The level of the pH meter is expressed in terms of the index (resolution or minimum display value) of its indicator (electrometer). For example: an instrument with a division of 0.1 pH is called a class 0.1 instrument; the minimum display value is 0.001 pH The instrument is called a 0.001 instrument, and so on. The accuracy of the instrument is the combined error of the electrometer and the electrode matching test standard solution. It is not only related to the electrometer, but also more relevant to the glass electrode and the reference electrode. From the actual use requirements, the current electrical division value is 0.1 ~ 0.001pH, if necessary, according to the current level of technology, can produce a more precise electrical meter. However, due to structural and manufacturing reasons, the performance of commonly used electrodes has not yet reached a completely ideal level.

Both the repeatability error of the glass electrode and the solution junction potential stability of the reference electrode are not better than 0.01 pH. Therefore, the resolution of the electricity meter is higher, and the accuracy of the instrument test is hardly superior to 0.01 pH. However, selecting a high-resolution instrument can maximally overcome or eliminate the effect of the meter on test errors. Because it is no longer a problem to achieve a satisfactory accuracy of the electricity meter, the innovation, improvement and improvement of the instrument's intelligence, user-friendliness, reliability, easy operation, and cost-effectiveness are all ongoing.

The "+" series model instruments such as the pHS-4C+ produced reflect this trend. The relationship between the level of the pH meter and its test accuracy is specified in the National Meter Verification Regulations (JJG119-84) of the pH meter as follows: Level of the instrument 0.2 Level 0.1 Level 0.02 Level 0.01 Level 0.001 Level division value or minimum display value (pH) 0.20 .10.020.010.001 Electrometer indication error (pH) ±0.1±0.05±0.01±0.01±0.002 Supporting test indication total error (pH) ±0.2±0.1±0.02±0.02±0.01 Note: The testing range for supporting test should be Within the range of pH 3 to pH 10. It can be seen from the above table that for a pH meter below 0.01, the value of the total error of the indication is equal to its level. For a 0.01 level acidometer, the total error of the indication is 0.02 pH. For a 0.001 acidity meter, the total error of the indication is shown. Can only reach ± ​​0.01pH, and at this time the need to use a first-level pH reference material can be guaranteed. (Note: pH national standard substances are divided into two grades: first grade and second grade, general pH meter is commonly used as secondary pH standard material.)

It also needs to be explained why the total error of the instrument indicates why the test range is pH3 to pH10. It is due to the fact that the junction potential of the reference electrode is related to the pH of the solution. The liquid junction potential in the range of pH3 to pH10 can be stabilized within ±0.01pH. If 10<pH<3, the error caused by the junction potential of the junction Up to ± 0.07 pH can be achieved. At the same time, glass electrodes can also cause alkali and acid errors in alkaline and acidic solutions. Therefore, it must be specified in the range of pH3 ~ pH10, otherwise it will not be able to distinguish the indication error of various levels of pH meter.