The composition measurement with by far the greatest sensitivity and rangeability is pH. Consider that pH routinely detects changes at 7 pH to the 9th decimal place and for a 0-14 pH scale covers 14 orders of magnitude of hydrogen ion concentration. New glass and reference designs have dramatically reduced drift even for the harshest conditions, such as high temperature and sterilization. References junctions can be easily replaced. Smart wireless transmitters have much better diagnostics and resolution. Here is a checklist to help you take advantage of these advances in pH measurement technology.
The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for automation component. The following list assumes the materials of construction have been properly specified and that the sensor will work safely and reliably with acceptable accuracy for the maximum possible temperature and pressure and the electrical connections and the enclosure will meet electrical area classifications and codes. The slides in parentheses below are in the ISA Automation Week 2011 tutorial pH-Measurement-and-Control-Opportunities.pdf. For a detailed understanding see Chapter 6 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries. Reliability and precision (noise, repeatability, resolution, and threshold sensitivity) are most important.
- Do O-rings and gaskets meet worst case corrosive and temperature operating conditions?
- Is the best glass used for the worst case temperature, pH, and chemicals that can attack glass? (e.g. general purpose, high pH, high temperature, sterilizable, and HF resistant)?
- Is a spherical bulb used for maximum accuracy?
- For pH < 1 or > 12 would conductivity or density give a better concentration measurement?
- Is the best reference design and fill used for the accuracy and speed requirement and worst case temperature and composition (low water or pure water solutions that have low conductivity, salts and chemicals concentrations that change junction potential, plug junction, and poison reference internals)?
- Is the accuracy and equilibration speed requirement or coating and poisoning problem so extraordinary that a flowing junction is needed to provide the most constant reference potential, the fastest junction equilibration, and eliminate plugging-poisoning?
- If plugging is not a problem, can an aperture junction be used to give lowest junction potential?
- Will double and triple junction references be sufficient to slow down contamination rate?
- Are special electrolytes needed to prevent precipitation of silver salt from contact with process component (e.g. silver-cyanide precipitate from process cyanide contact with silver)?
- Can a removable reference junction enable an electrode to be rejuvenated (reference junction and fill replaced) before plugging -poisoning becomes a problem (slide 26)?
- Can a large surface solid reference electrode essentially eliminate plugging, contamination, and poisoning if reference speed of equilibration is not a problem (slide 19)?
- Is the chemical attack, premature aging from high temperature, or dehydration (non-aqueous solvents or low water concentrations) so severe that automated retractable insertion assembly is needed to limit exposure to process just long enough to get periodic pH?
- Is the solution conductivity so low (e.g. condensate, boiler feedwater, deionized water) a special assembly is needed to provide low sample flow, diffuser, and electrolyte reservoir (slide 22)?
- Can a VP connector be used to quickly locally disconnect electrode cable eliminating the need to disconnect transmitter and retract cable through conduit or flex to prevent twisting of cable?
- Is a smart electrode with stored calibration record available for selected electrode (slide 40)?
- Is a solution ground needed for impedance diagnostics and ground potential elimination?
- Is a smart transmitter to detect glass and reference problems available (slides 27-32)?
- Is solution pH temperature compensation needed besides standard Nernst temperature compensation (slide 6,29)?
- Can a wireless transmitter be used to get latest features and enable portability of measurement to test the best electrode and location (least deadtime and least noise-bubbles) (slide 42)?
- Is the electrode installed with tip pointing down at a 30-60 degree angle to prevent bubble residing in tip or caught on internal electrode (slide 33)?
- Are electrodes always wetted even for batching and during shutdown of continuous operations?
- Is middle signal selection needed to eliminate response to single failure and noise (slide 16)?
- Is stream velocity and protective shroud design the best for process conditions (slides 17,18)?
- Is velocity 5-10 fps and exposure of glass to flow needed to prevent coatings?
- Is velocity 0.1-1 fps and shroud reducing flow impingement needed to decrease abrasion?
- Does the electrode tip extend into the center line of pipe and past baffles in vessel?
- Is electrode location free from bubbles (e.g. not near sparger ring) and flashing (e.g. not on pump suction or valve discharge)?
- Are electrodes sufficiently downstream from pump or static mixer to reduce concentration and pressure fluctuations but not so far as to increase deadtime by more than 3 sec (slides 33,34)?
- Are insertion electrodes in series used to ensure same velocity and composition (slides 33,34)?
- Is electrode location free from flashing (e.g. not on pump suction or valve discharge)?
- Should an insertion type of electrode assembly with ball valve and restraining strap be used to safely withdrawn (retract) the electrode from a pipe or vessel with process fluid?
- Is the electrode and transmitter location safely accessible for maintenance?
- Are electrode and transmitter signal connections always dry?
