How conductivity probes measure dissolved solids, what upsets them, and how to keep them calibrated.
Pure water barely conducts electricity. Dissolved salts split into ions, and the ions carry current, so the more dissolved solids, the higher the conductivity. In a cooling tower, conductivity rises as water evaporates and leaves its minerals behind — which is why it is used to decide when to bleed.
Conductivity is reported in microsiemens per centimeter (µS/cm), or millisiemens per centimeter (mS/cm) for concentrated water. 1 mS/cm = 1000 µS/cm. Older references use micromhos (µmho/cm), which are the same as µS/cm.
There are two ways to measure it.
- Two or four metal electrodes (commonly stainless steel, titanium, or graphite) sit in the water. The transmitter applies an alternating voltage and measures the current that flows between them.
- Cell constant (K). The electrodes’ size and spacing set how much water is “in the circuit”, expressed as a cell constant in cm⁻¹. Low-conductivity water needs a low cell constant (for example K = 0.01 or 0.1 for pure water); cooling water commonly uses K = 1.0; high-conductivity water uses K = 10. The transmitter must be set to the probe’s cell constant.
- Good for clean water and low conductivities. The electrodes touch the water, so coating affects them directly.
- Two wire-wound rings (toroids) are sealed inside a plastic body with a hole through the middle. One ring induces a current in the loop of water passing through and around the hole; the second ring measures that current.
- No metal touches the water, so the sensor tolerates coating, corrosive chemistry, and dirty water far better than a contacting sensor.
- Good for medium to high conductivity — cooling water, wastewater, and chemical solutions. Toroidal sensors are generally poor below a few tens to a few hundred µS/cm, depending on the model.
Conductivity rises roughly 2 % per °C for typical natural and cooling water (the exact coefficient depends on the water). Every conductivity probe has a temperature element, and the transmitter corrects the reading to what it would be at 25 °C (77 °F). Unless stated otherwise, readings are “compensated to 25 °C”. Make sure the compensation coefficient matches your water; the common default is 2 %/°C.
¶ Conductivity and TDS
Total dissolved solids (TDS, in ppm or mg/L) can be estimated from conductivity by multiplying by a factor, commonly 0.5 to 0.7 (0.65–0.67 is a common default for cooling water). The factor depends on which salts are present, so a TDS figure from a conductivity meter is an estimate, not a measurement. Conductivity also says nothing about which ions are present — it cannot tell hardness from sodium chloride.
- Coating and scale on a contacting sensor hide part of the electrode surface, which usually makes the reading low. Oil films do the same.
- Air bubbles in or around the cell displace water and read low or noisy. This is common just after a pump or where the sample line has air.
- Wall effect on toroidal sensors. A toroidal sensor’s field extends out into the water around it. A nearby pipe wall or fitting changes the reading, especially metal walls. Keep the manufacturer’s clearance, or calibrate installed.
- Wrong cell constant or temperature coefficient set in the transmitter gives a consistent error.
- Temperature element failure gives an error that grows with distance from 25 °C.
- Low-conductivity limit. Toroidal sensors become inaccurate at low conductivity; contacting sensors with too high a cell constant do the same.
- Polarization on contacting sensors at high conductivity, where the cell constant is too low for the water.
- Electrical noise and ground loops from pumps and heaters in the same water.
- Keep the sensing area fully submerged, with water flowing through it, and free to shed air. Mount in a pipe section that stays full, or in a flow cell.
- Orient so bubbles escape. Mount on the side of a horizontal pipe rather than the top (where air collects) or the bottom (where sediment collects). For toroidal sensors, align the hole with the flow so water sweeps through it.
- Clearance for toroidal sensors. Keep the sensor away from pipe walls and fittings by the clearance the manufacturer states — commonly at least an inch or so (a few cm) — or use the manufacturer’s installation factor.
- Sample representative water. For bleed control, sample the recirculating water, not a make-up line or a point right after a chemical injection.
- Isolation valves on an insertion point let the probe be removed without draining the system.
¶ Maintenance
- Rinse and wipe the electrodes or the toroid bore with a soft brush or cloth.
- Scale: soak in a dilute acid (commonly 5–10 % hydrochloric acid, or a mild acid descaler), then rinse well.
- Oil and organic film: warm water with detergent.
- Do not abrade platinized or coated electrodes; follow the manufacturer’s guidance.
Wear eye protection and gloves when using acid cleaners, and follow Chemical Handling. Isolate and depressurize before removing an insertion probe — see Isolation and Pressurized Systems.
- Zero in air. A clean, dry probe in air should read zero. Many transmitters use this as the zero point.
- Standard solution. Calibrate with a conductivity standard close to the normal process value (for cooling water, commonly 1000 or 1413 µS/cm; 1413 µS/cm is 0.01 M KCl at 25 °C). Standards are easily contaminated and change as they absorb carbon dioxide or evaporate: use fresh solution, poured out, never back into the bottle.
- In-process calibration. With the probe installed, measure a sample with a calibrated handheld meter and adjust the controller to match. This also corrects for toroidal wall effect. Check the handheld’s own temperature compensation matches the probe’s.
- Interval. Check monthly to start, then adjust based on drift. A clean conductivity probe is stable; drift usually means it needs cleaning.
For calibrating from the InteliHive controller, see Calibrating a conductivity probe on the InteliHive.
Conductivity sensors have no consumable reference, so they commonly last several years. Replace when the body or cable is damaged, the temperature element fails, or cleaning no longer recovers the reading.
Models documented in this wiki: