Ask an instrument technician in any Malaysian chemical plant, scrubber or effluent treatment facility what wears out first on a pH sensor, and the answer is rarely the glass. It is the reference. A pH sensor can have a perfect glass bulb and still read 0.5 pH units wrong, because the half of the sensor that is supposed to hold a constant voltage has quietly stopped doing so.
Over the past decades, pH sensor makers have tried three broad approaches to protecting the reference in harsh processes: the conventional liquid or gel reference, the solid-state reference, and multi-stage barrier designs. Understanding how each one works, and how each one eventually fails, is the most useful knowledge a buyer can have when specifying a pH sensor for aggressive service.
The short answer is this. A conventional liquid or gel reference fails quickly in harsh processes because contaminants reach the silver/silver chloride element easily and the electrolyte leaks away. A solid-state reference slows this down considerably by removing the liquid, but it still relies on a single medium to hold contaminants back. A multi-stage barrier reference, such as LECOL’s patented ION BARRIER, places several different defences in series, each designed to stop a different threat, which is why it keeps the reference stable for longer in the harshest service.
Why the reference electrode decides sensor life
A pH sensor measures the voltage difference between two electrodes. The glass electrode develops a voltage that changes with pH. The reference electrode, usually silver wire coated with silver chloride sitting in potassium chloride (KCl), must hold a fixed voltage no matter what the process is doing. The sensor can only report pH correctly if that reference voltage stays put.
To complete the circuit, the reference must be in electrical contact with the process through a junction. That contact is also its weakness. Anything that passes through the junction and reaches the silver/silver chloride element can change its voltage, and anything that blocks the junction breaks or destabilises the connection. LECOL attributes over 90% of pH sensor failures to deterioration of the reference cell, through three mechanisms:
Poisoning. Sulphides, cyanides, bromides, heavy metals and strong reducing or oxidising agents react with silver chloride, converting it into compounds with a different electrical potential.
Clogging. Particles, precipitates, oils and coatings block the junction, raising its resistance and making the signal noisy and slow.
Depletion and dilution. KCl leaks out into the process, or process liquid seeps in, changing the chloride concentration that sets the reference voltage.
Generation one: the liquid or gel reference
The conventional design fills the reference chamber with liquid or gelled KCl and connects it to the process through a small porous ceramic junction. In clean water this works well and is inexpensive, which is why it remains the most common design worldwide.
In harsh processes, every weakness appears at once. The small ceramic junction clogs easily. The liquid path gives poisoning ions a short, direct route to the silver wire. Pressure and temperature changes pump electrolyte out and process liquid in. The result is a sensor that needs frequent cleaning and recalibration and, in aggressive service, may last only weeks.
Generation two: the solid-state reference
Solid-state references replace the liquid with a solid or polymer matrix charged with KCl. This was a major step forward, and it is the basis of established industrial sensors such as ABB’s TB5 series, including the TB556 threaded sensor, which uses a totally solid inner reference chamber charged with KCl and describes it as all but eliminating the poisoning, plugging and pumping problems of liquid and gel designs.
Removing the liquid brings real advantages. There is no electrolyte to refill, no liquid to be pumped out by pressure changes, and contaminants diffuse through a solid far more slowly than through a liquid.
The limitation is that the solid matrix is still a single medium. Poisoning ions do not stop at its surface; they diffuse through it more slowly, and over time a front of contamination advances towards the silver/silver chloride element. Particles and oils that reach the junction surface are not filtered out by the solid itself. The solid-state reference delays the failure mechanisms of the liquid design, but it relies on one material to resist all of them.
Generation three: the multi-stage ION BARRIER reference
LECOL’s approach, protected by US Patent 12540912 B2, treats reference protection as a series of separate defences rather than a single barrier. In its B65 harsh environment pH sensor, process liquid passes through three stages before anything can reach the silver/silver chloride element:
A large-area liquid junction. Instead of a small ceramic pin, the junction is a large surface of porous PTFE, or wood for oily and hydrocarbon-fouling samples. A large area is much harder to block completely, so the electrical connection to the process stays stable even as fouling builds.
