How Does a Conductivity Sensor Work? EC Readings in Water Explained

Your display says 640 µS/cm. Is that high, low or normal? The short answer: an EC reading tells you how well water carries an electrical current, and that depends almost entirely on how many dissolved ions it contains – salts, minerals, acids and bases. A conductivity sensor measures exactly this electrical conductivity of water, refers it to a standard temperature and gives you one fast number for everything dissolved in it that carries a charge.

What it does not tell you is which substances those are. 640 µS/cm would be unremarkable for tap water in many parts of Germany, but in a rainwater cistern it would be a clear sign that something else has found its way in. Whether EC readings in water mean anything to you comes down to four things: the unit, the temperature, a sensible point of comparison and the spot where you measure.

What does an EC sensor measure?

Pure water barely conducts electricity; dissolved ions carry the charge, and the more there are, the better the solution conducts. A classic conductivity probe uses two electrodes with a fixed spacing and a defined surface area. The electronics apply an alternating voltage and measure the current flowing between them, which gives the conductance in siemens.

Why alternating voltage? With direct current, ions would pile up at the electrodes, the surfaces would polarize and the reading would drift downward over time. Switching polarity largely prevents that.

To turn conductance into a comparable conductivity value, you need the cell constant K. It describes the ratio of electrode spacing to electrode area and is given in 1/cm. Conductivity is the measured conductance multiplied by K. Probes for low-conductivity water use a small cell constant, while probes for saltier water use a larger one, because otherwise too much current would flow and accuracy would suffer.

The Dragino DR-ECK probes in our range follow this two-electrode principle: the DR-ECK1.0 uses a K1 electrode and the DR-ECK10.0 a K10 electrode. Both have a built-in temperature sensor and report conductivity and temperature digitally over RS485. There are also four-electrode probes, which are less prone to polarization across wide ranges, and inductive (toroidal) sensors with no direct electrode contact, used mainly in heavily fouled or very salty process liquids.

What is a normal EC reading for water?

“Normal” only makes sense for a specific kind of water. The electrical conductivity of water ranges from almost zero for ultrapure water to tens of thousands of µS/cm for seawater. The overview below shows typical orders of magnitude, all referenced to 25 °C. They are orientation values, not a standard for your case.

First, the water types sit orders of magnitude apart, so even a rough reading is telling: rainwater at 400 µS/cm is suspicious, tap water at 400 µS/cm is not. Second, the drinking water value is an indicator parameter, not a line between “safe” and “dangerous.” It signals that the water is unusually mineralized and deserves a closer look.

Your most useful reference is your own baseline. Water utilities usually publish an analysis that lists conductivity. For wells, cisterns, ponds or process water, measure carefully during the first few weeks and use that period to define what normal looks like at that exact spot. Deviations from it say more than any table value.

µS/cm vs mS/cm: which conductivity unit should you use?

The most common unit is microsiemens per centimeter, µS/cm. For saltier water, millisiemens per centimeter, mS/cm, is often used instead. The relationship is simple: 1 mS/cm equals 1,000 µS/cm. So 1.2 mS/cm and 1,200 µS/cm are the same value, not two readings that disagree by a factor of a thousand.

Less often you will see siemens per meter (1 S/m equals 10 mS/cm) and, in older US documentation, µmho/cm, which is numerically identical to µS/cm. For very pure water, resistivity is often quoted instead of conductivity: 18.2 MΩ·cm corresponds to the 0.055 µS/cm in the table. And when a garden or hydroponics meter shows “EC 1.8,” it almost always means mS/cm.

Most mistakes happen during processing, not measurement. According to Dragino’s documentation, the DR-ECK probes return an integer raw value that has to be scaled differently depending on the electrode. A raw value of 686 means 686 µS/cm on the DR-ECK1.0 but 6,860 µS/cm on the DR-ECK10.0. The resolution is correspondingly 1 µS/cm or 10 µS/cm.

