Gone are the days of measuring pH with litmus paper and a colour chart, today's modern production techniques require a far more accurate method that is reliable and cost-effective
The UK manufacturing industry has seen a decline in recent years. The days when companies could afford to employ a technician just to look after pH electrodes are a distant memory.
Even the water companies, which for decades used simple measurements supplied on a 0/4-20mA signal, now require instrumentation that provides as much diagnostic information as possible.
The latest pH transmitters to come onto the market offer far more than ever before in the way of diagnostics and user-friendly programming. Such instruments now provide predictive maintenance facilities that actually monitor the deterioration of the glass pH electrode and inform the user after each calibration when the electrode will actually fail - a useful feature if you are running a ten-day batch process and the system predicts failure after five days.
Now users have the option to change the electrode before entering into lengthy batching processes.
Other features include Sensor Check Systems, which monitor the electrode for both glass breakage and sensor fouling.
These systems can either retract the sensor from the process automatically and clean it in the case of fouling, or inform an operator to change it in the case of breakage.
All of these features are nice to have.
However, the main concern today, whether the measurement is on a triple validated drinking water plant or on ovarian cancer drug production, is reliability.
Users need to know that the reading is always 'as good as it gets' - and the only way to do this is to remove the possibility of human error in the system. Unlike a flowmeter, which may need calibration every 5-10 years no matter what the medium it is measuring, the pH calibration frequency is determined by the process conditions and could vary anywhere between one hour and one month.
Therefore a system that can think for itself, determine when it needs to be cleaned or calibrated and performs the function itself without any operator intervention is a definite advantage! Endress+Hauser's CPC300 is the first of a new generation of pH systems that offers the customer a self-contained measuring system with the advantage of on-board data logging.
The Windows-based software tool allows the user to configure the system via a PC and simulate the system set-up prior to installation of the software (using a Dat module).
The same Dat module then downloads the logged results ready for upload to the PC.
This Dat module can be used to configure any number of transmitters, either with the same instrument set-up or with an individual programme tailored to each measuring point.
In this way a permanent record of all pH and temperature data is generated which can be exported to any Windows compatible programme.
This data can confirm such information as the temperature and the time that the pH electrode was sterilised.
This is an invaluable tool for tracing possible causes of contamination in sterile conditions.
The units also record the date and time of calibration, along with the actual performance details of the electrode itself - again, a permanent record to prove the accuracy of the measurement.
Even with advanced transmitter technology, all pH measurements are still only as good as the electrode being used.
If we look at the way electrode technology has improved, we now see that instead of the old three-electrode systems comprising of pH glass, reference and temperature sensor, the industry standard is now a combination electrode with internal Pt100 or Pt1000.
Due to their compact design, these electrodes offer new possibilities when it comes to introducing them to the process.
They can now be inserted into a pipeline or process vessel either by hand or via pneumatically actuated housings.
A particular advantage is that they can be removed offline for cleaning and calibration or simply stored outside the process in water or buffer solution to be easily reinserted when required.
This also has the added benefit of offering extended life to the electrode, as it is not continually stressed by the process solution.
Even the way we manufacture pH electrodes has changed, with new automated glass blowing replacing the old labour-intensive manual method.
The new automated method means that electrodes now have a more uniform performance due to consistent quality - something highlighted by the fact that quality certificates stating electrode performance are now issued as standard by major manufacturers.
One of the more recent innovations in electrode technology has been the Isfet electrode (Ion sensitive Field Effect Transistor).
These electrodes use a transistor that is sensitive to hydrogen ions to measure the pH of a solution, thus removing any glass from the pH measurement.
These type of electrodes now give the opportunity to safely measure the pH of food or pharmaceutical products, where in the past it has not been possible due to the glass construction.
Any breakage of the electrode would lead to contamination of the product and potential loss of revenue.
The Isfet electrodes themselves also offer additional benefits. Unlike glass electrodes, Isfet electrodes do not suffer from long-term drift and can therefore go for longer periods without calibration.
This is due to the fact that the transistor is not affected by the solution it is measuring in the same way as the glass membrane of a conventional electrode - although there are limitations.
Hot caustic soda will greatly reduce the lifetime of an Isfet electrode and so it is still advisable to use a retractable housing during Clean In Place (CIP) applications for instance, that require flushing of production pipelines or vessels with caustic soda.
The measurement of pH is now becoming more commonplace within all segments of industry.
In fact, the advent of pH balanced healthcare products on supermarket shelves have at least given the measurement a higher profile with the average person in the street.
The need for pH measurement is not going away - if anything it is getting more prevalent as new transmitter and electrode technology is opening up applications that in the past have been impossible to measure.
So what has the future got in store for pH measurement? The real possibility of the smart sensor that does not need a transmitter is on the horizon and ultimately a sensor will be developed that is smart enough to clean and calibrate itself and communicate digitally or via an analogue signal.
This, coupled with an ever-increasing bias towards digital communications will lead to further advancements in pH measuring technology, along with an increased variety of suitable applications.
