Toroidal Conductivity Sensors

What Is a Conductivity Sensor and Why a Toroidal Design?

Conductivity is a liquid’s ability to carry an electric current. It depends on the type and concentration of the ions in solution and on temperature, and in industry it is usually stated in µS/cm, mS/cm or S/cm. While conductivity is very low in fluids close to pure water, it climbs to high values in acid, base and salt solutions. That makes conductivity measurement a practical way of continuously monitoring the amount of dissolved ions — and, indirectly, the chemical concentration.

Process conductivity sensors divide into two basic groups: contacting (electrode-type) sensors, which measure with metal electrodes in direct contact with the fluid, and the toroidal — also called inductive — sensors covered on this page. In a toroidal design the measuring coils are completely enclosed inside a chemically resistant polymer body, so the liquid only ever touches the body material. As there is no electrode surface, error sources that often affect contacting sensors, such as polarization, electrode coating and electrode corrosion, are largely removed.

On its own, the toroidal sensor delivers no usable output. It operates together with a compatible liquid analysis transmitter that excites the coils and processes the measured signal. The transmitter uses the sensor’s cell constant and the reading from the integrated temperature element to calculate conductivity, then forwards the result to the control system. Where concentration is being monitored, this conductivity value is interpreted through the conductivity–concentration relationship of the particular solution.

The Physics of Toroidal (Inductive) Conductivity Measurement

Toroidal measurement resembles the operation of a transformer: the process liquid flowing through and around the sensor provides the coupling between two ring-shaped (toroidal) coils in the sensor body. The measurement proceeds as follows:

1.      Excitation of the drive coil: The transmitter applies an alternating voltage to the first (drive) coil. The changing magnetic field set up in the coil core induces an electric field in the liquid around the coil.

2.      Closed current loop in the liquid: The liquid that passes through the bore in the sensor body and surrounds its outside forms a closed conductive loop. The current induced in this loop is directly proportional to the conductivity of the liquid.

3.      Signal generation in the receive coil: The current in the liquid loop also threads the core of the second (receive) coil, where it induces a signal proportional to that current. The two coils have no electrical contact with each other; the conductive liquid alone couples them.

4.      Conversion to conductivity via the cell constant: The receive-coil signal is evaluated with the cell constant (nominal 2.7/cm), which represents the geometry of the liquid loop, and turned into a conductivity value.

5.      Temperature compensation: Since conductivity varies strongly with temperature, a Pt 100 RTD integrated in the sensor measures the process temperature. Using this value, the transmitter corrects the reading to a reference temperature so that temperature swings are not mistaken for changes in concentration.

Why Is Electrodeless Measurement an Advantage in Dirty and Corrosive Media?

With contacting sensors, the measurement happens at the interface between the electrode surface and the liquid. At high conductivity, polarization at this interface becomes pronounced; once oil, sludge or crystalline deposits coat the electrode, the reading drifts, and aggressive chemicals corrode the metal electrode directly. In a toroidal sensor the polymer body is the only surface wetted by the liquid. Because a thin layer of deposit barely changes the total resistance of the current loop, the measurement tolerates fouling far better than electrode designs. Periodic visual inspection is still advisable for thick deposits that could narrow the sensor bore.

From Conductivity to Concentration

In solutions dominated by a single dissolved substance, conductivity and concentration are linked by a defined relationship, which is what allows acid, base and salt concentrations to be tracked continuously. In many solutions, however, conductivity rises with concentration only up to a peak and then falls again, so one conductivity value can correspond to two different concentrations. The measurement therefore has to be designed by checking that the process concentration range stays on one side of the curve (the monotonic part), and temperature compensation has to be configured specifically for the solution.

How the Installation Environment Affects the Measurement

The current loop sensed by a toroidal sensor also takes in the volume of liquid around its body. When the sensor sits too close to a pipe wall or to the inner surface of a tank, the loop geometry is altered and the effective cell constant deviates — a phenomenon known as wall effect. Sufficient clearance around the sensor is recommended; if that cannot be achieved, calibration should be performed with the sensor in its installed position. Air bubbles caught in the sensor bore can also depress the reading, so the mounting orientation should be selected with flow and gas separation in mind.

