Monitoring the quality of water is one of many important aspects in environmental control. With the increasing growth in human population, water for human consumption has become scarce and increasingly polluted. There are many parameters that define the quality of water but the most important of these is probably pH.
Electrical Interference pH is a deceptively simple measurement. However, there are many factors that need to be taken into account for a reliable reading. The most important characteristic of pH electrodes is it's very high impedance, of the order of 109 ohms. This is compounded by noisy factory environments and by long distances between the electrode and the controller.
A typical pH measuring device would be normally configured to operate in the single ended mode, also known as the asymmetrical mode. This means that the reference electrode would be connected to the ground potential of the amplifier. This configuration works very well as long as the environment is electronically noise-free. This is not the situation in an industrial environment. It is very commonly seen that the readings on a pH controller suddenly fluctuates, even to over-range or under-range condition. This situation arises, when for example, the mixing motor is switched on. An old leaky motor might inject some electrical interference of 1 to 2 volts into the liquid whose pH is monitored. This noise being a common signal, is picked up by both the pH and the reference electrodes. Since in the asymmetrical mode, the reference electrode is grounded, the electrical noise is present only on the pH electrode. This noise would be amplified along with the pH signal and thus the fluctuating readings. If the electrical noise was from a DC source, typically like those in an electroplating tank, the problem would not be fluctuating readings mostly stable but incorrect values.
A simple way to solve this problem would be to reconfigure the input to floating differential mode. In other words, not to ground the reference electrode. Therefore, the electrical disturbance will be present equally on both the pH and reference electrodes. It therefore becomes a common mode signal and hence can be rejected very easily by the operational amplifier. However, this brings in the necessity to use an additional grounding electrode commonly referred to as potential matching pin. Relay Hunting Hunting of the relays around the set point is a very common problem faced in the industry. This may even lead to the breakdown of expensive dosing pumps and solenoids at times. Let us look at what causes this problem. Let us assume that in atypical case, a low set point of 6 pH has been set. It would mean that when the pH drops to a value below 6, the caustic dosing pump starts. The addition of caustic solution would start to increase the pH. When the pH reaches 6, the pump would stop. The mixing pump would still continue operating and hence the pH would drop back below 6. This would start the cycle all over again and so on. This results in the hunting of the relay around the set point.
A simple way to overcome this problem would be allowing the pump to continue to dose even beyond the set point, say until 6.5 pH in the above example. In such a situation, when dosing pump stops, the pH might drop to probably 6.2 pH which is still above the set point and hence hunting is prevented. This extra band that has been introduced is known as the hysteresis band. In modern day controllers, independent and adjustable hysteresis bands are available for the high and low set points.
Limit and Proportional Control The function of a pH controller would be mainly to monitor the pH and activate the pumps if the pH value goes out of the set points, and dose the respective chemicals which would bring the pH back within limits. The manner in which this corrective action takes can be in two ways, namely limit control and proportional control.
Limit control is a coarse method since it keeps the relays on permanently if the pH is out of limits. The dosing of the chemical would not be regulated based on the deviation of the pH from the set point but at a steady and fixed rate. This would causeovershoot and undershoot of the process and hence the control will not be smooth.
In applications where fine control is required like those in food or pharmaceutical applications which usually operate within a narrow band, a limit control would not be acceptable. The best option would be to switch to 'Proportional Control'. This, as the name suggests, would offer a control action which is proportional to the deviation of the pH value from the set point. In other words, the further the pH value from the set point, the longer the dosing. As the pH approaches the set point, the dosing reduces and finally stops when the pH reaches the set point.
There are two methods of applying 'proportional control', Pulse Length and Pulse Frequency. In the pulse length mode of operation, the total time of the pulse can be fixed by the user typically anywhere between point 0.5 and 20 seconds. The 'ON' time of the pulse would vary depending on the deviation of the pH from the set point. The further away the pH, the longer the 'ON' time and hence, the higher the dosing. As the pH approaches the set point, the 'ON' time and hence dosing keep reducing.
In the Pulse Frequency mode of operation, the frequency can be set by the user typically anywhere between 60 and 120 pulses per minute. The frequency of the pulse would vary depending on the deviation of the pH from the set point. The further away from the pH from the set point, higher the frequency, hence higher dosing. As the pH approaches the set point, the frequency and the dosing reduces.
In-Line Calibration In most of the industrial applications, the pH controller would be calibrated at the beginning of the process. As the process progresses, bleed samples would be taken and analyzed separately in the laboratory. At times, it is noticed that there is a mismatch between lab results and the readings on the controller. This might have come about due to the soiling of pH electrode being used in an aggressive environment which might also be a long process running a few days in some cases. In order to correct this problem, the electrode may have to be taken out of the process tank which means halting the process. This may not be acceptable in most cases. A simple way to overcome this problem would be to provide a facility of a one-point calibration which can be done on-line. This does not affect the original slope of the calibration but offsets the line. alpha-pH1000 controller allows one-point on-line calibration to be done without having to disassemble the electrode housing.
Current Transmission—4/20 mA In most industries these days, it is essential to have a hard copy of the parameter that is monitored over the entire time frame of the process. The easiest and most economical way of achieving this would be to connect up the controller to a chart recorder. It is therefore necessary for the controller to have a 4/20 mA transmission capability. Most present day controllers come with this facility built in. However, there are only a few which offer many features in this mode.
