Industrial pH and ORP Sensors

What Are pH and ORP Sensors?

pH is the logarithmic expression of hydrogen ion activity in a solution and indicates whether a liquid is acidic, neutral or alkaline. Because the scale is logarithmic, a shift of one pH unit corresponds to a tenfold change in hydrogen ion activity. In process steps such as neutralization, chemical dosing and corrosion control, even a small measuring error can therefore turn into noticeable deviations in reagent consumption and product quality. ORP (oxidation-reduction potential) expresses in millivolts a solution’s tendency to gain or give up electrons, and it is used to monitor how effectively oxidizing or reducing chemicals are working.

An industrial pH sensor combines three elements: a pH-sensitive glass electrode, a reference electrode that supplies a stable comparison potential, and a liquid junction that brings the reference electrolyte into ionic contact with the process liquid. In ORP sensors the glass electrode is replaced by a metal measuring electrode. Either way, the sensor itself delivers only a raw millivolt signal, which travels via a preamplifier to a liquid analysis transmitter, is processed there with the calibration data and is then passed on to the control system.

The sensor group described on this page covers general-purpose sensors, immersion/insertion types mounted in pipelines or basins, metal-bodied retractable sensors that can be taken out of the line while the process is running, and high-performance versions whose reference junction and fill gel can be renewed. This allows one measuring principle to be adapted to very different mounting and maintenance requirements.

How Potentiometric pH and ORP Measurement Works

pH and ORP measurement is potentiometric: without drawing any meaningful current from the circuit, the sensor detects the potential difference between the measuring electrode and the reference electrode. The measuring chain can be summarized in five steps:

1.      Potential development at the glass membrane: The thin glass bulb at the tip of the pH electrode interacts with hydrogen ions at its surface and generates a potential that depends on the pH difference between the internal fill solution and the process. For this reason the electrode is given a gentle shake before start-up, so the bulb is completely filled with internal solution and no air is left at the tip.

2.      Reference electrode and liquid junction: The reference electrode produces a constant potential within its electrolyte fill, while the liquid junction forms the ionic path between this electrolyte and the process. A fouled junction or a depleted electrolyte causes a shift of the zero point (offset) in the measurement.

3.      Metal electrode for ORP measurement: In ORP sensors a noble-metal electrode such as platinum takes on a potential set by the balance of oxidizing and reducing species in the solution. Measured against the reference electrode, this potential is read directly as the ORP value in millivolts.

4.      Effect of temperature: The slope of the glass electrode, expressed in mV/pH, varies with temperature. Temperature compensation is applied to obtain a correct result, and during calibration the sensor is also given time to reach the temperature of the buffer solution.

5.      Preamplification and transmitter: Since the glass electrode has a very high internal impedance, its raw signal is susceptible to electrical noise. Sensors are configured with or without an integral preamplifier, and the wiring follows from that choice, the transmitter type and the cable color code. The transmitter then calculates the pH or ORP value from the slope and offset established during calibration.

Two-Point Buffer Calibration and Standardization of pH Sensors

The way to secure the accuracy of a pH measurement is to calibrate the sensor in two buffer solutions of known pH. The sensor is placed in the first buffer and left until it has reached the solution temperature and the reading has settled. After the transmitter has accepted this first point, the sensor tip is rinsed with a little of the second buffer, and the procedure is repeated in the second solution. From these two points the transmitter recalculates the zero offset (mV) and the slope (mV/pH). The first calibration is recommended at commissioning.

Once the buffer calibration is done and the sensor has conditioned to the process, the measurement can be standardized in-line or against a grab sample. The sample is drawn close to the sensor and measured with a second calibrated instrument that has temperature compensation; the transmitter reading is then set to this value. The slope is left unchanged by standardization — instead, liquid junction potential and similar interfering effects are balanced out so that the reading comes closer to the true process pH. Standardizing at process temperature is recommended.

Calibrating an ORP Sensor with Quinhydrone Solution

ORP sensors are calibrated in a standard solution whose potential is known. A practical method is to make a saturated quinhydrone standard by dissolving a few quinhydrone crystals in pH 4 or pH 7 buffer; because quinhydrone dissolves only sparingly, a handful of crystals is enough. Before calibrating, sensor and transmitter are briefly connected electrically so that any charge that has built up is released. The sensor is then immersed in the standard, given one to two minutes to stabilize, and the ORP value on the transmitter is adjusted to the reference value that applies at the solution temperature.