As a rule, store µS/cm as your working unit, and label every value with its unit and whether it is temperature compensated. A spreadsheet column labeled only “EC” turns into a puzzle the moment a second probe is added or someone’s export treats the decimal comma as a thousands separator.

EC vs TDS: what is the difference?

TDS stands for total dissolved solids, the total amount of dissolved material, given in mg/L or ppm. Many inexpensive TDS meters measure conductivity, not solids, and multiply it by a conversion factor. The TDS they display is an estimate that depends entirely on that factor.

The two most common factors are 0.5 and 0.7. The 0.5 factor is based on sodium chloride. The 0.7 factor is based on a mix of 40 % sodium sulfate, 40 % sodium bicarbonate and 20 % sodium chloride and is often used for nutrient solutions and fertilizers. The gap is large: 800 µS/cm gives 400 ppm TDS with a factor of 0.5, but 560 ppm with a factor of 0.7.

There is also a fundamental limit: dissolved substances without a charge, such as sugar or many organic compounds, are part of the true TDS but contribute almost nothing to conductivity. A lab determines the actual dissolved content by evaporating a sample and weighing the residue. For ongoing monitoring, it makes sense to store EC as your primary data and calculate TDS in the dashboard only as an extra, with the factor documented.

Salinity works in a similar way. Practical salinity in PSU is calculated from conductivity and temperature and assumes the ion composition of seawater. That is a useful approximation for brackish water and coastal wells, but not for inland tap or well water.

Why does conductivity need temperature compensation?

Ions move faster in warm water, so the solution conducts better. The effect is substantial: for neutral salts, conductivity rises by roughly 1.5 to 2.2 % per degree Celsius, according to Hanna Instruments. An uncompensated sensor in an outdoor tank will therefore show a daily cycle that comes from the sun and says nothing about the water itself.

Temperature compensation converts the reading to a reference temperature, usually 25 °C and sometimes 20 °C. With a typical coefficient of 2 % per degree, it works like this: water at 10 °C gives an uncompensated reading of 700 µS/cm. Referenced to 25 °C, that is 700 divided by 0.7, or 1,000 µS/cm. Without the conversion, you would underestimate the value by 30 %.

Regulations show how much the reference temperature matters: the EU Drinking Water Directive lists 2,500 µS/cm at 20 °C, while the German Drinking Water Ordinance lists 2,790 µS/cm at 25 °C. That is the same yardstick at different reference temperatures.

According to the datasheet, the DR-ECK probes have a built-in temperature sensor with ±0.5 °C accuracy and compensate to 25 °C between 0 and 60 °C. Two rules follow: do not compensate a second time in your software if the value already arrives compensated. And after immersing the probe, wait until temperature and reading are stable before you treat a value as valid.

What does a high EC reading mean?

A high EC reading first means only that there are many dissolved ions in the water. The cause can be entirely harmless, such as hard water rich in calcium, magnesium and bicarbonate. It can also point to something you should know about: road salt washing into a ditch or well in winter, seawater intruding into a coastal well, fertilizer or slurry from neighboring fields, wastewater, or a leaking line carrying a salty process liquid.

Concentration is often overlooked. When water evaporates from an open basin or a cooling circuit, the salts stay behind and conductivity rises slowly even though nothing new has been added.

The shape of the curve helps. A slow, steady rise fits evaporation or a gradual change in the source. A sudden jump points to an event: rain washing something in, a dosing pump running, a change of supply. The probe itself can also cause jumps. Air bubbles on the electrodes usually push the reading down, deposits change the effective cell constant, and a switch in units or compensation settings can create an apparent jump by a large factor.

The reverse is true as well: a low reading does not mean the water is clean. Bacteria, oils, many pesticides and other uncharged substances barely show up in conductivity. EC is an excellent early indicator of change, but it is not a drinking water analysis. When the value becomes unusual, the next step is a targeted test, not a conclusion drawn from the curve alone.

