Why does the displayed value of the pH transmitter fluctuate so greatly?
The displayed reading of the online pH transmitter is unstable and fluctuates drastically. After initial calibration with standard buffer solution at startup, a fault occurs within 2 hours. The specific symptom: the pH value of the measured medium ranges from 6.8 to 7.2. When the actual pH is 6.8, the transmitter reading fluctuates around 5.9. When the actual pH is 7.2, the transmitter reading starts to drop from 11 and slowly decreases further. This phenomenon disappears after replacing the transmitter with a new one.
Large fluctuations of readings on online pH transmitters are relatively rare. Imported pH meters seldom have such issues, though this problem may occasionally occur. Check whether a new probe has been installed. If a new probe is fitted, inspect its impedance. If the probe has not been replaced, check the salt bridge. Finally, consider replacing the probe. Loose wiring may also cause this issue; tighten the terminal connections.
Note: Salt bridge. After the reference electrode is immersed in the test solution, a small amount of KCl solution seeps out through porous ceramic. It internally connects to the reference electrode and externally contacts the sample solution, acting as an electrical bridge - hence the name salt bridge. The leakage rate of the salt bridge solution greatly affects the liquid junction potential. Too slow leakage leads to unstable liquid junction potential. Moreover, rising pressure of the sample solution will further slow down leakage. The sample liquid may penetrate into the reference electrode and cause the acidometer to malfunction.
Between pH 6.8 and 7.2, the change in hydrogen ion quantity corresponds to roughly 4 units, which translates into an extremely weak mV signal. If the amplifier amplifies the signal at this ratio and another buffer solution is used, the reading may show approximately -2 or 16.
Since the fault disappeared after transmitter replacement, I judge this to be an accidental failure. If you had not changed the buffer solution at that time, the pH reading would probably also jump randomly; it was just a coincidence. Buffer solution pH 7.2? Common standard buffers are pH 4.0, 6.86 (or 7.0), and 9.18 (or 9.0). For pH calibration, the pH difference between the two buffer solutions must be greater than 2.0.
