The pH sensor is part of an instrument that measures the pH of water. It is made from a specially formulated, pH sensitive glass in contact with the solution, which develops a potential (voltage) proportional to the pH of the solution. The internal reference electrode is designed to maintain a constant potential at any given temperature, and serves to complete the pH measuring circuit within the solution. It provides a known reference potential for the pH electrode. The difference in the potentials of the pH and reference electrodes provides a millivolt signal proportional to pH. The most common application is the measurement of pH which in turn controls an acid feed pump. It is important to control the pH of the water because pH impacts both scale forming and corrosive tendencies of water. Equipment life and operating efficiency can be reduced when the pH gets too high or too low.
pH, which is always written with a lower case “p,” is a scale of how much ionic hydrogen is in a substance. The scale runs from 0 to 14 where a pH of 7 is neutral. The lower the number on the scale, the more acidic the substance is. The higher the number on the scale, the more basic, or alkaline, the substance is. The scale is logarithmic, so a substance with a pH of 9 is 10 times more alkaline than a substance with a pH of 8.
The pH sensor is most often controls an acid feed pump which is used to manage the pH of recirculating water. If the sensor fails, the controller will not receive the correct pH information and either feed too much acid, too little acid, or perhaps no acid at all. All of these conditions should be avoided. Too much acid can accelerate corrosion. Too little or no acid can cause scaling to impact efficiency.
A predictable failure pattern would help operators prepare for problems. Unfortunately, the list of problems causing sensor failure is too long to produce a pattern. Some problems cause the sensor calibration to drift high, and some cause the calibration to drift low. Routine preventative maintenance is the best approach.
The clearest clues are varying pH readings or the inability to calibrate the pH value. Physical damage is usually obvious, but even a scratch on the pH glass will cause faulty and unreliable readings. Discoloration of the reference junction can signal contamination of the junction and approaching high impedance problems.
A pH sensor should last 12-18 months, but there are some handling, maintenance, and storage practices that can shorten that considerably. Storing the sensor dry or storing the sensor in deionized water can dramatically shorten its life. The side stream piping in seasonal applications may drain and care should be taken to avoid exposing the pH sensor to air by removing it and storing it in pH buffer. Not cleaning the sensor can prematurely clog the reference junction, but touching or physically cleaning the sensing glass can also shorten sensor life. Solvent contamination in the water can damage sensor seals and allow the electrolyte to leak out. Freezing or boiling can crack the sensor glass. Hydrofluoric acid will attack the sensor glass and strong caustic solutions, as in pH approaching 14, can ruin sensors in just hours. Oil contamination in the water will cause a film over the pH glass and often permanent fouling. Many different ions in solution can impact the reference junction when excessive concentrations are present, but this is generally limited to off-line contamination and proper sensor storage can prevent reference junction problems.
The overall impedance of the reference junction is a sum of the resistances of its components with the largest being the liquid junction. This is due to the limited volume of current carrying electrolyte within the liquid junction. Coating or blockage of the liquid junction further increases the impedance of the liquid junction. The most common causes of high reference impedance are iron fouling and biofilm.
The life expectancy of a pH sensor can be shortened by a number of factors discussed above, so the absence of these factors can make some sensors last longer than others. Sensor design can impact life expectancy too. Sensors with electrolyte volume will generally last longer than sensors with small volumes.
Replacing the sensor is easy. Simply pull the insulating cover back on the BNC connector and twist it counterclockwise to remove the cable from the controller. Disconnect the solution ground spade connector. Reverse the procedure to connect the new sensor.
