Toroidal Conductivity Sensors: Electrodeless Measurement, Range and Selection
Non-contact measurement in coating, fouling or corrosive solutions: how it works, what range it covers, and which controller it pairs with.
When Is a Toroidal Sensor Selected?
- The solution coats, fouls or corrodes electrodes: oily cleaner baths, chromates, rinse tanks, fume scrubbers.
- Conductivity is high: 500 µS/cm – 2,000 mS/cm (from 0.1 mS/cm on active narrow-band models); concentrated chemicals and salt solutions.
- The target is concentration: the result can be displayed as µS/cm, mS/cm, ppm or % concentration.
- Do not select it: in very low conductivities such as pure water, RO permeate or boiler feedwater, where a contacting or UPW-type sensor is required.
No Electrodes: A Pair of Coils Measures the Solution
A toroidal sensor consists of two encapsulated coils (toroids). One coil induces a current loop circulating through the solution; the second coil reads the magnitude of this current. The current is proportional to the ionic content of the solution and is therefore measured as conductivity. Because the sensor makes no electrical contact with the solution, there is no exposed electrode surface; thin coatings, fouling or polarization on the surface do not distort the reading.
Walchem states that encapsulated, non-contacting toroidal technology is immune to the coating, fouling and calibration problems to which direct-contacting sensors are prone. The WEC310 series electrodeless conductivity controller datasheet notes that the sensor measures reliably across a wide dynamic range, that the PEEK construction withstands high temperatures and chemical attack, and that the electrodeless design provides longer maintenance-free operation against fouling.
| Feature | Contacting (electrode) | Toroidal (electrodeless) |
|---|---|---|
| Solution contact | Electrodes contact the solution | Encapsulated coil, non-contact |
| Thin coating / fouling | Can shift cell constant and calibration | Unaffected |
| Lower limit | Low ranges including 0–0.3 mS (300 µS/cm) with 0.01 cell constant | 500 µS/cm on passive model; 0.1 mS/cm on active narrow band |
| Upper limit | Up to 300 mS with cell constant 10.0 | Up to 2,000 mS/cm |
| Typical medium | Cooling tower, boiler, RO, oil-free condensate | Oily bath, chromate, rinse tank, scrubber, concentrated chemical |
Walchem Electrodeless Sensors: Range, Material, Temperature, Pressure
Values are taken from the Walchem conductivity sensors brochure (180555.E); bar and °C equivalents are approximate.
| Type | Range | Body | Temperature | Pressure | Mounting | Controller |
|---|---|---|---|---|---|---|
| Passive, wide range | 500 µS/cm – 2,000 mS/cm; cell constant 6.286 | CPVC | 20–180 °F (approximately -7…82 °C) | 140 PSI (approximately 9.7 bar) | 1" NPTM submersion, 2" NPTM inline | W600, W100 |
| Passive, wide range | 500 µS/cm – 2,000 mS/cm; cell constant 6.286 | PEEK | 32–190 °F (approximately 0…88 °C) | 140 PSI | 1" NPTM submersion, 2" NPTM inline | W600, W100 |
| Active, narrow band | 0.1–1; 1–10; 10–100; 100–1,000 mS/cm | CPVC | 20–158 °F (approximately -7…70 °C) | 140 PSI | 1" NPTM submersion, 2" NPTM inline; 20 ft cable, extension up to 250 ft | W400, WebMaster |
| Active, narrow band | 0.1–1; 1–10; 10–100; 100–1,000 mS/cm | PEEK | 20–190 °F (approximately -7…88 °C) | 250 PSI (approximately 17 bar) | 1" NPTM submersion, 2" NPTM inline | W400, WebMaster |
| Wide range, WEC310 compatible | 500 µS/cm – 1,000 mS/cm | CPVC or PEEK (including ring-shaped "donut" PEEK) | CPVC 20–180 °F; PEEK 20–250 °F (approximately -7…121 °C) | CPVC 140 PSI; PEEK -15…250 PSI | 1" or 3/4" NPTM submersion, 2" NPTM inline | WEC310 |
Temperature multiplier. Ranges are given at 25 °C. In the toroidal sensor table, the range multiplier is 181.3% at 0 °C, 100% at 25 °C, 75.5% at 40 °C, 55.6% at 60 °C, 43.5% at 80 °C, 35.7% at 100 °C, 30.4% at 120 °C, 25.5% at 150 °C, and 22.9% at 180 °C. Temperature compensation in the sensor is provided by a thermistor (10K or 100K) or RTD (PT1000). For a sensor operating in a hot bath, the measuring range must be checked against this derated value.