Conventionally, in the controllers, 4 mA would correspond to 0 pH and 20 mA to 14 pH. When such a device is connected to a 1/4 DIN panel mounted chart recorder, the pH value can be constantly monitored. If the process requires a stringent control and is also operating within a narrow band of set points, say 1 pH; the recording on the chart paper will not be well resolved. This is due to the fact that in the conventional controller the range of 14 pH is distributed over 16 mA. It can therefore be seen for a 1 pH variation, the current varies only by 1.15 mA approximately. This on the chart paper would be a movement across a width of less than 1/2 cm for the recorder mentioned above It is clear from this example that recording would not be well resolved and hence would not be of much use.
An easy and economical solution to the problem would be to have a controller with 'zoom' facility for the current output. This in simple terms would mean that the user should have the facility to fix the pH values he wants to the 4 and 20 mA output. Let us consider a process operating within 1 pH band say 6 and 7 pH. If it was possible to se the controller to deliver 4 mA at 6 pH and 20 mA at 7 pH, it can be seen that we now have the entire 16 mA over just 1 pH band. This has now given us a 'zoom' of almost 14 times. It would be an added advantage if this 'zoom' band could be set anywhere on the entire pH scale.
pH Measurement in Liquids with Hydrofluoric Acid In certain processes Hydrofluoric (HF) is bound to be present. Typical cases would be like those in glass industries. pH measurements in such situations poses a problem. Since conventional pH electrodes are themselves made of glass, especially the 'bulb', in a short span of time depending on the concentration of the HF, the electrode dies. Most often the solution to this problem would be using electrodes made of high HF resistance glass. This, as one may see does not totally eliminate the problem. Logically, the best solution would be to use a material that is not attacked by HF. Antimony electrode, which is basically an ORP electrode can be used here. The solution is not as simple as it sounds. This is due to the fact that the property of the glass electrode is very different from that of the Antimony electrode. Therefore these electrodes cannot just be swapped, one in place of the other.
The alpha-pH1000 controller has the facility to switch from one to another and in that process change internally all the settings associated with the type of electrode, either glass or Antimony. This method, though not the best, is still a good and economical way to measure pH in HF environment.
Alarm Function Most of the mid-range controllers these days come with a separate relay for the 'alarm' function. This relay generally activates if the measured parameter is out of either of the set points. What this means is that in the event there is a control action taking place, the alarm relay is also active and hence the alarming device be it a siren or a flashing light. As a result of this, generally there is not much attention given when the lights starts to flash or the siren starts to hoot. Invariably there is a reset switch wired up externally, which will be operated to stop the noise. We see therefore in such situations, the alarm device is looked at as a nuisance rather than a facility. How could we better utilize this feature.
Let us look at a practical situation. Assume that the low set point is fixed at 6 pH. When the pH drops to a value below 6, the set point relay gets activated and hence the chemical dosing starts. Under normal circumstances, the alarm relay also would have got activated and as long as the alarm condition exists, the siren keeps hooting. If by experience one would know how long it would take for the dosing action to correct the situation, say 2 minutes in this example, it would be great advantage if one could set a delay time of 2 minutes for the alarm relay to function. What this means in reality is that once the set point is exceeded, only the dosing pump starts and the alarm relay circuit starts the counter. If in 2minutes the problem did not get corrected, which may be due to airlock in the pump or empty chemical tank, then the alarm relay activates and therefore the siren or the flashing light This would be more meaningful.
Prevention of Chemical Wastage In certain industries it becomes essential to monitor two parameters simultaneously and carry out corrective action based on one parameter first followed by the other. pH and ORP measurements would be a good example in 'electroplating' and 'swimming pool'. The pH is first adjusted to a specific level and then the ORP control is done. Thepractical problem one would face here is that the pH and ORP are independent. While the pH is being corrected, the ORP gets affected and vice versa. If it were possible to HOLD the ORP controller while the pH is being corrected and let it start the control only when the pH is within the acceptable limits, then that would result in a large saving of the chemicals. If this control process could be carried out automatically, it would be an added advantage.
ORP Measurements ORP or Redox is the oxidation-reduction potential usually measured in millivolts. This is not a specific ion measurement since all the ions present in the liquid will contribute to the ORP potential. It can be seen therefore that in areas like wastewater treatment, the ORP in actual millivolt would not make much meaning. Instead if it was possible for the controller to measure the millivolt and display it as a relative percentage, then the transition from one state to another can be more easily seen. For this to be possible, the controller should be capable of operating in what is known as the ORP % mode, which is available in the alpha-pH1000 controller. The user can calibrate the unit with two liquids whose relative toxic levels are known and then use it to measure and control the ORP % value of the water being processed.
Manual Temperature Compensation There are many instances where a temperature probe is not being used. This typically would be when there is no large variation in temperature of the process. It is still necessary to apply a compensation for correcting for the effect of temperature if the process temperature is going to be anything other than 25°C. Most of the controllers these days have the facility of Manual Temperature Compensation (MTC). The drawback however in most of them is that there is only one setting for the MTC. If for example, the MTC is set to 40°C and measurement is being carried out, the compensation corresponding to 40°C would be applied. If now the user decides to carry out a calibration, care should be taken to reset the MTC to 25°C. If this step is forgotten, then calibration would be wrong. This error may not be very high in case of pH measurements. However in Conductivity and Resistivity measurements, assuming that the temperature coefficient of the liquid is around 2% per°C, the error for a 15°C variation could be as high as 30%. This would in no way be acceptable. A simple solution to this problem is by providing two settings for the MTC namely one for Process temperature and another for the Calibration. This way the controller can apply the correct compensation based on the mode, 'measurement' or 'calibration', in which it is operating.
Monitoring the quality of water is one of many important aspects in environmental control. With the increasing growth in human population, water for human consumption has become scarce and increasingly polluted. There are many parameters that define the quality of water but the most important of these is probably pH.