Design and Construction Features

·         High conductivity measuring capability: Conductivity can be measured up to 2,000,000 µS/cm (2 S/cm), a working range that suits concentration monitoring in high-conductivity acid, base and salt solutions.

·         Chemically resistant body options: The sensor body is molded from glass-filled PEEK, glass-filled ETFE or unfilled ETFE; the material is chosen to match the chemical composition and temperature of the process.

·         High-temperature and high-pressure version: The high-temperature PEEK version can be used at process temperatures up to 200 °C and pressures up to 2,135 kPa (absolute).

·         Vibration-resistant internal structure: A reinforced internal metal frame raises the mechanical robustness of the sensor at mounting points that are exposed to vibration.

·         Integrated Pt 100 temperature element: A Pt 100 RTD built into the body senses the process temperature right beside the measuring zone for temperature compensation.

·         Two process connection standards: The ¾-inch MNPT threaded version is installed directly or with an adapter, while the ⅝-11 UNC threaded version is used with its standard EPDM gasket on a mounting adapter. FKM and FFKM gasket alternatives are also offered for the UNC version.

·         Shielded and unshielded cable: The standard integral cable is 6.1 m long; alongside 3 m, 10 m, 15 m and 30 m options, a shielded cable that improves EMI/RFI protection can be specified.

·         Removal without stopping the process: Optional mechanical (lead-screw driven) or manual retraction assemblies allow the sensor to be withdrawn from pipelines and tanks while the process stays in operation.

·         Documentation options: Calibration, loop calibration and electronic calibration certificates can be requested, as well as a material traceability certificate.

Technical Specifications

The table combines the defined data for the toroidal sensor itself and for the adapters and retraction assemblies used with it. All pressure values are absolute. The usable limit of the complete system is set by the lowest rating among the selected body material, adapter and retraction assembly.

Retraction assemblies are only compatible with the sensor configuration that has the ⅝-11 UNC connection and the short cable. The lower end of the measuring range and the achievable accuracy depend on the transmitter and must be confirmed for the sensor–transmitter combination at project stage.

Key Advantages

·         High tolerance to fouling: Without an electrode surface, thin deposits have practically no influence on the measurement, and cleaning is needed far less often than with electrode sensors.

·         No metal electrode to corrode: Because the wetted surface is a chemically resistant polymer, the sensor offers a long service life in aggressive acid, base and salt solutions.

·         Polarization-free measurement at high conductivity: A measuring capability that reaches 2 S/cm delivers stable data in the high-conductivity region, where polarization becomes a problem for contacting sensors.

·         Continuous data for concentration control: Changes in acid, base or salt concentration can be followed in real time — and dosing adjusted accordingly — without waiting for laboratory sample results.

·         Suited to demanding process conditions: The high-temperature version, the 2,135 kPa pressure limit and the vibration-resistant internal structure allow the sensor to serve on heavy-duty lines.

·         Process continuity during maintenance: With a retraction assembly, the sensor can be removed for inspection, cleaning or replacement without draining the line.

·         Easy integration into existing plants: Two thread standards and insertion adapters in different materials make it simpler to fit the sensor to existing process connections.

Processes Using Toroidal Conductivity Measurement

Toroidal conductivity sensors are used in processes that handle high-conductivity, dirty or corrosive fluids and wherever chemical concentration has to be monitored continuously. The headings below show how the sensor configuration is matched to the process condition.

Acid and Base Concentration Monitoring

When acid and base solutions are prepared, diluted or dosed, conductivity serves as a continuous indicator of concentration. The body material — PEEK or ETFE — is selected according to the type of solution and the operating temperature.

Salt Solutions and High-Conductivity Electrolytes

In high-conductivity electrolytes such as salt solutions, toroidal measurement provides stable readings up to 2 S/cm without any electrode polarization problem.

Dirty Fluids Prone to Build-Up

In fluids that carry suspended solids, oil or precipitates, the absence of an electrode surface reduces the impact of coating on the measurement and extends maintenance intervals.

