Conductivity Measurement: Sensor Types, Cell Constant and Selection
Conductivity is the indirect indicator of dissolved ion concentration in water, and it underlies decisions from pure water quality to boiler and cooling tower cycles of concentration. Continuous monitoring makes loss of cycles or a leak visible immediately.
What Does Conductivity Measure?
Conductivity is an indirect indicator of dissolved ion concentration in water. In aqueous solutions it arises from dissolved acids, bases and salts splitting into positive and negative ions. In ultrapure water, where almost no ions are present, only a few H₃O⁺ and OH⁻ ions remain and conductivity is extremely low; this intrinsic conductivity of water forms the lower limit of the scale. The practical units are µS/cm and mS/cm (1 mS = 1,000 µS); the SI unit is S/m.
The measurement applies an alternating voltage to a cell of two or four electrodes and measures the resistance. A cell constant compensates for the cell geometry and is determined by calibration against a conductivity standard. Because conductivity depends on temperature, the sensor contains a temperature element and the value is normalized to a 25 °C reference.
Four Questions That Decide Sensor Selection
- Range: ultrapure water sits at a few µS/cm or below, boiler and cooling tower water at tens of thousands of µS/cm, concentrated solutions at hundreds of mS/cm. The range sets the cell constant and the number of electrodes.
- Medium: clean water, or an aggressive solution that leaves coatings or fouling? In the second case a toroidal (electrodeless) sensor comes into play.
- Process conditions: temperature, pressure, CIP/SIP or autoclave requirement, and the wetted material (stainless steel, PEEK, graphite, titanium, platinum).
- Connection and output: PG13.5, Tri-Clamp, NPT or Varivent; Arc digital, 4-20 mA, passive or active sensor; and which controller it connects to.
2-Electrode, 4-Electrode and Toroidal Sensors Compared
| Type | Principle | Strength | Limit | Example products |
|---|---|---|---|---|
| 2-electrode contacting | Two electrodes, resistance measured with alternating voltage | Simple and economical; sensitive at low conductivity (pure and ultrapure water) | One cell does not cover a wide range; the cell constant is chosen for the range | Conducell UPW (0.01-1,500 µS/cm Arc), Conducell 2DC-PG (10 µS/cm - 20 mS/cm) |
| 4-electrode contacting | Four electrodes, good linearity over a wide range | Trace level to very high conductivity with one sensor | For the ultrapure water range, a 2-electrode UPW type is more suitable | Conducell 4UxF (1 µS/cm - 500 mS/cm analog), Conducell 4US (0.1 µS/cm - 500 mS/cm) |
| Electrodeless (toroidal) | Contactless inductive measurement with encapsulated coils | Immune to coating and fouling; long life in aggressive and concentrated solutions | Ranges start at hundreds of µS/cm; not suitable for pure water | Walchem CPVC and PEEK toroidal sensors (500 µS/cm - 2,000 mS/cm) |
| Contacting, tower and boiler type | Graphite or stainless electrode, cell constant 0.01 - 10.0 | Cell constant option covers low and high conductivity; tower and boiler mounting accessories available | Not suitable for oily or coating media | Walchem contacting sensors (cell constant 1.0: 0-30,000 µS/cm; 10.0: 0-300,000 µS/cm) |
Range, Temperature, Pressure and Connection by Family
Values are taken from the manufacturers' catalogues and brochures and are referenced to 25 °C. Send the model number for the order code and the latest technical values.
