table only works for Apera branded pH standards. Other brands' products may have a slight difference.
| Temp./pH | 7.00 pH | 4.00 pH | 10.01 pH | 1.68 pH | 12.45 pH |
| 5˚C / 41˚F | 7.09 | 4.00 | 10.25 | 1.67 | 13.23 |
| 10˚C / 50˚F | 7.06 | 4.00 | 10.18 | 1.67 | 13.01 |
| 15˚C / 59˚F | 7.04 | 4.00 | 10.12 | 1.67 | 12.81 |
| 20˚C / 68˚F | 7.02 | 4.00 | 10.06 | 1.68 | 12.62 |
| 25˚C / 77˚F | 7.00 | 4.00 | 10.01 | 1.68 | 12.45 |
| 30˚C / 86˚F | 6.99 | 4.01 | 9.97 | 1.68 | 12.27 |
| 35˚C / 95˚F | 6.98 | 4.02 | 9.93 | 1.69 | 12.11 |
| 40˚C / 104˚F | 6.97 | 4.03 | 9.89 | 1.69 | 11.96 |
| 45˚C / 113˚F | 6.97 | 4.04 | 9.86 | 1.70 | 11.83 |
| 50˚C / 122˚F | 6.96 | 4.06 | 9.83 | 1.71 | 11.70 |
| 55˚C / 131˚F | 6.96 | 4.07 | 9.81 | 1.72 | 11.58 |
| 60˚C / 140˚F | 6.97 | 4.09 | 9.78 | 1.72 | 11.47 |
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.