pH Controller for Water/Food Process

The pH meter and pH/ORP electrode selection

Wiring diagram:

Definition of the ports

• INPUT: Measuring terminal of the electrode
• REF: Reference terminal of the electrode
• NC: Undefined
• A: Temperature compensation terminal A, connect to NTC 10K or PT1000
• B: Temperature compensation terminal B, connect to NTC 10K or PT1000
• C: Temperature compensation terminal C, connect to 3-wire PT1000 GND (3-wire PT1000), or connect to B (2-wire PT1000), or NC (NTC 10K)
• RS485 (A+): RS485 communication interface A+
• RS485 (B-): RS485 communication interface B-
• 4-20mA (+): 4-20mA output terminal +
• 4-20mA (-): 4-20mA output terminal -
• AC 220V/110V (L) or DC 24V (-): AC 220V/110V live wire or DC 24V (-)
• AC 220V/110V (N) or DC 24V (+): AC 220V/110V neutral wire or DC 24V (+)
• HO: Normally-open relay for high alarm
• HC: Normally-closed relay for high alarm
• LO: Normally-open relay for low alarm
• LC: Normally-closed relay for low alarm
• COM: Common terminal

Communication

RS485 communication data and register address as follows:

Q1: What is the working principle of a pH meter?
A1: The pH meter works on the principle of a galvanic battery. The electromotive force between the two electrodes of the galvanic battery is based on Nernst's law, which is not only related to the properties of the electrodes, but also to the concentration of hydrogen ions in the solution. There is a the corresponding relationship between the electromotive force of the primary cell and the hydrogen ion concentration, and the negative logarithm of the hydrogen ion concentration is the pH value.

Q2: The same sample is measured on two pH meters at the same time, why are the readings inconsistent?
A2: Because the calibration conditions of the two pH meters are different (such as calibration done at different times), the measured values are different. Therefore, the pH meter should be calibrated at the same time with the same buffer, and then measured at the same time.

Q3: How do I know if the pH meter is accurate?
A3: Take three standard buffer solutions: pH 6.86, pH 4.00, pH 9.18 (preferably freshly prepared and at the same temperature), position calibration at pH 6.86, slope calibration at pH 4.00, and then test pH 9.18 to know the pH meter Is it accurate.

Tips: What is a two-point calibration of a pH meter?

For precision pH meters, in addition to "positioning" and "temperature compensation" adjustment, there is also an electrode "slope" adjustment, which requires two standard buffers for calibration. Generally, the "positioning" calibration is carried out at pH 6.86 or pH 7.00, and then according to the acid-base condition of the test solution, pH 4.00 (acidic) or pH 9.18 or pH 10.01 (alkaline) buffer solution is used for "slope correction. The specific operation steps are as follows:

1. The electrode is washed and dried, immersed in a pH 6.86 or pH 7.00 standard solution, and the temperature compensation knob is placed at the solution temperature. After the displayed value is stable, adjust the positioning knob so that the displayed value of the instrument is the pH value of the standard solution.
2. Take out the electrode, wash it, spin dry, and immerse it in the second standard solution.
3. After the displayed value is stable, adjust the slope knob of the instrument so that the displayed value of the instrument is the pH value of the second standard solution.
4. The electrode is washed and dried, and then immersed in the standard solution of pH 6.86 or pH 7.00. If the error exceeds 0.02 pH, repeat steps 1 and 2. Until the two standard solutions do not need the adjustment knob to display the correct pH value.
5. Take out the electrode, wash it and spin it dry, adjust the pH temperature compensation knob to the sample temperature, immerse the electrode in the sample solution, shake it and place it at rest, and display a stable reading.

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