At 25 °C, for example, saturated quinhydrone solution gives approximately 264 mV in pH 4 buffer and approximately 87 mV in pH 7 buffer. Using a saturated KCl/AgCl reference and a clean platinum electrode, the reading is expected to stay within ±20 mV of these values. Since the potential of quinhydrone solution loses its stability over time, a fresh standard has to be prepared for every calibration, and because the solution is acidic it should be handled with suitable personal protective equipment.

Tracking Sensor Health with Calibration Data

Calibration does more than correct the reading — it also provides objective information on how much service life the sensor has left. If, in a two-point calibration, the slope falls below the typical lower limit of 40–50 mV/pH, the glass electrode is worn out. A zero offset whose absolute value exceeds roughly 60 mV signals an exhausted reference system. Sluggish response or unsteady readings are often caused by a fouled electrode surface and can be cleared by cleaning; if cleaning does not bring an improvement, the sensor should be replaced.

On the mounting side, every sensor should be installed with its tip pointing downward and no more than 80° from vertical. This orientation keeps the air bubble inside the sensor from settling at the tip of the glass bulb and disturbing the measurement; horizontal or tip-up installations should be avoided.

Sensor Design Highlights

The sensor group offers design options that carry one measuring principle into a range of mounting, wiring and maintenance scenarios.

·         Three mounting concepts: An insertion type threaded directly into the pipeline, an immersion type fixed to the end of a pipe and lowered into a basin, and a retractable type that can be withdrawn through a ball valve — selected according to the measuring point and the maintenance strategy.

·         Dual-direction NPT threaded body: Insertion sensors carry a forward-facing 1-inch NPT process connection (on some versions an additional ¾-inch NPT) and a rear-facing 1-inch NPT thread. The rear thread is used to attach an immersion pipe, which keeps the cable entry dry.

·         Metal-bodied retractable sensors: Sensors with a 25 mm (1-inch) outside-diameter metal body come in several lengths and are connected to the process with a ferrule-type compression adapter or a ball valve retraction assembly.

·         Safe retraction mechanism: A retraction stop integrated into the body prevents the sensor from slipping out of its mounting hardware and being ejected by process pressure. A guard at the electrode tip shields the glass from damage when the sensor rests against the closed valve.

·         Preamplifier options: Sensors can be configured without a preamplifier or with an integral standard preamplifier; pH sensors are also offered with a smart preamplifier option. With a remote junction box and an extension cable, the sensor can be placed at a distance from the transmitter.

·         Renewable reference system: On some high-performance sensors the liquid junction and the reference fill gel are replaceable. When calibration reveals a high offset, only the reference side needs to be renewed instead of the complete sensor.

·         Diagnostics based on calibration data: The slope and offset values that the transmitter calculates at every calibration give a concrete yardstick for the condition of the glass electrode and the reference system.

·         Intrinsically safe design: Depending on configuration, versions marked with the Ex ia IIC type of protection and equipment protection level Ga can be used at sites classified as potentially explosive atmospheres.

·         Alternative ferrule and simple seal maintenance: If the standard PTFE ferrule in the compression adapter does not hold the sensor firmly enough, a stainless steel ferrule can be fitted instead; should the adapter start to leak, a new O-ring restores the seal.

Technical Data and Configuration Options

The table below summarizes what the sensor group has in common in terms of mounting, connection, calibration and hazardous-area suitability. Measuring range, accuracy, process temperature and pressure, and wetted materials vary with the sensor type, so these values are confirmed separately for the selected configuration at project stage.

The temperatures given in the hazardous-area markings are ambient limits and must not be read as process temperature limits. Whether the wetted materials suit the process composition and operating conditions is the user’s responsibility and should be assessed separately during selection.

Key Advantages

·         Mounting that fits the measuring point: Separate mounting concepts for pipelines, basins and uninterrupted processes mean the sensor is matched to the process — the measuring point does not have to be rebuilt around the sensor.

·         Maintenance without stopping the process: Within the permitted pressure limits, the ball valve retraction assembly lets the sensor be taken out of the line for cleaning, calibration or replacement.

·         Predictable sensor replacement: Thanks to slope and offset criteria, sensor replacement can be scheduled on measured performance data rather than on a fixed calendar.

·         Lower life-cycle cost: On sensors with a renewable reference system, changing only the liquid junction and fill gel can cut consumable costs compared with replacing the whole sensor.

·         Verification against real process conditions: Standardization, which complements buffer calibration, balances out effects such as liquid junction potential so that the reading agrees with a process sample.

·         Suitability for hazardous areas: Intrinsically safe (Ex ia) approved configurations make it possible to set up measuring points in process zones classified as potentially explosive atmospheres.

