What is Conductivity?Electrical conductivity is a measure of theability of a solution to carry a current. Currentflow in liquids differs from that in metalconductors in that electrons cannot flowfreely, but must be carried by ions. Ions areformed when a solid such as salt is dissolvedin a liquid to form electrical componentshaving opposite electrical charges. Forexample, sodium chloride separates to formNa+ and Cl- ions. All ions present in thesolutions contribute to the current flowingthrough the sensor and therefore, contribute tothe conductivity measurement. Electricalconductivity can therefore be used as ameasure of the concentration of ionizablesolutes present in the sample.
Conductivity UnitsElectrical resistivity uses the unit of ohmmeter or Ω⋅m. Electrical conductivity is thereciprocal of electrical resistivity. Rather thanuse the units Ω−1⋅m−1, in 1971 the unit“siemens” (symbolized by the capital letter S)was adopted by the General Conference onWeights and Measures as an SI derived unit.The unit for electrical conductivity becomessiemens per meter. The siemens unit is namedafter Werner von Siemens, the 19th centuryGerman inventor and entrepreneur in the areaof electrical engineering.
North American practice continues to see theuse of unit mho/cm to measure conductivity,where the unit “mho” is a reciprocal ohm. The
word “mho” is the word “ohm” spelledbackwards. Because of the history ofconductivity measurements in micromho/cmand millimho/cm, it is common to see thesemeasurements translated to microsiemens/cmand millisiemens/cm because there is a one-to-one correspondence between these units.
Conductivity Terminology andFormulas
conductivity=K cell
R1
1/100∗T−25where
conductivity is the temperature-compensated reading in siemens/cm;
Kcell = cell constant in cm-1 , typically inthe range 0.01/cm to 50/cm;
R = measured resistance in ohms;
α = temperature compensation factor as% change per °C, typically close to 2.0;
microsiemens per centimeter (µS/cm)millisiemens per centimeter (mS/cm)
resistivity= 1conductivity
resistance= 1conductance
cell constant=K cell=lA
wherel = distance in cm between the electrodesA = area in cm2 of the electrodes
siemenscm
=mhocm
= 1ohm⋅cm
1 S /m=104 S /cm=103 mS /m
Conductance Data forCommonly Used ChemicalsExamples of conductance of various materialswith changing concentration are shown inillustration 1. Sodium hydroxide (NaOH)exhibits variable temperature-related rates ofconcentration change. It is clear from thegraph that both Sulfuric acid, (H2SO4), andnitric acid, (HNO3), have unusual conductivityvs weight relationships as well. It clearlyshows that there is no “conductivity constant”between chemical combinations.
Cell ConstantTo determine the amount of current that willflow through a known amount of liquid, thevolume between the two electrodes must beexact and the current must be kept consistentand moderate. This is known as the cellconstant. Any effective volume changechanges the cell constant and current, toomuch volume will result in noise (low
Illustration 1 Conductivity (µS/cm) vs chemical concentration
current), or too little volume in electrolyticeffects (high current). The cell constantrecommended will vary depending on theconductivity range of the solution. Highconductivity requires a high cell constant andlow conductivity requires a low cell constant.
Industrial users may have a wide range ofapplications with unpredictable variables.Ideally, IC Controls would check the ranges ofconductance for all the applications andrecommend appropriate cell constants.However, we may be dealing with unknownsand upsets. To provide for unknowns theIC Controls model 455 conductivity analyzerauto range capability allows for a tenfoldincrease or decrease in range by themicroprocessor. The user can achieve fullaccuracy at a far greater range than washistorically possible. For example, a 1.0/cmconstant recommended for 0-1,000 µS/cm canread accurately up to 0-10,000 µS/cm or downto 0-100 µS/cm full scale. Not only is accuracyassured over a greater conductivity range, butyou can use fewer cell constants.
Temperature Compensation
Ionic movement, and therefore conductivitymeasurement, is directly proportional totemperature (see illustration 2). The effect ispredictable and repeatable for most chemicals,but unique to each chemical. The effect isinstantaneous and quite large (typically 1%-3% / ºC) with reference to the value at 25ºC(see table 2 and illustration 3). Also refer toformula on page 1 of the report where α refersto % change.
