Accurate conductivity, resistivity, salinity and TDS measurements
Achieve consistent readings of a range of related parameters using conductivity meters and probes. Thermo Scientific Orion bench conductivity meters, portable conductivity meters and conductivity probes are suitable for wastewater, drinking water, water quality lab applications requiring ultrapure or deionized water, manufacturing processes including clean-in-place, industrial wash and rinse water, power generation, mining, healthcare including water for injection and other uses such as aquaculture and food and beverage.
What is conductivity and why should it be measured?
Conductivity depends on the concentration of ions and temperature.
Conductance is defined as the reciprocal of resistance and is measured in Siemens (S), which was formerly referred to as mho (ohm spelled backwards). Conductivity is an inherent property of any given solution.A measurement results in the conductance of the sample and it is converted to conductivity. This is done by determining the cell constant (K) for each setup using a known conductivity standard solution.
Conductivity = (Cell conductance X Cell constant)
The cell constant is related to the physical characteristics of the measuring cell. For a cell comprised of two flat, parallel measuring electrodes, K is defined as the electrode separation distance (d) divided by the electrode area (a).
In practice, measured cell constant is entered into the meter (directly or by user calibration) whereby the conversion from conductance to conductivity is calculated and presented.
Conductivity meters measure the ion capacity in aqueous solution to carry electrical current. As the ranges in aqueous solutions are usually small, the basic units of measurements are milliSiemens/cm (mS/cm) and microSiemens/cm (μS/cm). Conductivity is used widely to determine the level of impurities in water supplies for domestic consumption, wastewater, water quality testing, as well as industrial use. Industries that employ this method include the chemical, semi-conductor, power generation, hospitals, textile, iron and steel, food and beverage, mining, electroplating, pulp and paper, petroleum and marine industries. Specific applications include chemical streams, demineralizer output, reverse osmosis, stream boilers, condensate return, waste streams, boiler blowdown, cooling towers, desalinization, laboratory analysis, fruit peeling and salinity level detection in oceanography. In the table below are examples of solutions and their known conductivities.
|Ultrapure water||0.055 μS/cm|
|Power plant boiler water||1.0 μS/cm|
|Good municipal water||50 μS/cm|
|Ocean water||53 mS/cm|
|Distilled water||0.5 μS/cm|
|Deionised water||0.1 - 10 μS/cm|
|Demineralised water||0 -80 μS/cm|
|Drinking water||0.5 - 1 mS/cm|
|Wastewater||0.9 - 9 mS/cm|
|10% HCI||700,000 μS/cm|
|32% HCI||700,000 μS/cm|
|31% HNO3||865 mS/cm|
|*mS/cm = milliSiemen per centimeter|
μS/cm = microSiemen per centimeter
- We use conductivity measurements to determine the amount of dissolved ions present in a sample, which in water, serves as a measure of water quality
- Although conductivity measurements are generally simple, not accounting for tempature will greatly affect the validity of the data generated. Applying temperature compensation is a way to account for temperature effects, and helps ensure the reliability and accuracy of your measurements. Temperature compensation uses the measured conductivity and temperature readings of the sample and applies a coefficient or algorithm to calculate and report the conductivity value of the sample at the selected reference temperature. When reported at 25 degrees C, this is known as specific conductance.
What is resistivity and why should it be measured?
The resistivity of a solution describes how strongly it resists an electrical current; in other words, it’s the inverse of conductivity. Another common application for measuring resistivity is when making ultrapure water. Ultrapure water has a high resistivity (>18.18 MΩ·cm at 25° C) and therefore very low levels of conductivity (0.055 µS/cm at 25° C), which can only be accurately measured with a conductivity probe and meter to achieve confidence in its inability to conduct electricity.
This is an important parameter to measure when working with, or making, purified water, such as deionized, distilled, or reverse-osmosis water. Depending on the application, purified water may also be known as reagent water, reagent grade water, clinical lab reagent water, or Type I water. Other terms may apply depending on the purity. Ultrapure water has a high resistivity (>18.18 MΩ/cm at 25°C) and therefore very low levels of conductivity (0.055 μS/cm at 25˚C). Ultrapure water is often used for laboratory, pharmaceutical, semiconductor, or boiler applications.
Find out more about how conductivity impacts the creation of ultrapure water by watching the webinar: Myths and Truths: pH and Conductivity of Ultrapure Water.
|Organic compound||Conductivity, µS/cm||Temp (°C)|
|Formic acid (4.94%)||5500||18|
|Acetic acid (50%)||740||18|
|Water, New York City||72||25|
|Vodka, 100 proof||4||25|
|Sugar solution, pure||3||10|
|Acetic acid (99.7%)||0.040||18|
|Oils: vegetable, fuel, 100% biodiesel||<0.010||25|
What are total dissolved solids and why should they be measured?
The term Total Dissolved Solids (or TDS) refers to the total amount of minerals, salts, and/or metals dissolved in water.
When drinking water has a high level of TDS, it will be unpleasant to drink; therefore, many countries have established a maximum recommended level for TDS in a drinking water. TDS is also used to monitor the quality of watershed source waters, such as rivers, lakes, and ponds. High TDS can indicate hard water, brackish or saline water, and/or nutrient loading of water. Hard water may be unsuitable for industrial, aquarium, spa, swimming pool, and reverse osmosis water treatment systems. Brackish or saline water may be unsuitable for agriculture, hydroponics, and aquaculture. Nutrient loading may compromise the health of a water body and impact its use as a potable water source
TDS is commonly determined by gravimetry, chemical analysis, or conductivity.