From Conductivity to Bath Concentration
In an oily cleaner, chromate or rinse bath, the actual goal is to keep solution concentration within the target band. The WEC310 electrodeless conductivity controller monitors one or two baths simultaneously and provides a control output and alarm relay for each. One of four conductivity ranges is selected (50–1,000 µS/cm, 1,000–10,000 µS/cm, 10–100 mS/cm, 100–1,000 mS/cm) and the result is displayed as µS/cm, mS/cm, ppm or % concentration. The automatic temperature compensation factor can be adjusted to match the behavior of most chemicals; the temperature input is a 100 K thermistor.
Modern Walchem controllers provide the same measurement: the Intuition-6 and Intuition-9 electrodeless input reads 500 µS/cm – 2,000,000 µS/cm across five sub-ranges with 1 percent accuracy; temperature compensation is automatic across -5…260 °C using a temperature-dependent multiplier table. This allows a single controller to manage bath concentration alongside pH or other parameters.
Where Is a Toroidal Sensor Used?
| Application | Why toroidal | Related page |
|---|---|---|
| Oily cleaner baths | Oil and film coat electrodes; contacting sensors drift | Walchem conductivity sensors |
| Chromate solutions and rinse tanks | Concentrated and aggressive; CPVC or PEEK body | Surface treatment and plating |
| Fume scrubbers | One of the demanding chemical applications listed by the manufacturer; fouling-immune measurement | Intuition-9 |
| Concentrated chemical tanks (up to 1,000 mS/cm) | Dosing feed and tank concentration control | Chemical pump selection |
Toroidal Conductivity Sensors: Frequently Asked Questions
Operating principle, range, body materials, units and controller compatibility.
What is a toroidal conductivity sensor?
A toroidal (electrodeless, inductive) sensor is a conductivity sensor with encapsulated coils that makes no direct electrical contact with the solution. Walchem's sensors have CPVC or PEEK bodies and are immune to the thin coating buildup and calibration instability issues of contacting sensors. For this reason, they are used in oily cleaner baths, chromate solutions, rinse tanks, fume scrubbers, and concentrated chemicals up to 1,000 mS/cm.
What is the difference between a toroidal sensor and a contacting sensor?
A contacting sensor touches the solution with electrodes; if the electrode surface becomes coated or fouled, the cell constant changes and the reading drifts. A toroidal sensor has no electrodes, so surface coating does not affect it. Conversely, the measuring range of toroidal sensors starts at hundreds of µS/cm; for very low conductivities such as pure water, RO permeate or boiler feedwater, a contacting or UPW-type sensor is used.
In what range does a toroidal sensor measure?
Walchem passive toroidal sensors cover 500 µS/cm – 2,000 mS/cm. Active models have narrow bands: 0.1–1, 1–10, 10–100 and 100–1,000 mS/cm. As temperature increases, the range narrows: 100% at 25 °C, 35.7% at 100 °C, and 22.9% at 180 °C. On Intuition-6 and Intuition-9 controllers, the electrodeless input reads 500 µS/cm – 2,000,000 µS/cm across five sub-ranges.
Why can the reading also be displayed in ppm or percent concentration?
In chemical baths, the goal is solution concentration rather than conductivity itself. The Walchem WEC310 electrodeless conductivity controller displays the reading in one of the units µS/cm, mS/cm, ppm or % concentration, and the automatic temperature compensation factor can be adjusted to match the temperature behavior of most chemicals. The conductivity-to-concentration conversion is specific to the solution; share your chemical details with us.
Should I select CPVC or PEEK?
The choice depends on the temperature, pressure and chemical nature of the solution. In Walchem's table, CPVC models are rated for 140 PSI (approximately 9.7 bar) and at most approximately 70–82 °C; PEEK models reach up to 250 PSI (approximately 17 bar) and approximately 88–121 °C. PEEK also stands out for its mechanical strength and chemical resistance. Both materials are used in aggressive solutions; chemical and temperature data are required to confirm compatibility.
Which controllers accept a toroidal sensor?
Intuition-6 and Intuition-9 (universal input, contacting and electrodeless), WebMasterOne (electrodeless conductivity directly), W600 and W100 series (passive sensor), P100 / W100P (contacting or electrodeless), and the WEC310 controller designed specifically for electrodeless conductivity. The W400 and WebMaster series require an active (powered) toroidal sensor, whereas the W100 and W600 operate with a passive sensor.
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