Typical Problems In pH Process Control

Monitoring the quality of water is one of many important aspects in environmental control. However there are common issues that are often overlooked or unintentionally taken for granted. This is not the usual case that most conventional controllers have necessary functions that can take care of them. Except for some controller makers that incorporate such useful features to counter-measure below process problems and provide stable readings even in demanding applications.

Electrical Interference
Most pH electrodes have very high impedance, generally 10⁹ ohms. This can be compounded by noisy operating environments and by high cable resistance due to long distances between the electrode and the controller. Hence electronical noise has to be well taken care in such instances.

Typical pH measuring device is normally configured to operate in the single ended mode, known as the asymmetrical mode. Such system works very well as long as the environment is electronically noise-free. But in a practical industrial environment, it is commonly noted that readings on a pH controller may suddenly fluctuate and vary around measured value; often with a mixing motor powered on. A leaky motor or ungrounded mixer running on mains power may inject some electrical interference of 1 to 2 volts into the sample liquid where pH electrode is installed immersed in same tank. Often than not, this noise being a common signal and is picked up by pH electrode. In a typical asymmetrical operation mode, this electronical noise would be amplified along with pH signal and thus produce fluctuating and unstable readings. If the electrical noise was generated from a DC source, the problem would not be fluctuating readings, mostly stable but incorrect values.

A simple way to resolve such problem would be to reconfigure the input to floating differential mode. Therefore, symmetrical operation mode is more viable solution with the introduction of using additional grounding pin (known as liquid ground pin or potential matching pin) to effectively shield off any possible electrical disturbance present, resulting in regulated, stable readings.

Relay Hunting
Often contact relays may cause chattering of switches around set point (relay hunting) which is a very common problem faced in the industry. This may even lead to breakdown of expensive dosing pumps and solenoids at times due to frequent wear and tear condition. To overcome such problem the controller should allow the pump to continue to dose even slightly beyond the set point, while pH value might drop after mixing subsequently, hence relay hunting could be prevented. This safety band, known as the hysteresis band prevents over-shoot or under-shoot of measured values, coupled with preset delay time duration of setpoint relays being activated or deactivated instanteanously. Thus, independent and adjustable hysteresis bands with relay preset time delay will be best solution available for any high and low set points contact relays.

In-Line Calibration
In many industrial applications, pH controller is usually precalibrated at the beginning of the process. As the process progresses over time, samples are taken and analyzed separately in laboratory. At times, there might be a mismatch between lab results and measured readings on the controller. This could be due to the soiling and condition of pH electrode being used continuously. In order to correct this problem, the electrode may have to be taken out of the process tank which result in halting the process operation and this may not be acceptable in most cases. A feasible solution is to provide a single-point calibration which can be done online (without having to remove electrode from tank) that does not affect the original slope of the calibration but offsets the entire pH line to corrected value.

Limit, Proportional And Proportional-Integral Control
Limit control activates contact relays instantenously once the measured value is out of its set limits. The chemical dosing is not regulated and this would cause overshoot or undershoot of process, thus control is not smoothly managed. To have a fine control operation within narrow band, the proportional control action is recommended. The best option is to have Proportional-Integral control - useful for very fine control in food, pharmaceutical or dedicated manufacturing applications. This control action is proportional to the deviation of the pH value from the set point. That means based on the deviation of the pH from the set point, control is fine tuned at a steady and fixed rate to ensure contact relays is activated accordingly. As measured pH approaches the setpoint, the dosing reduces and finally stops once pH reaches the setpoint.

Two types of Proportional Control - namely the Pulse Length and Pulse Frequency. In the Pulse Length mode, the total time of the pulse can be fixed by user, say between 5 to 20 seconds. The 'ON' time of the pulse will vary depending on the deviation of the pH from the setpoint. The further away the pH, the longer the 'ON' time and hence, the longer the
dosing until once the pH approaches the setpoint, the dosing keep reducing accordngly. In the Pulse Frequency mode, the frequency can be set by user, typically between 60 and 120 pulses per minute. The frequency of the pulse would vary depending on the deviation of the pH from the setpoint. The frequency of dosing is high if the pH is far from the set point, and pulse duration reduces as the pH approaches the setpoint.

pH Measurement In Hydrofluoric Acid Application
In aggressive cleaning processes, Hydrofluoric acid chemical (HF) is bound to be present. Most glass bulb pH electrodes pose problem as the HF acid would etch the glass membrane gradually resulting in fouling of electrode or quickly shortening the useful lifespan of pH electrode. Common solution to this problem is to use pH electrodes made of high HF resistance glass but this does not totally eliminate the problem. The best solution is to use an inert material that is not attacked by HF. Antimony electrode, basically like an ORP electrode, can be employed in HF application. However the property of antimony pH electrode is very different from that of the conventional glass pH electrode. Therefore these electrodes cannot just be swapped, one in place of the other. There is very few controllers in the marketplace (e.g. Eutech Alpha pH 1000) that offer dual Antimony-Glass electrode selection option as meter needs to configure internally all the settings associated with the type of electrode, either glass or Antimony in order for process to be measured accurately. This method, though not the best, offers a good and economical way to measure pH in HF environment.

Manual Temperature Compensation
There may be instances where a temperature sensor is not being used, assuming no large variation in temperature of the process. However it is necessary to apply a compensation
correction for the effect of temperature if process temperature is operating other than 25 °C. Most conventional controllers have the facility of setting Manual Temperature Compensation (MTC). But most compensate the process temperature and often ignore the calibration temperature of solution used. Often than not, user would not reset MTC to 25 °C for calibration or might forget to set back correct process temperature. If this sequence is missed out, then entire calibration would be incorrect. This error may not be very high
in case of pH measurements. However in Conductivity and Resistivity measurements, the temperature coefficient of the liquid is around 2 %/°C, the error for 15 °C variation could be as high as 30% which is no way be acceptable. To overcome this problem, few controllers provide separate settings for MTC, namely one for Process Temperature and another for the Calibration Temperature. In this way the controller can apply the correct compensation based on :Measurement' or 'Calibration' mode, for more accurate process operation.