·         Signal integrity over long distances: Integral preamplifier and remote junction box options help the high-impedance electrode signal reach the transmitter more reliably.

Use Cases and Application Areas

Depending on the mounting concept, pH and ORP sensors can be adapted to very different measuring points. The scenarios below show how the sensor type is chosen for a given measuring point; suitability for a specific industry is assessed separately on the basis of process chemistry and wetted-material compatibility.

Process Pipelines

The insertion sensor is screwed into the line, or into a mounting adapter, with its forward-facing NPT thread. On lines where chemicals are dosed in proportion to flow, the pH measurement can be combined in the control loop with a flow measurement — for example from magnetic flow meters — to drive the dosing pump or valve.

Basins, Sumps and Open Tanks

The sensor is fixed to an immersion pipe by its rear-facing thread and lowered to the required depth; since the cable connection sits inside the pipe, it stays protected from moisture. Retractable sensors can also be used on an immersion pipe by fitting the compression adapter in the reverse direction.

Processes That Must Run Without Interruption

Where draining the line is costly or not possible, a retractable sensor can be taken out for maintenance via a ball valve assembly, as long as the line pressure is below the permitted retraction pressure.

Fluids Prone to Oil and Scale Build-Up

Oily films can be cleaned off using a mild, non-abrasive detergent, and scale by a short soak in dilute hydrochloric acid. In processes of this kind, retractable mounting makes routine cleaning practical.

Control of Oxidizing and Reducing Chemicals

ORP measurement follows, in millivolts, how active oxidizing or reducing chemicals are in the process; combined with pH measurement, it helps confirm whether a reaction has reached its intended end point.

Sites Classified as Potentially Explosive Atmospheres

Intrinsically safe (Ex ia) approved configurations can be used in process zones with a hazardous-area classification, together with a transmitter and wiring that fit the intrinsically safe circuit concept.

Parameters to Consider When Selecting a pH Sensor

The right sensor results from looking at the measuring task, the mounting point and the available maintenance capacity together. Before a technical assessment we recommend clarifying the following:

Measuring Task and Process Chemistry

·         Variable to be measured: pH, ORP or both

·         Expected pH/ORP range and the resolution needed for control

·         Process composition and chemical compatibility with the wetted materials

·         Tendency towards oil, scale or solids build-up

·         Minimum / normal / maximum process temperature and pressure

Mounting Concept and Mechanical Conditions

·         Mounting type: insertion, immersion or retractable

·         Existing connection thread (1-inch or ¾-inch NPT) and need for a mounting adapter

·         Required immersion depth and sensor length

·         For retractable mounting: line pressure compared with the permitted retraction pressure for the sensor length

·         Whether the tip-down, maximum 80° from vertical condition can be met, and enough working space for retraction

Electrical Connection and Hazardous Area

·         Transmitter type to be used and preamplifier preference

·         Cable distance between sensor and transmitter; need for a remote junction box and an extension cable

·         Area classification, required type of protection, temperature class and ambient temperature range

·         Whether further parameters will be monitored at the same point with dissolved oxygen sensors or toroidal conductivity sensors

Calibration, Maintenance and Storage

·         Planned calibration frequency and the buffer solutions to be used

·         Possibility of drawing a sample close to the sensor for standardization

·         Preference for a renewable reference system and a spare consumables plan

·         Storage conditions for spare sensors (temperature-controlled room, protective cap)

Where several liquid analysis parameters have to be monitored at one point, running the sensors on a common transmitter platform simplifies both wiring and maintenance planning.

Hazardous-Area Approvals and Compliance Information

The hazardous-area approvals and compliance information defined for the sensor group are summarized below. The applicable marking changes with the sensor type and with whether, and which, preamplifier is fitted, so it must be confirmed against the ordered configuration.

Hazardous-area suitability has to be verified for every project by considering the complete intrinsically safe circuit — sensor, preamplifier, transmitter and wiring. TLY Enerji supports project teams in this assessment.

pH/ORP Measuring Point Engineering with TLY Enerji

TLY Enerji approaches pH and ORP measurement as a measuring chain made up of sensor, mounting hardware, wiring and transmitter. At the outset we review the fluid composition, temperature and pressure conditions, build-up tendency and hazardous-area classification together, and decide which of insertion, immersion or retractable mounting suits the measuring point. For retractable solutions, we check the sensor length and the permitted retraction pressure against the actual line conditions.

