How to Calculate Electrical Conductivity from TDS?

Did you know the Dead Sea has a 200,000 µS/cm electrical conductivity (EC)? It is the most conductive natural water body in the world. The Dead Sea's salinity exceeds 34%. Salts in the Dead Sea cause high total dissolved solids (TDS) and electrical conductivity (EC), establishing a close relationship between them. This article will explore how these terms relate and how electrical conductivity sensors can calculate them.

Our target is to develop a deep understanding of EC and TDS, critical parameters for all major industries that use water in their processes. Measuring EC and TDS in an industrial setting requires a rapid estimation that provides results closest to the actual values. EC sensors offer a reliable and fast method for calculating them. In contrast, higher accuracy and precision methods are tedious and time-consuming. This article will explore all our options for calculating electrical conductivity from TDS and vice versa, starting with how EC represents water quality.

1. Water Quality Relationship with Electrical Conductivity

Water is a lifeline for the living and a vital material for industries. Humans and Earth's mechanisms revolve around water. It is a source of hydration for all living organisms and an equally crucial universal solvent for chemical processes.

Considering the wide range of water uses, the definition of water quality changes with application. While it is safe for humans to consume 100-500 ppm of water, chip manufacturing uses 1 ppm precisely controlled using EC sensors, which makes it barely conductive. The article will explain why chip manufacturing requires water with such low EC. First, we need to understand the basic definitions.

1.1. Understanding Total Dissolved Solids (TDS)

TDS is a term that is more relevant to water for living organisms. Organizations like the WHO provide comprehensive guidelines on safe drinking water for humans. Similarly, using electrical conductivity sensors to approximate TDS in industrial applications is critical to ensure safe operation and avoid scaling or corrosion.

1.1.1. Definition and Composition of TDS

TDS represents the amount of organic and inorganic solids in water. These can be minerals, salts, metals, and other ions. TDS is expressed in milligrams per liter (mg/L) or ppm (parts per million).

An electrical conductivity sensor can provide TDS using a conversion formula. However, the formula changes with the type of liquid, so careful evaluation of the liquid is required. A more accurate and precise method is evaporating water from a 0.1-liter sample and weighing the residue minerals left on the surface.

1.1.2. Sources of TDS in Water

There are various sources of TDS in water. Some occur naturally, while others result from industrial or process pollution. Here are their details:

• Urban Runoff: During the rainy season, the water washing down cities can contain petroleum products, fertilizers, pesticides, metals, salts, and more. These contribute to the rise in water TDS.
• Industrial Wastewater: Industrial processes may require adding chemicals that can increase the TDS of water. For example, synthetic dyes, heavy metals, salts, suspended solids, acids, alkalis, and microfibers can increase the TDS of water in clothing factory wastewater.
• Piping: In plumbing, the hardware, including the pipe and fittings, can contribute to the rise in TDS levels, especially metal piping, which can add metal oxides due to the corrosive nature of water.
• Seawater: Seawater is a vast body of water with high salt content. The average seawater level is 35,000 ppm TDS.
• Irrigation and Agriculture: Fertilizers and pesticides can add nitrates and phosphates, common in agricultural land. Some irrigation might use well water, which can increase the salt content of the field, thus contributing to the increase in the water's TDS.

1.2. Electrical Conductivity (EC) Explained

The EC also changes as the number of inorganic or organic solids in water changes. Higher solid content, such as metal, salts, and ions, can contribute to the increase in EC, making it a reliable method for judging water quality.

1.2.1. Definition and Measurement of EC

Electrical conductivity is the ability of a material to conduct electricity. It is written in microSiemens per meter (mS/m). Materials that can pass electricity are called conductors. Water, in its purest form, is not a conductor. However, the addition of dissolved solids makes it conductive.

Electrical conductivity can be measured using EC sensors, which can be handheld or installed in a line for continuous measurement. These sensors can have electrodes 1cm apart and pass small currents through the liquid. The meter measures the resistance between the probes, which is directly related to the liquid's conductivity.