A wood-cell filter. Behind the junction, natural wood fibres trap larger contaminants within their cellular structure, removing particles and organic material before they go further into the reference.
The ION BARRIER matrix. The final stage is a matrix of randomised ionic pathways. Instead of a direct route, aggressive ions must follow a long, tortuous path, and the matrix restricts the passage of harmful ions and neutralises chemical attack along the way.
Why the layered design holds up better than both
The advantage comes from the principle of defence in depth. Each failure mechanism is handled by the stage best suited to it, rather than one material trying to resist everything.
Clogging is addressed at the front, by a junction large enough that partial fouling does not break the connection. Particles and organics are removed by the wood-cell filter before they can reach the inner reference. Poisoning ions face a path that is both longer and more restrictive than a straight path through a single solid, which slows the advance of the contamination front further. Compared with a liquid or gel reference, every failure mechanism is slowed. Compared with a single solid-state medium, poisoning and fouling are handled by dedicated stages instead of one material, which is the basis of LECOL’s claim of greater stability and longevity than conventional solid-state systems.
| Criteria | Liquid or gel reference | Solid-state reference (e.g. ABB TB5 series) | LECOL ION BARRIER |
|---|---|---|---|
| Reference medium | Liquid or gelled KCl | Solid matrix charged with KCl | Multi-stage: junction, wood-cell filter, ION BARRIER matrix |
| Path for poisoning ions | Short and direct | Slowed through a single solid | Long, tortuous and restricted |
| Particle and oil protection | Small junction, clogs easily | Depends on junction | Large junction plus internal filter |
| Electrolyte refill | Often required | Not required | Not required |
| Effect of pressure changes | Electrolyte pumped in and out | Greatly reduced | Greatly reduced |
The rest of the sensor matters too
Glass chosen for the process. The B65 measuring electrode can be specified as hydrofluoric acid resistant, coating resistant hemispherical or flat glass, low temperature glass, high temperature glass (15 to 135°C, tolerating 140°C temporarily), or platinum for ORP. Flat glass resists coating, and HF resistant glass survives fluoride that dissolves ordinary pH glass.
Temperature measured at the glass. LECOL places the PT100 or PT1000 element directly in front of the glass electrode, so temperature compensation follows process changes immediately rather than lagging behind.
A chemically resistant body. The B65 body is PPS (Ryton), rated to 10 bar at 70°C and 2.8 bar at 135°C, with a 3/4 inch NPT thread for in-line or immersion mounting, the same installation format used by threaded industrial sensors such as the ABB TB556.
Where these sensors are used in Malaysia
Threaded harsh-environment pH sensors are found wherever process chemistry is aggressive: neutralisation tanks, gas scrubbers, flue gas desulphurisation at power stations, sewage and industrial effluent treatment, chemical pulping, dyes and pigments, and semiconductor wastewater. In Malaysia, that covers a wide range of sites, from electronics and semiconductor fabs in Penang and Kulim to chemical, power and wastewater facilities across Selangor and Johor.
What to ask when specifying a harsh-process pH sensor
Before choosing any sensor for aggressive service, ask four questions. What will poison the reference in this process, and how does the reference design slow it down? What will foul the junction, and how large is the junction area? Which glass suits the chemistry and temperature? And how is temperature measured? A sensor that answers all four for your process will hold calibration far longer than one chosen on price or habit.
Autoflo Technology is the authorised distributor of LECOL pH and ORP sensors in Malaysia. For help specifying a pH sensor for a harsh process, contact us at info@autoflotechnology.com.
ABB, TB556 and TB5 are trademarks or product designations of ABB, referred to here for technical comparison only. Autoflo Technology and LECOL are not affiliated with ABB.