Where should you install a conductivity sensor?

Dragino describes several mounting options for the DR-ECK probes: through the side wall of a tank, from above via a flange, in a pipe bend, in a flow-through fitting, or freely submerged. The basic rules are the same for all of them.

The electrodes must be fully and permanently submerged, in a place where the water moves and is well mixed. Dead corners, the spot directly downstream of a dosing point and the bottom where sludge collects are all poor choices. The manufacturer explicitly names two installation mistakes. If the electrode sits deep inside a fitting that is too long, a dead space with stagnant water forms. And at high points in a pipe, or in lines that are not completely full, air collects and causes wrong or unstable readings.

Distance from walls matters too. Hanna Instruments recommends keeping at least about 2.5 cm (one inch) from walls and bottom, because otherwise the measuring field is disturbed: metal makes the reading appear too high, glass and plastic make it appear too low. Angle the conductivity probe so that air bubbles can escape upward instead of getting trapped between the electrodes.

The probe head is rated IP68 and, according to the datasheet, withstands up to 0.6 MPa. The cable connector, however, is only specified for below 85 % relative humidity. So mount the plug dry and above the water. The 5 m cable determines how far away the measuring electronics can be, and the probe should sit where you can pull it out for cleaning without taking the system apart.

How do you monitor water conductivity continuously with LoRaWAN?

Conductivity becomes truly useful as a trend over days and weeks, and at wells, basins or ditches that usually means a wireless link without mains power or Wi-Fi. LoRaWAN suits this well because the data volumes are small and, depending on the surroundings, links can reach several kilometers.

The chain looks like this: the probe connects to a LoRaWAN node with an RS485 interface, the node transmits to a gateway, the gateway forwards to a network server, and from there the values reach your dashboard. According to Dragino, the DR-ECK probes speak Modbus RTU at 9600 baud, 8 data bits, no parity and one stop bit. The default address is 0x12 for the K1 model and 0x11 for the K10 model. The probes need 7 to 30 V DC at under 0.5 W. Your node therefore has to supply a suitable voltage and, on battery power, should switch it on only for the measurement, with a little lead time so the reading can settle.

Because conductivity usually changes slowly, a transmit interval of 10 to 15 minutes is enough in many cases. Alerts work best in two stages: one threshold on the compensated value and a second on the rate of change, each triggered only after two consecutive unusual readings. The decoder has to know the K factor and divide the temperature by 10; otherwise the numbers in your dashboard will be wrong.

Dragino is clear about maintenance: calibrate before first use and about every three months in continuous operation. Calibration is single-point, with 1,413 µS/cm for the K1 model and 12.88 mS/cm for the K10 model. If an electrode has been stored dry for a long time, soak it in distilled water for several hours first. Remove organic deposits with warm water and detergent or with alcohol, and scale with 10 % citric acid. Never wipe the electrodes themselves, because that damages the platinum black coating. Log every calibration and cleaning in the dashboard so that a step change afterward is not mistaken for a water event.

According to the datasheet, the DR-ECK1.0 measures 0 to 2,000 µS/cm and the DR-ECK10.0 10 to 20,000 µS/cm, so seawater, for example, falls outside both ranges; which probe suits your water is covered in a separate article on choosing between K1 and K10.

Where can you buy a conductivity sensor?

The measuring range has to cover your normal value and realistic peaks without the resolution becoming too coarse at the low end. A built-in temperature sensor with compensation to 25 °C is essential unless the water sits in a climate-controlled room. The output has to match your data chain, which for the DR-ECK probes means RS485 with Modbus RTU and a 7 to 30 V supply. Also check how the probe mounts, whether the cable reaches your node and where the connector will stay dry. And plan for maintenance: a suitable calibration solution, access to the measuring point and a log for cleaning and calibration.

If you are looking for a conductivity sensor to track EC readings in water over time, you will find our models in the conductivity sensors category.