1.2.2. Factors Affecting EC

The electrical conductivity sensor may provide varying values depending on the liquid condition. The molecules and their behavior may change due to varying chemical and physical properties. Here are some factors that can affect EC:

• Temperature:As the temperature of a liquid, such as water, increases, the electrical conductivity increases. The increase rate is 2% per °C. Therefore, EC sensors or controlling programs incorporate temperature compensation for accurate readings.
• Ion Concentration:The EC can increase if the ion concentration increases due to the mentioned factors. Some examples are the presence of Na⁺, Cl⁻, Ca²⁺, Mg²⁺, and SO₄²⁻ in water.
• Organic Matter:Adding oil and organic substances to water can decrease its EC, as these materials are non-conductive.
• Geological Effects:Water that passes through limestone and rocks can pick up calcium and bicarbonate, changing its EC.
• Instrument Limitations:Probe polarization and interference can also affect the EC value. Modern instrumentation eliminates inaccuracies by using frequency conversion methods.

1.3. Correlation Between TDS and EC

The central theme of our article is that TDS and EC are closely related. These terms are convertible using a simple formula. However, the conversion factor can change depending on various factors.

1.3.1. The Direct Relationship: More Ions, Higher EC and TDS

You might have established a direct relationship between EC and TDS. As electrical conductivity increases, TDS also increases. In cases like wastewater and urban runoff, organic matter can increase TDS while the EC starts to decrease. In most industrial settings, the addition of solids is controlled; therefore, the correlation is also well-established for accurate results.

1.3.2. Limitations of the Correlation (Water Type, Ion Types)

Before we dive into the correlation between TDS and EC, it's vital to understand its limitations. The correlation can provide rapid and accurate results if the limitations do not pose an error threat.

• Conversion Factor:The conversion factor will change based on the presence of ions in water. Therefore, the correlation is not just multiplication or division by the same number for all cases.
• Not All Solid Conduct Electricity:Dissolvable solids such as sugars, alcohols, oils, and organic matter can increase TDS but can cause EC to decrease. Therefore, the correlation is valid for fixed scenarios.
• Temperature Affects EC but Not TDS:As the temperature of a liquid or water increases, the EC will increase, but TDS will remain the same, as no solids are added to the liquid.
• Ion Types:the presence of different types of solids can have varying effects on the EC. Some ions may affect the EC more than others.
• Salinity and TDS:The TDS equals salinity in clean water, as salts are the only solids. However, in polluted water, the presence of organic solids can change the values of salinity and TDS.
• Suspended Solids Do Not Change EC:Suspended solids, such as sediments, clay, and plastics, increase solids' presence but do not affect the EC, thus invalidating the correlation.

2. How to Calculate Electrical Conductivity from TDS?

2.1. The Conversion Factor

Converting TDS to electrical conductivity requires a conversion factor. As we discussed earlier, this formula has limitations. The relation remains valid if electrical conductivity and TDS are directly related. However, we still need to have an approximate idea of the nature of water.

2.1.1. Understanding the K Factor (Typical Values)

The K factor in the conversion formula utilizes the directly proportional nature of EC and TDS. Most TDS meters are EC sensors that check the electrical conductivity of the liquid and apply a conversion formula to give the result in ppm or ml/g.

2.1.2. Variations in K Factor based on Water Type

The value of the K factor changes with water type. Here are some of the examples:

• Seawater (~0.5 K) → More NaCl conducts electricity efficiently.
• Groundwater (~0.65 K) → Includes Ca and Mg, which conduct less efficiently.
• Pure water (~0.7 K) → Fewer salts, more bicarbonates, and organic matter.

2.2. The Formula

EC (µS/cm) = TDS (ppm) / Conversion Factor (K)

The formula simply divides TDS by a conversion factor, resulting in EC. Most EC sensors have this formula embedded in their hardware or software. The software may adjust the conversion factor based on values from other sensors to obtain more accurate values.

2.3. Considerations for Accurate Calculation

As we mentioned earlier, some factors can affect conversion. We must ensure proper compensation to the electrical conductivity sensor readings for accurate calculation. Here are the two main factors that can affect accuracy:

2.3.1. Temperature Compensation

Electrical conductivity increases as temperature increases, while the TDS remains the same. In this case, the conversion factor (K) value needs to decrease to ensure the relationship between TDS and EC is accurate.

2.3.2. Calibration of Measuring Instruments

Electrical conductor sensors require calibration. Every manufacturer may use different calibration frequencies, or the user can set a frequency themselves to ensure accuracy in results. The calibration can be done using different batches of standard solution with varying electrical conductivity and known TDS. It will check the performance and allow calibration of the equipment. The EC sensor should give the same value as the known solution EC.

