Conductivity measurement and testing

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 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.


Conductivity-10151497-270x195
resistivity-ph
dissolved-solids
salinity-ph

What is conductivity and why should it be measured?

Conductivity-10151497-270x195

Conductivity depends on the concentration of ions and temperature.

Electrical conductivity is an inherent property of most materials and ranges from extremely conductive materials, such as metals, to non-conductive materials, like plastic or glass. In between the two extremes are aqueous solutions, such as sea water and plating baths. In metals, the electrical current is carried by electrons while in water it is carried by charged ions. In both cases, the conductivity is determined by the number of charge carriers, how fast they move, and the capacity of the carrier. Thus for most water solutions, the higher the ion concentration from dissolved salts, generally the higher the conductivity. Conductivity will increase with an increase in ion concentration until the solution becomes too crowded, thus restricting the freedom of the ions to move. Thereafter, conductivity may actually decrease with increasing ion concentration. This can result in two different concentrations of a salt having the same conductivity.

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 and is derived from conductance by the geometry of the measuring cell. 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 is comprised of 2 flat parallel measuring electrodes separated a fixed distance (d). The Cell constant (K) is related to the physical characteristics of the measuring cell and is a function of the separation distance (d) divided by the electrode area (a). In practice, the measured cell constant is a fixed value and is entered into the meter whereby the conversion from conductance to conductivity is calculated and presented.

Applications

Conductivity meters measure the ion capacity in aqueous solution to carry electrical current. As the ranges in aqueous solutions are usually small, the basic untis 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 as well as industrial use. Industries that employ this method include the chemical, semi-conductor, powr 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.

SolutionConductivity
Absolute pure water 0.055μS/cm
Power plant boiler water1.0μS/cm
Good city water50μS/cm
Ocean water53mS/cm
Distilled water0.5μS/cm
Deionised water0.1 - 10μS/cm
Demineralised water0 -80μS/cm
Drinking water0.5 - 1mS/cm
Wastewater0.9 - 9mS/cm
Seawater53mS/cm
10% NaOH355mS/cm
10% H2SO4432mS/cm
31% HNO3865mS/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 raw 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.

What is resistivity and why should it be measured?

Resistivity

The resistivity of a solution is measured by how strongly it resists an electrical current; in other words, it’s the inverse of conductivity. This is an important parameter to measure when working with organic compounds that don’t conduct electrical current very well and have very low conductivity such as oil, solvents, phenol, alcohol and sugar. The below table lists some examples of organic liquid conductivities. These conductivities are for comparison only, because values will vary depending on temperature, concentration, and the presence of other compounds. As shown below, the differences in the conductivity values can be many orders of magnitude.

Organic compoundConductivity, µS/cmTemp (°C)
Formic acid (4.94%) 5500 18
Acetic acid (50%) 740 18
Latex paint 700 25
Water, New York City 72 25
Corn syrup 16 32
Acetonitrile 7 20
Vodka, 100 proof 4 25
Isopropanol 3.5 25
Sugar solution, pure 3 10
Benzyl alcohol 1.8 25
Methanol 0.44 18
Glycol 0.30 25
Glycerol 0.064 25
Acetic acid (99.7%) 0.040 18
Ethanol <0.010 25
Oils: vegetable, fuel, 100% biodiesel <0.010 25
Pain, enamel <0.010 25
measuring resistivity


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.

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.


What are total dissolved solids and why should they be measured?

dissolved solids

The term Total Dissolved Solids (or TDS) refers to the total amount of substances in a sample, which are dissolved and cannot be filtered out. A common solution that is frequently measured for TDS is 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.

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 very little training for good results.

The standard method of determining TDS (total dissolved solids) is by evaporating the sample to dryness at 180°C and weighing the residue. Conductivity can be used to estimate TDS. This is done by calculating what concentration of sodium chloride would have the same conductivity as the sample at the same temperature. Using an Orion Star portable conductivity meters or a Versa Star or Orion Star Bench Meter combined with an Orion conductivity probe it’s simple to generate a report of a sample’s TDS value in mg/L of sodium chloride by comparing the conductivity and temperature of the sample to data stored in the meter’s memory. The stored data was obtained from the International Critical Tables. For temperature values between 5 and 70°C, and TDS values between 0 and 19,999mg/L, the displayed TDS value agrees with the International Critical Tables values with an accuracy of 0.87% RSD.