Corrosive Chemical Lines

On lines where metal electrodes would corrode within a short time, the polymer-bodied sensor meets the process liquid only through chemically resistant material. The gasket material (EPDM, FKM or FFKM) must also be chosen to suit the process chemistry.

High-Temperature and Pressurized Lines

The high-temperature PEEK version can be used at process temperatures up to 200 °C together with a suitable stainless steel adapter or the mechanical retraction assembly. The system limit must be verified together with the ratings of the selected mounting hardware.

Removable Mounting in Tanks and Pipelines

On tanks and pipelines that run continuously, a mechanical or manual retraction assembly allows the sensor to be removed through a ball valve without stopping the process; the mechanical assembly has the higher pressure and temperature limits.

Conductivity Sensor Selection Guide

Selecting a toroidal sensor calls for a combined look at the chemistry of the solution, the process temperature and pressure envelope, the mounting geometry and the integration requirements. Answering the following questions before quotation and engineering work leads quickly to the right configuration:

Process Chemistry and Conductivity Range

·         Type of solution to be measured (acid, base, salt or mixture) and the expected conductivity range

·         If concentration is to be measured: whether the process range stays on the monotonic part of the conductivity–concentration curve

·         Whether a contacting sensor should be considered instead of a toroidal one for very low-conductivity fluids

·         Chemical compatibility of body (PEEK / ETFE) and gasket (EPDM / FKM / FFKM) materials

·         If pH control is needed alongside concentration: a measuring point designed together with pH and ORP sensors

Temperature and Pressure Envelope

·         Minimum / normal / maximum process temperature (above 120 °C the high-temperature PEEK version is required)

·         Maximum process pressure and the permitted limit for the body material

·         Whether Canadian pressure equipment registration (CRN) is required

·         Temperature-pressure limits of the selected adapter or retraction assembly

Mounting and Mechanical Conditions

·         Whether the sensor will be fixed in the line (insertion/immersion) or removable with a retraction assembly

·         Existing process connection: ¾-inch MNPT, ⅝-11 UNC, or 1½-inch / 2-inch MNPT via an adapter

·         Pipe diameter and clearance around the sensor (wall effect); calibration in the installed position if needed

·         Vibration, flow velocity and the risk of trapped air bubbles

·         Insertion travel and working space required for a retraction assembly

Cable, Transmitter and Documentation

·         Distance between sensor and transmitter and the matching cable length; shielded cable at electrically noisy sites

·         Compatibility with the liquid analysis transmitter to be used and any need for concentration calculation

·         Whether other liquid analysis measurements, such as dissolved oxygen sensors, will be handled on the same platform

·         A separate measuring point with temperature transmitters if process temperature must also be monitored independently

·         Requirements for calibration, loop calibration, electronic calibration and material traceability certificates

Conductivity data is also a useful guide when assessing whether instruments that rely on the conductivity of the liquid — such as magnetic flow meters — can be applied. If use is planned at a site with a hazardous-area classification, suitability should be assessed separately at project stage.

Certificates, Pressure Registration and Connection Standards

The documents that can be ordered with the toroidal sensor, the limits that apply under pressure registration and the thread standards in use are as follows:

If the sensor is to be used at a site classified as a potentially explosive atmosphere, the suitability of the sensor and transmitter combination should be discussed separately with TLY Enerji at the design stage.

End-to-End Conductivity Measurement Support from TLY Enerji

A dependable conductivity result depends as much on the fluid chemistry, the mounting geometry and correctly configured temperature compensation as on the sensor’s measuring capability. TLY Enerji engineers look at the type of solution and its concentration range, the process temperature-pressure envelope and the pipe or tank geometry at the mounting point together, and on that basis define the combination of body material, connection type and mounting hardware. The system limit is verified against the lowest rating among sensor, adapter and retraction assembly.

For concentration applications we check whether the conductivity–concentration relationship is unambiguous across the process range, and we support project teams with transmitter selection and configuration, cable and junction box planning, control system integration and commissioning. The result is a conductivity measuring point that reflects changes in process chemistry correctly from the first calibration onwards and serves as a control input with predictable maintenance.

