Not every high-pressure pH measurement needs a retractable assembly. On many pressurised pipelines in Malaysian plants, the line is isolated for maintenance on a planned schedule anyway, and the measurement point needs a sensor that simply survives pressure and temperature reliably between those shutdowns. That is the role of the high-pressure in-line pH sensor, and its design follows a principle that is worth understanding before choosing one: separate the part that carries pressure from the part that measures.
This article explains why ordinary pH sensors struggle in pressurised lines, how two-piece high-pressure construction works, how the main reference technologies compare under pressure, and when an in-line design is a better choice than a retractable one.
The short answer is this. A normal pH sensor fails in pressurised lines because its plastic body is not built to carry the load and its liquid or gel reference is contaminated when pressure drives sample through the junction. High-pressure in-line sensors use a two-piece design, with a chemically resistant sensing element inside a stainless steel housing that carries the mechanical load, combined with a non-liquid reference. LECOL’s B85 uses this construction with its multi-stage ION BARRIER reference, which adds dedicated protection against fouling and poisoning.
Why ordinary pH sensors struggle in pressurised lines
The body carries a load it was not designed for. A normal pH sensor body is moulded plastic with a single seal, sized for tanks, open channels and low-pressure flow cells. Installed directly in a pressurised pipe, it must hold back the full line pressure, often at elevated temperature, and plastics creep under that combination over time. Seals extrude and threads loosen.
The reference is pushed and pulled. In a liquid or gel reference, process pressure higher than the internal pressure forces sample through the junction into the electrolyte, contaminating it and shifting the reference voltage. As pressure rises and falls with pumps and valves, the flow through the junction changes direction and the reading moves with it.
The temperature limits are low. General purpose glass and gel references are typically limited to around 60 to 80°C, below the operating temperature of many pressurised process lines.
Two-piece construction: separating pressure from measurement
High-pressure in-line sensors solve the mechanical problem by splitting the sensor into two parts. A stainless steel housing threads into the process and carries the pressure load. The sensing element, with its glass, reference and temperature element, sits inside the housing in a chemically resistant plastic body, sealed with O-rings.
This arrangement has three advantages. The stainless steel housing handles pressure and temperature that a plastic body could not. The plastic sensing element only has to resist the process chemistry, not carry the load. And when the sensing element reaches the end of its life, it can be replaced while the housing stays in place, reducing the cost of each replacement.
Choosing the reference for a pressurised line
The housing solves the mechanical problem, but the reference inside still determines how long the measurement stays accurate.
Liquid or gel references are poorly suited to pressurised lines because pressure drives sample into the electrolyte and pressure changes move the reading.
Solid-state references remove the liquid that pressure acts on, which largely solves the pressure problem. Established two-piece high-pressure sensors such as ABB’s TB567, part of the TB5 series, combine a solid-state reference with a Ryton (PPS) electrode body, a 316 stainless steel outer sleeve, Viton O-rings and a wood or PTFE junction, with published ratings of 200 psi at 140°C and 250 psi at 100°C. The limitation that remains is chemical: contaminants diffuse slowly through the single solid medium over time, and the junction surface is not protected from fouling by the solid itself.
Multi-stage barrier references keep the pressure tolerance of a non-liquid design and add dedicated protection. In LECOL’s patented ION BARRIER design, process liquid passes through a large-area junction, then a wood-cell filter that traps particles and organics, and finally a matrix of randomised ionic pathways that forces aggressive ions along a long, restricted path to the silver/silver chloride element. Because poisoning and fouling are each handled by their own stage, the reference stays stable for longer than with either a liquid reference or a single solid medium. Our article on how pH sensor reference technology decides life in harsh processes covers the three generations in more depth.
How the LECOL B85 is built
The LECOL B85 is a two-piece high-pressure sensor: a PPS (Ryton) sensing element inside an optional 316 stainless steel housing, rated to 20 bar maximum and 135°C maximum, with the ION BARRIER reference. The junction is notched PTFE or notched wood, and the measuring glass can be hydrofluoric acid resistant, coating resistant hemispherical, high temperature (15 to 135°C, tolerating 140°C temporarily) or platinum ORP, with PT100 or PT1000 temperature compensation.
| Criteria | Normal pH sensor | Two-piece sensor, solid-state reference (e.g. ABB TB567) | LECOL B85, ION BARRIER reference |
|---|---|---|---|
| Construction | Single plastic body | Ryton electrode body, 316 SS sleeve | PPS sensing element, 316 SS housing |
| Effect of pressure on reference | Sample forced into electrolyte | Largely eliminated | Largely eliminated |
| Protection against poisoning and fouling | Short direct path, small junction | Single solid medium | Large junction, wood-cell filter and ION BARRIER in series |
| Published rating | Typically a few bar, 60 to 80°C | 200 psi at 140°C, 250 psi at 100°C | 20 bar, 135°C |
| Replacement | Whole sensor | Electrode, sleeve reused | Sensing element, housing reused |
The B85’s 20 bar rating sits above the published pressure ratings of typical two-piece solid-state sensors at temperature, while its maximum temperature is 135°C. For lines running continuously close to 140°C, the actual operating temperature should be confirmed during selection.
In-line or retractable?
An in-line sensor can only be removed after the line is depressurised and isolated. Where the process is shut down for maintenance on a regular schedule, this is the simpler and more economical choice, because there is no extraction housing or valve to maintain. Where the line runs continuously and cannot be interrupted, a retractable design is worth its extra cost, and we cover that option in our article on measuring pH under high pressure with retractable sensors. Operating patterns at many Malaysian plants change over the life of an installation, so this choice is worth reviewing whenever a sensor is replaced.
Where high-pressure in-line sensors are used in Malaysia
High-pressure in-line pH sensors are used on high-pressure pipelines and other pressurised process points. In Malaysia, these include process lines in petrochemical, oleochemical and chemical plants, high-pressure process and utility water systems, and power generation, where continuous and reliable pH measurement is needed under conditions an ordinary sensor would not withstand.
Autoflo Technology is the authorised distributor of LECOL pH and ORP sensors in Malaysia. For help specifying pH measurement for pressurised process lines, contact us at info@autoflotechnology.com.
ABB, TB567 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.