Hydrogen ion concentration in aqueous solutions, or pH for short, is one of the most difficult of process measurements, but it is also one of the most important parameters to measure and control. Because pH is measured on a logarithmic scale, relatively large amounts of chemicals are needed to adjust pH up and down the scale, explained Christopher Roderick, OEM accounts manager for the instrumentation division of ABB.
For example, if one unit of acid is needed to reduce pH from 7 to 6, then 10 units will be required to reduce pH from 6 to 5. The best way to minimize chemical additions is to measure pH continuously and make adjustments as needed to control the setpoint near its desired value.
Selecting the right pH measurement solution can improve plant productivity, cut chemical costs and reduce required maintenance. ABBs Christopher Roderick discussed the dos and donts of industrial pH measurement.
Controlling pH to setpoint not only minimizes required chemical additives, but it also results in better plant operations because various plant processes run more efficiently when maintained at correct pH levels, explained Roderick.
pH measurement instruments have three main parts. The first is the measurement electrode and the second is a reference electrode. The two electrodes are connected to an analyzer which determines pH based on signals received from the probes.
The most critical step in pH instrument selection is picking the right reference electrode as various process fluids can react with and/or coat the electrode and make the instrument inoperable, cautioned Roderick. To cope with reference electrode problems, manufacturers such as ABB make many different kinds of pH probes that combine reference and measuring electrodes for various applications.
The first step in selecting the right pH instrument is to understand the process. Specifically, the probability of the reference probes plugging, poisoning and pumping. Other parameters that must be determined include the anticipated pH range, temperature and pressure, along with the conductivity for pure-water applications. Mounting options include in-line, hot-tap, submersible, tri-clamp and others.
Once data is gathered and the process is understood, the best solution can be identified. This will include specifying the reference probe, the type of measuring electrode glass and the required process connections. At the analyzer side of the instrument, installation conditions, such as operating temperature, ambient moisture and the presence of hazardous vapors or dusts, must be considered.
The analyzer will typically have a local readout and will also send a signal to the plants control and monitoring system. This signal can be 4-20 mA, or it can be some type of digital signal, such as those sent from Foundation fieldbus, HART or Profibus.
Instruments with digital signal capability are more expensive to purchase and install than 4-20 mA instruments, but they provide many benefits, including two-way communication, which is required for remote calibration, Roderick said. Digital signals can also transmit much additional information in addition to pH, such as parameters related to instrument health.
Selecting the right pH measurement solution, Roderick concluded, can improve plant productivity, cut chemical costs and reduce required maintenance.