- Gravimetry – The gravimetry protocol requires that a volume of filtered sample be evaporated to dryness at about 100°C, then dried to a constant weight at 180°C. The increase in dish weight represents the total dissolved solids per the filtered sample volume.
- Chemical analysis – The chemical analysis protocol requires that the sample be measured for major ions (such as sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, and fluoride) and other parameters, such as nitrate and alkalinity. The results are used to calculate TDS.
- Conductivity – The conductivity protocol requires only a conductivity measurement to be made. This measurement is multiplied by a factor following the simple formula, i.e., TDS = k EC (in 25°C) whereby k is a function of type of water being measured, and EC is the conductivity.
Of the three common TDS measurement protocols, only conductivity is suited for field testing and continuous monitoring. In addition, it’s a much quicker and simpler measurement, which requires little training for good results.
Using an Orion Star portable conductivity meters or a Versa Star Pro or Orion Star A bench meter combined with an Orion conductivity probe it’s simple to obtain a measurement of a sample's estimated TDS value in mg/L. The meter automatically reads the conductivity and multiplies by the selected TDS factor. Data can be automatically or manually stored in the meter memory for later download.
|111.9mS/cm Conductivity Standard (mS/cm)||12.9mS/cm Conductivity Standard (mS/cm)||1413µS/cm Conductivity Standard (µS/cm)||147µS/cm Conductivity Standard (µS/cm)||100µS/cm Conductivity Standard (µS/cm)|
What is salinity and why should it be measured?
Because of its high sensitivity and ease of measurement, conductivity is said to be the most commonly used method to determine the salinity of seawater. When the Practical Salinity Scale was adopted by oceanographers, they defined salinity as follows: a seawater of salinity 35 (S = 35) has a conductivity ratio of unity with a solution of 32.4356 grams of potassium chloride in 1 kg of solution at 15C and 1 atmosphere. This value for salinity was determined by extensive testing of seawater samples. Therefore, a practical salinity reading is a relative value based on a standard potassium chloride (KCl) solution.
Since salinity is a ratio, the measured value is dimensionless and has no units. However, salinity is commonly reported in units known as “practical salinity units” or psu, or in the traditional units of “parts per thousand” or ppt.
When properly calibrated, a conductivity probe and meter can be used to determine salinity in seawater and brackish estuarine water. Salty solutions, such as brines or irrigation water, are better measured by using the TDS mode. Orion Conductivity Meters automatically calculate salinity using oceanographic equations compensated to 15°C per accepted conventions. When using an Orion Conductivity Probe, which has an integrated temperature sensor, and a conductivity meter, like a Orion Star A Portable Meter or Orion Versa Star Pro Bench Meter, salinity can be reported as practical salinity units (psu) or parts per thousand (ppt), depending on user preference.
Tips for accurately measuring conductivity, resistivity, TDS and salinity
Although conductivity measurements are generally simple and easy to take, mistakes can still affect the validity of the data generated. By understanding and avoiding the most common measurement mistakes, you can help ensure you are on the path to accurate and reproducible readings.
- Use a suitable conductivity sensor. Sample composition, location (i.e. need for durability in the field), and the purity of the water sample can all influence the type of conductivity sensor you choose. Select the right conductivity probe ›
- Understand and anticipate the effects of temperature. Conductivity measurements are strongly affected by the temperature of the sample.
- Accurately use the temperature compensation (TC) function. Temperature compensation (TC) will calculate and display the conductivity at the chosen reference temperature. If TC is off, the displayed value is the actual conductivity at that temperature.
- Carefully set the temperature compensation settings. Whether to apply a TC or not, or what type of TC selected, can affect the accuracy of your readings.
- Take a conductivity reading only after temperature equilibrium is achieved. Conductivity is temperature-dependent, so time must be allowed for the conductivity sensor to equilibrate to the same temperature as the sample.
- Minimize the use of elaborate multi-point calibrations. According to ASTM, a one-point calibration of the cell constant at a representative conductivity is sufficient for accurate conductivity readings. If the samples cover a large range of conductivity levels, one or more points can be made.
- Use additional care and caution when handling low-level conductivity samples. The stability and purity of the sample and how it is handled can affect the accuracy of the sample reading. Low-level samples can be easily affected by contamination, CO2 absorption, and degassing.
- Avoid setting calibration standards that are too low. Low-level standards are prone to contamination and difficult to use successfully. Tighter accuracy can be achieved by calibrating at 100 µS/cm or above.
- Follow storage and maintenance guidelines for your conductivity sensor. Improper long-term and short-term storage of conductivity sensors can change the surface and adversely affect their performance.
- Understand how to calculate resistivity, total dissolved solids (TDS) or salinity factors. Conductivity readings can be used to determine an estimate of these parameters in a sample by applying the related function through the meter setup. The meter will provide accurate conductivity and resistivity measurements, but the TDS and salinity values are an estimate as the true TDS is determined by gravimetric testing.
Conductivity measurement and testing products and solutions
The accuracy and reliability of your conductivity measurements depends on the instrumentation you use. Learn more about how you can identify the best match based on your required feautres, performance, spcifications, and budget.
Ensure high-quality water and ultrapure water and battle the constant and costly threat of impurities with Thermo Scientific Online Conductivity Water Measurement Systems.
Thermo Scientific pocket testers provide budget-friendly on-the-go temperature, conductivity and TDS testing.
An extensive assortment of conductivity probes are available to meet the needs of your sample measureents. Conductivity standards, TDS standards, and conductivity probe conditioning solutions for your conductivity measurement needs.
Knowledge Base: Certificates of Analysis and SDS for Orion, ROSS, AQUAfast, AquaMate, and AquaSensors products.
Orion Meters Software and Upgrades
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