We provide engineering support for sensor–transmitter matching, preamplifier selection and cable routing, integration into the control system, the first buffer calibration at commissioning, and the preparation of calibration and cleaning procedures for maintenance teams. Our aim is a measuring point whose condition can be followed through slope and offset data over its entire operating life — with maintenance that is planned rather than reactive.

·         Measuring point analysis and choice of mounting concept

·         Project-specific assessment of hazardous-area suitability

·         Transmitter, preamplifier and wiring engineering

·         PLC / DCS / SCADA integration and commissioning support

·         Planning of calibration, cleaning and spare consumables

Frequently Asked Questions

What is a pH sensor and how does it work?

A pH sensor is a potentiometric sensor that continuously measures how acidic or alkaline a liquid is. Its pH-sensitive glass electrode develops a potential that depends on the hydrogen ion activity in the process, while the reference electrode provides a fixed comparison potential. A compatible transmitter converts the millivolt difference between the two electrodes into pH units, using the slope and offset determined during calibration, and passes the result to the control system.

What is the difference between pH and ORP measurement?

pH measurement shows hydrogen ion activity — in other words, whether a solution is acidic or alkaline — and uses a glass electrode. ORP measurement expresses in millivolts whether a solution tends to oxidize or reduce, and uses a metal electrode such as platinum. Because both rely on the same potentiometric principle, similar mounting hardware is used; however, the calibration methods differ, and each answers a different process question.

How do you calibrate a pH sensor?

The most dependable method is a two-point buffer calibration. The sensor is first placed in one buffer and left until it has reached the solution temperature and the reading is steady. After the transmitter has accepted this point, the sensor tip is rinsed with the second buffer and the step is repeated in the second solution. From the two points the transmitter calculates the zero offset (mV) and the slope (mV/pH). Calibration is recommended at commissioning.

What is pH standardization and how does it differ from buffer calibration?

Standardization means aligning the reading with a known process pH after buffer calibration, once the sensor has conditioned to the process. A sample taken near the sensor is measured with a second calibrated, temperature-compensated instrument, and the transmitter reading is set to that value. Unlike buffer calibration, standardization leaves the slope as it is; it balances out interfering effects such as liquid junction potential. It is recommended to standardize at process temperature.

How do you calibrate an ORP sensor?

An ORP sensor is calibrated in a standard solution of known potential. A common choice is saturated quinhydrone solution, made by adding a few quinhydrone crystals to pH 4 or pH 7 buffer. The sensor is immersed, left for one to two minutes, and the transmitter value is set to the reference value for the solution temperature — for example about 264 mV in pH 4 buffer at 25 °C. Because the solution loses its stability over time, prepare it fresh each time.

How can you tell when a pH sensor needs replacing?

Calibration data give the clearest indication. If the slope in a two-point calibration has dropped below the typical lower limit of 40–50 mV/pH, the glass electrode has come to the end of its service. A zero offset above about 60 mV in absolute value shows that the reference system is exhausted; on sensors with a renewable reference, the liquid junction and fill gel can then be replaced and the sensor recalibrated. Slow or erratic response should first be tackled by cleaning.

Can a pH sensor be removed while the process is running?

Retractable sensors used with a ball valve retraction assembly can be withdrawn from the line for cleaning or replacement. This must only be done while the line pressure is under the maximum retraction pressure that applies to the sensor length: 5.4 barg for a 533 mm sensor and 3.4 barg for a 914 mm sensor. Do not close the valve until the sensor has been pulled all the way back to its stop; wear a face shield and keep a safe working position. In line with the safety warnings, pressure and temperature should be brought down to a safe level wherever possible.

How should a pH sensor be cleaned and stored?

Oily deposits are cleaned off with a mild, non-abrasive detergent; scale is removed by soaking the electrodes in 5% hydrochloric acid for one to five minutes. Metallic ORP electrodes can be polished with moistened baking soda (sodium bicarbonate). For storage, keep the sensor in its original packaging in a temperature-controlled room; once unpacked, store it tip down with its protective cap, which contains KCl solution. For short overnight breaks, tap water or pH 4 buffer can be used.

Can pH and ORP sensors be used in hazardous areas?

Yes, in suitable configurations. Intrinsic safety approvals exist under ATEX, UKEX, IECEx and EAC with Ex ia IIC T4/T5 Ga marking, and for the USA and Canada with Class I, Division 1 and Zone 0 markings. The applicable marking depends on the preamplifier type. Plastic surfaces should be wiped with a damp cloth only, and metal housings must be protected from impact and friction. Suitability of the complete circuit has to be assessed for each project.