Conductivity and TDS Probes

How EC probes work, why TDS in ppm depends on a hidden 0.5–0.9 conversion factor, temperature compensation to 25 °C, salinity via PSS-78, KCl calibration standards and probe cleaning.

What a conductivity probe measures

Water conducts electricity through its dissolved ions: distilled water is almost an insulator, while salt water is a reasonable conductor. A conductivity (EC) probe measures how easily current passes between electrodes of known geometry and reports the result in siemens per metre (S/m) or, in practice, microsiemens or millisiemens per centimetre (1 mS/cm = 1,000 µS/cm; 1 S/m = 10,000 µS/cm). Ultrapure water reads about 0.055 µS/cm, typical drinking water 200 to 800 µS/cm and seawater roughly 50 mS/cm. The probe applies an alternating voltage, typically 1 to 3 kHz, so that the electrodes do not electrolyse the water or polarise. Two-electrode cells serve low to moderate conductivities and four-electrode cells stay accurate in highly conductive water such as seawater. The cell constant, the ratio of electrode spacing to electrode area, is fixed by calibration. Electrodes are made of graphite, platinum or platinum-iridium, and modern cells carry a thermistor for temperature compensation.

EC versus TDS: the conversion factor problem

Total dissolved solids (TDS) is the mass of all dissolved inorganic and organic matter that passes a 2 µm filter, expressed in mg/L or ppm. The reference method is gravimetric: evaporate the water and weigh the residue. A "TDS meter" does not do this; it is a conductivity meter that multiplies EC by a fixed factor, TDS = k × EC, and the conductivity route is only accurate to about 10 percent. The factor depends on which ions are present: Wikipedia gives 0.55 to 0.8, the USGS expects 0.5 to 0.9 for natural waters, and an NDSU Extension table shows the factor for natural waters rising with conductivity, from 0.50 at 100 µS/cm to 0.65 at 1,000 µS/cm, 0.75 at 3,000 µS/cm and 0.80 at 7,000 µS/cm. A sodium chloride solution converts at roughly 0.64 mg per litre per µS/cm.

Handheld TDS meters are built around one factor chosen by the manufacturer. Two meters with different factors give different ppm readings for the same water: 400 µS/cm becomes 200 ppm at a factor of 0.5, 256 ppm at 0.64 and 280 ppm at 0.7. A ppm value is therefore only meaningful together with the factor used, and comparing tanks, RO output or published targets is more reliable in µS/cm. Conductivity-based TDS also misses everything that does not carry charge: dissolved organics, microorganisms and volatile compounds are invisible to the probe. For the practical use of these readings in RO/DI and remineralisation work, see the separate TDS meter guide.

Temperature compensation

Conductivity rises with temperature by 0.5 to 3 percent per °C depending on the ions present; most meters assume a linear 2 percent per °C (a coefficient of 0.019 to 0.020) or use the nonlinear ISO 7888 algorithm. To make readings comparable they are corrected to 25 °C and reported as specific conductance: κ25 = κ / (1 + α × (t − 25)). Meters with automatic temperature compensation do this internally, and the USGS states that the correction is normally accurate to ±5 percent for water between 5 and 35 °C with a pH between 4 and 11, which covers aquarium conditions.

Salinity from conductivity in marine tanks

Oceanographers define salinity through conductivity. The Practical Salinity Scale of 1978 (PSS-78) expresses salinity as the ratio of the sample’s conductivity to that of a standard potassium chloride solution; seawater with a chlorinity of 19.37 g/kg has a conductivity of 42.9 mS/cm at 15 °C, and standard seawater has a practical salinity of about 35. In 2010 the TEOS-10 standard introduced absolute salinity in g/kg, which combines the conductivity measurement with information about the water’s ionic composition, because conductivity alone assumes the ion proportions of natural seawater. A marine conductivity probe that reports salinity or specific gravity is running the PSS-78 conversion and therefore also assumes seawater-like ion ratios. Refractometers and hydrometers, which measure refractive index and specific gravity, are the common alternatives; their use and calibration are covered in the refractometer guide.

Calibration standards

Conductivity cells are calibrated with potassium chloride solutions of known conductivity. The USGS uses NIST-traceable KCl standards with certified values between 50 and 50,000 µS/cm; meters are commonly calibrated at 500, 1,413 or 12,900 µS/cm, and the standard closest to the expected reading should be used. Key points from the field manual:

For a multipoint calibration, start with the lowest standard and work upward; standards should be between 5 and 35 °C and within their expiry date.
A reading is accepted if it is within ±5 µS/cm of the certified value for standards up to 100 µS/cm, or within ±3 percent above 100 µS/cm; if it passes, recalibration is unnecessary.
Track the cell constant (or slope): a change of more than 2 percent between calibrations points to a contaminated standard or a dirty probe.
Check the zero: a clean cell in air or in deionised water should read below 3 µS/cm.
Conductivity meters hold calibration well, so frequent recalibration adds little. NDSU Extension recommends recalibrating after a battery change or after the meter has been dropped.

Probe fouling and care

A conductivity cell only reads correctly when the electrodes are clean, unscratched and wetted by nothing but the sample. Dirt and biofilm build up on immersed probes and require regular cleaning.

Rinse with plenty of deionised water before and after each measurement; in most cases this is all the cleaning needed.
Epoxy and graphite probes may be wiped gently with a laboratory tissue. Never wipe or scour platinum-coated electrodes.
Remove oily or chemical residues by soaking in a detergent solution, for hours if necessary, then rinse with deionised water.
For stubborn deposits, soak in 5 percent (v/v) hydrochloric acid for up to 2 hours and rinse thoroughly, after checking the manufacturer’s guidance.
Never clean platinum or platinum-iridium electrodes with nitric acid, aqua regia or anything that etches platinum or gold.
Most conductivity probes, including graphite types, may be stored clean and dry.