| Sensor | Measuring range | Cell constant | Temperature | Pressure | Application |
|---|---|---|---|---|---|
| Hamilton Conducell UPW | Arc: 0.01-1,500 µS/cm; analog: 0.02-2,000 µS/cm | below 0.1/cm (2-electrode) | 0-130 °C | 10 bar (130 °C) | Ultrapure water, pure water, WFI |
| Hamilton Conducell 4UxF | Arc: 1 µS/cm - 300 mS/cm; analog: 1 µS/cm - 500 mS/cm | 0.36/cm (4-electrode) | -20 to 150 °C | 20 bar (135 °C); 10 bar (150 °C) | CIP, bioprocess, chemical and pharmaceutical |
| Hamilton Conducell 4US | 0.1 µS/cm - 500 mS/cm | 0.147/cm (4-electrode) | 0-135 °C | 0-20 bar (135 °C) | CIP monitoring, fermentation |
| Hamilton Conducell 2DC-PG | 10 µS/cm - 20 mS/cm | 1/cm (2-electrode) | -5 to 80 °C | 6 bar | Water and wastewater |
| Walchem contacting (tower) | 0-30,000 µS/cm (cell constant 1.0) | 1.0 | Typical tower 0-70 °C | Polypropylene 10 bar; stainless steel + PEEK 20 bar | Cooling tower blowdown control |
| Walchem contacting (boiler) | 0-30,000 µS/cm; cell constant 10.0 with W100/W600 reaches 0-300,000 µS/cm | 1.0 or 10.0 | Typical boiler 0-205 °C | 17.2 bar (250 PSI) | Boiler surface blowdown |
| Walchem contacting (general) | Cell constant 0.01: 0-0.3 mS; 0.1: 0-3 mS; 1.0: 0-30 mS; 10.0: 0-300 mS | 0.01 / 0.1 / 1.0 / 10.0 | Polypropylene 0-100 °C; stainless steel 0-120 °C | PP 6.9 bar; SS 13.8 bar | RO system, condensate line |
| Walchem toroidal | 500 µS/cm - 2,000 mS/cm; active narrow bands 0.1-1, 1-10, 10-100, 100-1,000 mS | 6.286 (passive model) | CPVC and PEEK body; roughly 70-120 °C by model | CPVC 140 PSI (about 9.7 bar); PEEK 250 PSI (about 17 bar) | Chemical bath, chromate, scrubber |
Matching Cell Constant and Range
One sensor does not work equally well at every conductivity: a low cell constant suits low conductivity and a high cell constant suits high conductivity. With Walchem contacting sensors compatible with W100/W600 controllers, a 0.01 cell constant reaches 0-0.3 mS, 0.1 reaches 0-3 mS, 1.0 reaches 0-30 mS and 10.0 reaches 0-300 mS.
Temperature affects the range directly. In Walchem's range multiplier table the value is 100% at 25 °C, 181.3% at 0 °C, 75.5% at 40 °C, 55.6% at 60 °C and 35.7% at 100 °C, and 22.9% at 180 °C for the toroidal sensor: on a hot line the upper limit a sensor can measure is clearly lower than the catalogue figure. Check the range at process temperature, not at 25 °C.
The integrated temperature element of Hamilton Conducell sensors is for conductivity compensation only and should not be used to control process temperature. The 4UxF manual also notes that the cell constant depends on installation geometry, so the sensor should be calibrated in an installation similar to the process, and O-rings are wear parts to be replaced after one year at the latest.
Which Sensor for Which Application?
| Application | Why conductivity | Sensor approach | More |
|---|---|---|---|
| Pure water, WFI, ultrapure water | Accept/reject criterion of the pharmaceutical water grade; continuous record | USP 645 compliant 2-electrode UPW sensor, Arc or VP6 | Pure water conductivity and USP 645 |
| RO permeate and EDI outlet | Membrane performance and outlet quality | Low cell constant contacting sensor or UPW type | RO systems · EDI systems |
| Boiler water and condensate | Surface blowdown, carryover and foaming control; condensate purity | Boiler-type contacting sensor, cell constant 1.0 or 10.0 | Boiler water conductivity |
| Cooling tower | Cycles of concentration and blowdown setpoint | Cell constant 1.0, polypropylene or stainless steel + PEEK body | Cooling tower automation |
| CIP and food-beverage | Phase separation, rinse-out, chemical concentration | 4-electrode 4UxF or 4US; FDA-compliant wetted parts | CIP and SIP explained |
| Chemical bath, chromate, scrubber | Bath concentration, chemical feed | Electrodeless toroidal sensor; result in µS/cm, mS/cm, ppm or % concentration | Toroidal conductivity sensor |
Controllers, Transmitters and Outputs
Walchem: WebMaster controllers need an active sensor with electronics that converts the signal to voltage; for contacting sensors the W400, W600 and W100 series work with passive sensors. Intuition-6 and Intuition-9 have a universal sensor input and accept both contacting and electrodeless conductivity sensors. See Walchem water treatment controllers.
Hamilton: Arc sensors deliver a fully temperature-compensated signal directly as Modbus and 4-20 mA, so no transmitter is needed; analog (VP) sensors connect to a transmitter input.
For portable measurement, see the handheld water analyzer guide. For the wider online range, see online analyzers and the Hamilton and Walchem brand pages.
If you are looking for the equivalent of your current instrument, send the make and model to [email protected] and our engineers will propose the equivalent with the same connection and measuring range.
Frequently Asked Questions
Technical answers from Mikron Proses.
How do I choose a conductivity sensor?
Four questions decide. 1) Expected range: ultrapure water and RO permeate are at a few µS/cm or below, boiler and tower water at tens of thousands of µS/cm, concentrated solutions at hundreds of mS/cm; the range sets the cell constant and number of electrodes. 2) Medium: clean water, or an aggressive solution that coats or fouls the electrodes, where a toroidal (electrodeless) sensor is considered. 3) Process conditions: temperature, pressure, CIP/SIP and autoclave requirement. 4) Connection and output: PG13.5, Tri-Clamp, NPT, Arc digital, 4-20 mA, passive or active sensor.
What is the difference between 2-electrode and 4-electrode conductivity sensors?
Both apply alternating voltage to electrodes and measure resistance. A 2-electrode cell is simple and economical and works very well at low conductivity such as ultrapure and pure water; Hamilton Conducell UPW is 2-electrode and measures 0.01-1,500 µS/cm in the Arc version. A 4-electrode cell gives a wide range and good linearity: Conducell 4UxF measures from 1 µS/cm up to 500 mS/cm on the analog output and up to 300 mS/cm on the Arc output. As a general rule, use 2-electrode for trace-level pure water and 4-electrode for wide-range or high-conductivity processes.
What is a cell constant and why does it matter?
Conductivity is found by applying alternating voltage to the measuring cell and measuring resistance; a cell constant compensates for the cell geometry and is determined by calibration with a conductivity standard. A low cell constant suits a low conductivity range and a high cell constant a high range. Walchem contacting sensors come in 0.01 / 0.1 / 1.0 / 10.0 cell constants; the Hamilton catalogue gives below 0.1/cm for UPW, 0.147/cm for 4US, 0.36/cm for 4UxF and 1/cm for 2DC-PG. The cell constant also depends on the installation, so Hamilton recommends calibrating the sensor in the geometry in which it is installed.
When is a toroidal (electrodeless) conductivity sensor preferred?
When the solution coats, fouls or corrodes electrodes, as in oily cleaner baths, chromate solutions, rinse tanks and scrubbers. Walchem's encapsulated toroidal sensor makes no direct electrical contact with the solution and is not affected by thin coatings. Its ranges are around 500 µS/cm - 2,000 mS/cm (narrow-band active models start at 0.1-1 mS/cm), so very low conductivity waters such as pure water and RO permeate need a contacting or UPW-type sensor. See the toroidal conductivity sensor guide.
Why is conductivity quoted at 25 °C?
Conductivity of solutions changes markedly with temperature, so the measurement is made together with a temperature element (thermistor or RTD) and the value is normalized to a 25 °C reference. In Walchem's range table, 25 °C is taken as 100%; the multiplier is 181.3% at 0 °C, 55.6% at 60 °C and 35.7% at 100 °C, so the upper limit a given sensor can measure gets smaller as temperature rises. Check the range at process temperature when selecting a sensor.
How are conductivity and TDS related?
TDS is not measured directly; it is calculated from conductivity with a factor. The factor differs for each water type, and RO manufacturers' operating manuals give the TDS = K × EC25 relationship and state that K should be determined for each stream. The Pyxis SP-600 handheld meter uses 0.67 × µS/cm by default, and the factor is user-adjustable. In boiler and tower control, conductivity is usually used directly as the setpoint, and TDS conversion is only for reporting.
What standard is used to calibrate a conductivity sensor?
A conductivity standard close to the expected range. The Hamilton catalogue lists standards at 1.3 / 5 / 15 / 84 / 100 / 147 / 706 / 1,413 / 12,880 / 100,000 µS/cm (25 °C); the 1.3 and 5 µS/cm standards are offered with certified 1% accuracy. Conventional low-conductivity standards drift after the bottle is opened because they absorb CO2, and the glycerin-matrix Hamilton formulation is stated to avoid this problem. For pure water the detailed method is in the pure water conductivity guide.
Related pages
- Hamilton Conducell Conductivity Sensors
- Online analyzers
- Walchem water treatment controllers
- Pure Water Conductivity and USP 645
- Toroidal conductivity sensor guide
- Boiler water conductivity guide
- Cooling tower automation guide
- Hamilton
- Walchem
- Contact page
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