3. Process of Calculation

3.1. Measuring Electrical Conductivity

The first step in calculating electrical conductivity from TDS is measuring the EC using either of the two methods:

3.1.1. Using an EC Meter

EC meters can be industrial-grade or for domestic use. Depending on the design, these meters can have a detachable or inbuilt probe. They provide direct values with an onboard display. The mixture must be properly mixed, and the sample should represent the whole batch. Simply insert the probe and follow the EC sensor's instruction manual to get the results.

3.1.2. Using an Online Electrical Conductivity Sensor

An online electrical conductivity sensor is the fastest and most efficient way to monitor and control a process. The most modern EC sensors are temperature compensated and use frequency conversion method for accurate results. They also consider the electrode polarization and external interference that can affect the readings. Their outputs are generally analog (4-20mA) or digital (RS485) signals with a detection range of 0–200,000 µS/cm. A single probe can provide EC, Salinity, and TDS. These are ideal for process plants, wastewater treatment, water purification plants, or any other industry that requires online monitoring and control.

3.2. Selecting the Appropriate Conversion Factor

As mentioned earlier, selecting the appropriate conversion factor ensures accurate results. Here are the factors to consider:

3.2.1. Determining Water Type and Ion Composition

Determine the type of water, whether seawater, groundwater, or pure water. Use the proper conversion factor based on the observation.

3.2.2. Consulting Reference Tables and Guidelines

Consult the following table for the proper conversion factor:

3.3. Performing the Calculation and Interpretation

Finally, you can apply the formula to get the required TDS or EC. If your EC sensor provides conductivity, convert it into TDS; in another case, if your TDS meter provides ppm, you can easily convert it into µS/cm.

4. Uses of Electrical Conductivity Sensor/EC

4.1. Water Quality Monitoring

4.1.1. Drinking Water Quality Assessment

The EC and TDS values must be measured to ensure drinking water safety. WHO recommends a TDS of 300 parts per million (ppm) and an electrical conductivity (EC) level of less than 400 micro Siemens per centimeter (µS/cm).

4.1.2. Industrial Wastewater Monitoring

Industrial wastes can contain contaminants that can be hazardous to living things. Online monitoring can help address the ingress of pollutants head-on. In industries like clothing and mining, online monitoring using EC sensors is an essential requirement.

4.1.3. Environmental Monitoring (Rivers, Lakes)

In the case of lakes and rivers, which become the water consumption for living creatures, monitoring their EC and TDS can provide helpful insight. Ensuring the water is safe for consumption requires measurement, which EC sensors can perform rapidly and accurately. However, we still need to consider the conversion factor selection criteria.

4.2. Agriculture and Hydroponics

4.2.1. Nutrient Solution Management

In agriculture, monitoring the nutrients in a solution to enhance plant growth can have financial and health benefits. Using TDS and EC to ensure the proper mixture of nutrients for agricultural land can result in high yields. EC sensors and meters can help manage them effectively.

4.2.2. Soil Salinity Measurement

Soil salinity is measured using a salinity meter, an inherent part of EC sensors. The modern frequency conversion method allows the EC sensor to detect TDS, EC, and Salinity by slightly modifying the calculation methods and formulas.

4.3. Industrial Applications

4.3.1. Boiler Water Monitoring

Boilers in industrial power plant operations need a high-quality water feed. High-dissolved solvents can cause slug formation, hindering boiler efficiency and life. Therefore, EC sensors in the feedwater line are critical for monitoring plant health.

4.3.2. Chemical Process Control

Industries such as textiles, dyeing, consumable drinks, and dipping sauces need EC and TDS meters to monitor their ongoing processes. Any change in the EC and TDS can lead to variations in output, leading to high waste and inefficient operation.

Final Words: Calculating EC from TDS using EC Sensors

Calculating electrical conductivity (EC) and total dissolved solids (TDS) requires an EC meter or online EC sensor. The value that it produces can be converted into TDS and vice versa. However, the user needs to incorporate all the factors that can affect the conversion factor within the formula. Factors such as temperature, ion, solid type, water type, organic matter, etc, are vital to consider before making any approximations. Finally, after including and understanding all the factors, the user can safely utilize the formula to get the desired conversion of EC from TDS using a simple EC sensor.

If you want an advanced EC sensor with broad detection capability and robust outputs, visit the RIKA website to explore the RK500-13 Rika Online Electrical Conductivity (EC) / Salinity Sensor. Using the latest frequency conversion method, the Rika sensor can accurately provide TDS, EC, and Salinity values. We hope you find value in the article and Rika products.