 

milli-Siemens/cm

micro-Siemens/cm

Temperature
(°C)
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)
065.107.1357768154
166.847.3447998356
268.597.5558228658
370.357.7688468859
472.127.9838709161
573.918.2008949363
675.708.4189189664
777.508.6389439866
879.328.86096810168
981.159.08499210370
1082.989.309101710672
1184.839.535104310873
1286.699.763106811175
1388.569.993109411477
1490.4510.22111911679
1592.3410.46114511981
1694.2410.69117112283
1796.1510.93119812585
1898.0811.16122412787
19100.011.40125113088
20102.011.64127713390
21103.911.88130413692
22105.912.12133113894
23107.912.36135814196
24109.912.61138614498
25111.912.851413147100
26113.913.101441150102
27115.913.351468153104
28117.913.591496156106
29120.013.841524159108
30122.014.091552161110
31124.114.341580164112
32126.214.591608167114
33128.314.851636170117
34130.415.101665173119
35132.515.351693176121
36134.615.611722179123
37136.715.861751182125
38138.916.121780185127
39141.016.371808188129
40143.216.631837191131
41145.416.891866194134
42147.617.151896197136
43149.817.401925200138
44152.017.661954203140
45154.217.921983206142
46156.418.182013209145
47158.718.442042212147
48160.918.702071215149
49163.218.962101219151
50165.419.222130222154

What is salinity and why should it be measured?

salinity

Like TDS, salinity is a measurement of dissolved solids in water, specifically salt and the ions that comprise it. The most common form of measurement for salinity is based on the Practical Salinity Scale (PSS), which was adopted by oceanographers in the 20th century. After extensive testing of seawater, the PSS defined salinity as follows: a seawater of salinity 35 (S = 35) has a conductivity ration of unity with a solution of 32.4356 grams of potassium chloride (KCl) in 1 kg. of solution at 15°C and 1 atmosphere. Therefore, a practical salinity reading is a relative value based on a standard 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. Salinity may also be defined as a unit of electrical conductivity.

When properly calibrated a conductivity probe and meter can be used to determine the rate of salinity in substances such as water, soil or brine. The conductivity measurement (and therefore the salinity measurement) is dependent on the temperature of the sample. Conductivity and salinity meters, such as the Orion Star or Orion Versa Star conductivity meters, will automatically temperature compensate and report the expected practical salinity result at 15°C. If the conductivity probe has an integrated temperature sensor, such as an Orion conductivity probe, no further actions are required by the user to obtain the correct practical salinity measurement

Conductivity Electrodes

 Rugged DuraProbe 4-electrode conductivity cell2-electrode conductivity cell for ultra-pure waterHigh range
2-electrode conductivity cell
Chemical resistant
3-electrode conductivity cell
 
 Orion DuraProbe 
4-Electrode Conductivity Cells
Orion 2-Electrode Conductivity CellsOrion 2-Electrode Conductivity CellsOrion 3-Electrode Conductivity Cell
 
  • Widest conductivity range
  • For lab and field applications
  • Rugged epoxy/ graphite body
  • Cable lengths from 1.5 to 10 meters
  • Low conductivity range
  • For ultra pure water applications
  • Includes detachable glass flow cell
  • High conductivity range
  • For lab applications
  • Chemical resistant glass body
  • Flow through or dip cell
    • Rugged epoxy/ stainless steel
    • Removable guard
    Product SKU number013005MD A
    013010MD 
    A
    013020MD 
    A
    013025MD 
    A

    013016MD A

    018020MD A *

    014005 H **

    Measurement range1μS/cm to 200mS/cm0.01μS/cm to 300μS/cm10μS/cm to 2000mS/cm0μS/cm to 200μS/cm
    Approximate Cell Constant0.475cm-10.1cm-110cm-11.0cm-1
    Cell MaterialEpoxy/graphiteSteel, V4AGlass/ platinum, platinizedEpoxy/steel
    ConnectorMiniDIN connector (Star)MiniDIN connector (Star)MiniDIN connector (Star)BNC and 2.5 mm Phono Jack connector (Russell)
    Meter CompatibilityStar SeriesStar SeriesStar SeriesRL060C Russell
    Dimensions
    (D) Dia
    (L) Length
    (V) Flow cell volume
    15mm (D),
    120mm (L)

