pH Probes and pH Controllers used in automated pH Neutralization Systems
There are many styles, architectures, and housings available for industrial pH probes ranging from large wet tap assemblies to simple laboratory type glass electrodes with prices ranging from less than $100 to over $2,000. A few of the most commonly used probes and housings are discussed here. In essence all pH probes are simply a hydrogen ion selective electrode used in conjunction with a reference half-cell. We have available a detailed description of a simple pH electrode in our Technical Articles. Some of the common terms that are used with industrial pH probes:
A hydrogen ion selective half cell electrode which directly measures free hydrogen ion activity (free acidity) against the reference half cell. The potential generated by this electrode directly relates (logarithmically) to the pH of the solution.
The half cell containing the neutral liquid reference cell against which the pH half cell electrode is referenced. To make a pH measurement the positive lead of a voltmeter is connected to the pH Electrode while the negative lead is connected to the reference cell.
This is the porous fluid barrier between the fluid in the reference half cell and the process fluid within which we are attempting to make a measurement. This must be free flowing and clogging is a common probe failure. The component of the reference cell that you see is the junction.
Combination pH Probe
This is a single probe that contains both a pH Electrode and a Reference Cell. Almost all pH probes on the market are combination pH probes. Purchasing individual pH electrodes and Reference Electrodes is largely a thing of the past. All pH probes on this site are combination pH probes.
The sensitivity of the pH Electrode to hydrogen ion activity is a function of temperature. For accurate pH measurements outside a range of 5.0 < pH < 8.0 with temperature fluctuations > 15C then temperature compensation should be considered. For more info see our article on pH probe architecture.
The energy (voltage and current) generated by a pH probe is extremely low and must be converted to a usable signal through a logarithmic calculation. A pH transmitter is a dedicated voltmeter displaying the pH probe voltage in a linearized pH display usually along with a 4-20ma signal output. The pH transmitter will provide an excellent interface through which the user can calibrate the probe and the transmitter will provide temperature compensation if required by the application. pH Transmitters are normally NOT pH controllers and provide little to no ability to control pH.
There are many different styles of pH Probes used in an automated pH neutralization system. The style of the pH probe used is normally a function of the application within which it is used. In a lab application where the pH probe is used in a laboratory setting a simple glass pH electrode (such as the one shown here) is quite appropriate. This is a relatively cheap pH electrode that will offer excellent response time and repeatability. However, due to the all glass body, lack of pH sensing bulb protection, and very low reference junction surface area this is not a good choice for a wastewater neutralization system.
pH Probes used in a wastewater neutralization system need to be more rugged than a typical laboratory glass pH probe. pH Electrodes used in waste water neutralization systems will be exposed to a myriad of difficult conditions including high solids, fat / oil and grease, high temperatures, acids and alkalis (sometimes in a concentrated form) and more.
These applications will require hardened pH probes such as the CPVC body pH electrode shown here. In this case the standard glass electrode is encased in a rugged CPVC body, the glass bulb is replaced by a flat surface that is easily cleaned and not easily broken. Additionally the reference junction is very large making it easy to clean and prolonging the time before it must be cleaned.
pH Probes are generally the weakest link of any pH monitoring or pH control system. These are sensitive devices that are easily fouled, easily broken, and do not hold a calibration over a period of time. Digital Analysis strives to install all of our pH probes in an "Optimized" environment.
pH electrodes should never be installed in a tank, either through the top or sidewall, if at all possible. The Optimum environment for any pH probe is inline where the environment (flow velocity and pressure) can be controlled. We install all of our pH probes inline in a recirculation loop where the environment can be Optimized.
A pH Transmitter converts the very low level signal (-700mv to +700mv) to a linearized pH display and provides a high level signal output such as a 4-20ma output for use by other devices such as a PLC.
The pH Transmitter also provides an excellent interface through which the operator calibrates the pH probe to transmitter loop. Since pH probes are inherently unstable a calibration must be performed frequently, typically anywhere from one week to one month intervals.
A pH Controller must provide many functions not normally encountered in simple three perm PID controllers. For example: A pH Controller must recognize that the process to be controlled is not linear and, at best, is logarithmic. The pH controller must also recognize that the process does not respond instantaneously with some reactions occurring quicker than others. Mixing is not instantaneous, chemical injection is not precise, and pH probes are relatively slow. The pH controller must be sophisticated enough to follow a polynomial curve and must be capable of understanding that the "brakes" must be applied often otherwise the reaction will get ahead of the control response leading to an overshoot.
All of our pH control systems are based upon a PLC that provides complete control over the entire process. Additionally an Operator Interface Unit (OIU) provides the operator with an excellent window into the process.
For additional information we have an excellent technical write-up which covers these challenges in more detail.