·         Analysis of process chemistry and concentration range

·         Selection of body, gasket and mounting hardware

·         Assessment of wall effect and mounting geometry

·         Transmitter configuration and PLC / DCS / SCADA integration

·         Commissioning and maintenance planning support

Frequently Asked Questions

What is a toroidal conductivity sensor?

A toroidal conductivity sensor is an inductive sensor that measures the electrical conductivity of a liquid without using metal electrodes. Of the two coils inside its polymer body, one induces a current loop in the liquid and the other detects that current, whose magnitude is proportional to the liquid’s conductivity. With no electrode surface, it measures reliably in dirty, corrosive and high-conductivity solutions and is used to monitor acid, base and salt concentrations.

What is the difference between toroidal and contacting conductivity sensors?

Contacting (electrode-type) sensors measure the current between metal electrodes in direct contact with the liquid; this is an advantage at low conductivity, but coating, polarization and corrosion affect them. In a toroidal sensor the measuring elements are sealed inside a polymer body and the measurement is based on a current induced in the liquid. Toroidal designs are therefore preferred for high-conductivity, dirty and corrosive fluids, while contacting sensors should be considered for very low-conductivity fluids.

At what conductivity levels is a toroidal conductivity sensor used?

The sensor described on this page can measure high-conductivity electrolyte solutions up to 2,000,000 µS/cm (2 S/cm). Its nominal cell constant is 2.7/cm. The lower end of the measuring range and the accuracy that can be achieved depend on the transmitter used with the sensor, so if the application is in the low-conductivity region, the sensor–transmitter combination should be assessed separately at project stage.

How is concentration determined from a conductivity measurement?

In acid, base or salt solutions dominated by a single dissolved substance, conductivity and concentration follow a defined relationship; the measured, temperature-compensated conductivity is converted into concentration through this relationship. Because in many solutions conductivity peaks at a certain concentration and then declines, the process range must stay on one side of the curve. Otherwise the same conductivity could correspond to two different concentrations.

How do you choose the sensor body material?

The body material is selected for the process chemistry and the temperature and pressure conditions. A glass-filled PEEK body can be used up to 120 °C in the standard version and up to 200 °C in the high-temperature version, in both cases up to 2,135 kPa. Glass-filled or unfilled ETFE bodies are limited to 120 °C and 1,480 kPa and are chosen where chemical compatibility requires them. The gasket material (EPDM, FKM or FFKM) should be included in the same assessment.

Why must clearance be left around the sensor when it is installed?

The current loop measured by a toroidal sensor also extends through the liquid around its body. If the sensor is too close to a pipe or tank wall, the loop geometry changes and the effective cell constant deviates — this is called wall effect. Leaving adequate clearance around the sensor is recommended. Where that is not possible in narrow pipes, calibrating the sensor in its installed position is a practical way to compensate for the wall-effect error.

How does temperature affect conductivity measurement?

The conductivity of solutions rises significantly as temperature increases, so in an uncompensated measurement a temperature change can look like a change in concentration. The Pt 100 RTD integrated in the sensor measures the process temperature, and the transmitter uses this value to correct conductivity to a reference temperature. For concentration measurement, the compensation needs to be configured specifically for the solution being measured to obtain a correct result.

Can the sensor be removed without shutting down the process?

Yes, with a retraction assembly. The mechanical assembly is rated to 200 °C and 2,135 kPa, the manual assembly to 130 °C and 343 kPa; their maximum insertion travel is 267 mm and 305 mm respectively. The assemblies connect via a 1½-inch MNPT thread to a separately supplied ball valve and are used with the sensor configuration that has the ⅝-11 UNC connection and short cable. Once the sensor is retracted, the valve is closed and maintenance can proceed without opening the line.

Should shielded or unshielded cable be used?

At electrically noisy sites with motor drives, pumps or high-power equipment, a shielded cable that improves EMI/RFI protection should be chosen. Unshielded cable is recommended for certain transmitters, and some transmitters cannot be used with the shielded version, so the choice has to follow the transmitter. The standard integral cable is 6.1 m long; other lengths between 3 m and 30 m are available, and the run can be extended through a junction box.