    13mm (D), 
    120mm (L),
    8-12mL (V)

    16mm (D),
    115mm (L)

    13mm (D),
    120mm (L)
    Min/Max Immersion Depth35/NAmm35/110mm55/110mm25/120mm
    Key information
     
     A MiniDIN connector (Star)   B 8 Pin DIN oonnector (A+ Series)
     F 8 Pin Waterproof DIN connector (A Series)
     H BNC and 2.5 mm Phono Jack oonnector (Russell)
     *Conductivity cell does not have temperature compensation
     **3 -Bectrode ConductMty Cell

    View all
      Increase your productivity with conductivity probe accessories

     

     Rugged DuraProbe 4-electrode conductivity cell2-electrode conductivity cell for ultra-pure waterHigh range
    2-electrode conductivity cell
    Chemical resistant
    3-electrode conductivity cell
     
     Orion DuraProbe 
    4-Electrode Conductivity Cells
    Orion 2-Electrode Conductivity CellsOrion 2-Electrode Conductivity CellsOrion 3-Electrode Conductivity Cell
     
    • Widest conductivity range
    • For lab and field applications
    • Rugged epoxy/ graphite body
    • Cable lengths from 1.5 to 10 meters
    • Low conductivity range
    • For ultra pure water applications
    • Includes detachable glass flow cell
    • High conductivity range
    • For lab applications
    • Chemical resistant glass body
    • Flow through or dip cell
      • Rugged epoxy/ stainless steel
      • Removable guard
      Product SKU number013005MD A
      013010MD 
      A
      013020MD 
      A
      013025MD 
      A

      013016MD A

      018020MD A *

      014005 H **

      Measurement range1μS/cm to 200mS/cm0.01μS/cm to 300μS/cm10μS/cm to 2000mS/cm0μS/cm to 200μS/cm
      Approximate Cell Constant0.475cm-10.1cm-110cm-11.0cm-1
      Cell MaterialEpoxy/graphiteSteel, V4AGlass/ platinum, platinizedEpoxy/steel
      ConnectorMiniDIN connector (Star)MiniDIN connector (Star)MiniDIN connector (Star)BNC and 2.5 mm Phono Jack connector (Russell)
      Meter CompatibilityStar SeriesStar SeriesStar SeriesRL060C Russell
      Dimensions
      (D) Dia
      (L) Length
      (V) Flow cell volume
      15mm (D),
      120mm (L)

      13mm (D), 
      120mm (L),
      8-12mL (V)

      16mm (D),
      115mm (L)

      13mm (D),
      120mm (L)
      Min/Max Immersion Depth35/NAmm35/110mm55/110mm25/120mm
      Key information
       
       A MiniDIN connector (Star)   B 8 Pin DIN oonnector (A+ Series)
       F 8 Pin Waterproof DIN connector (A Series)
       H BNC and 2.5 mm Phono Jack oonnector (Russell)
       *Conductivity cell does not have temperature compensation
       **3 -Bectrode ConductMty Cell

      View all
        Increase your productivity with conductivity probe accessories

       

      Tips for accurately measuring conductivity, resistivity, TDS and salinity

      Measure conductivity accurately

      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.

      1. 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 ›

      2. Understand and anticipate the effects of temperature. Conductivity measurements are strongly affected by the temperature of the sample.

      3. 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.

      4. 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.

      5. 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.

      6. 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.

      7. 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.

      8. 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.

      9. 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.

      10. 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

      Conductivity Bench Meters
       

      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.

      Conductivity Portable Meters
       

      Our portable meters are rugged and waterproof, and provide dependable performance in the field without sacrificing accuracy and reliabilty.

      Online Conductivity Water Measurement Systems

      Ensure high-quality water and ultrapure water and battle the constant and costly threat of impurities with Thermo Scientific Online Conductivity Water Measurement Systems.

      Conductivity Testers

        

      Thermo Scientific pocket testers provide budget-friendly on-the-go temperature, conductivity and TDS testing. 

      Probes, Standards & Solutions

         

      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.


      Support

      Knowledge Base: Certificates of Analysis and SDS for Orion, ROSS, AQUAfast, AquaMate, and AquaSensors products.

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