In industrial applications, temperature oftenfluctuates and requires temperaturecompensation. This is generally accomplishedby using an automatic, linear temperaturecompensation method. In most cases, thevariations in temperature are corrected usingautomatic temperature compensation, 2% perºC is deemed acceptable.
Without temperature compensation big errorscan result, 50ºC x 2%/ºC = 100% off, or 50%error in reading! In laboratory applications,where measurements must be made withaccuracy and consistency in various chemicalcombinations, manual temperature
compensation can be considered for eachapplication. The temperature is set in themanual TC mode.
For on-line process applications theIC Controls 455 conductivity analyzer allowsthe user to set the TC constant in %/ºC tomatch the curve in the known temperaturerange of the known process chemical. Wherethe process mixture produces an unknownthe default 2% per ºC can be used;
Illustration 3 Temperature response of typicalsolutions
substance % change per ºC
acids 1.0 to 1.6
bases 1.8 to 2.2
salts 2.2 to 3.0
neutral water 2
Table 2 Typical temperature response
Illustration 2 Temperature compensation values
alternatively, tests can be performed and acustom value can be set in the 455. Somechemicals that are frequently diluted for usehave changing non-linear temperaturecompensation requirements, so IC Controlshas programmed special versions with TC in agraph in the memory; e.g. NaOH 455-21,H2SO4 455-22, HCl 455-23, NaCl 455-24, thatread out in % concentration. Additionally the455-63 offers calibration and TC for high-purity water with very low conductivity.
Sensor CleaningAs the volume (distance) between the twoelectrodes is exact, fouling of the sensor canalter the distance between the electrodes andchange the cell constant. Therefore, keepingthe electrode clean is important. The 455analyzer will determine the cell constant at thepoint of calibration and condition of use andcompensate accordingly. Changing cellconstants will then not be a factor adverselyaffecting repeatability.
Conductivity CalibrationAs mentioned above, a calibration scheduleshould be adhered to. While it is a quick,single point process, it is important that allapplications be accurately reflected withacknowledgment of the set TC and cellconstant. The 455 will keep in memory arecord of calibration dates, values, and cellconstants that can be downloaded to yourcomputer for proof of performance or to trendthe sensor cell condition. If the user calibratesusing a laboratory bench top unit as acalibration standard, there is a grab samplemethod incorporated into the 455 analyzer thatenables the user to standardize the reading tocorrespond with the lab unit. Using labstandardization or the high integrity of themodel 455 calibration is the conveniencechoice of the user.
Theory of CalibrationPeriodic calibration of conductivity sensors incontinuous use is recommended. Variousfactors can affect the physical limits on the
liquid and the apparent cell constant (scale,biological growths, oils, wax, gum, etc).Reducing the area for current-carrying liquid.
A conductivity cell's physical size and shapeare important. The only restrictions on an ion'smovement are the physical limits of the liquid.A conductivity analyzer measures all thecurrent that will flow between two electrodes;thus if there are no restrictions not only willthe shortest path between the electrodes carrycurrent, but also other roundabout paths willcarry a smaller share of current. The controlledvolume of a good conductivity sensor placesphysical limits on the liquid and controlscurrent paths, which is identified by the cellconstant. The cell constant can be accuratelydetermined by dipping the sensor in arecognized conductivity standard, (preferablytraceable to NIST since literature referencesare frequently in conflict over conductivityvalues). The standard should be near the highend of the range of operation for the cellconstant of the sensor or in the range ofinterest.
Sensors with low cell constants like 0.01/cmtend to have large electrode surfaces which areclose together, making for fairly large sensors.They need a long, slim container to be fullyimmersed in liquid for calibration. Sensorswith medium cell constants like 0.1/cm and1.0/cm are much smaller and more compactand can usually be calibrated in a beakersuspended above the bottom. High range cellswith 10/cm, 20/cm and 50/cm constantsusually include an internal liquid passage thatrequires a long thin vessel to be immersed ormay require a pumped sample for calibration.
Use of NIST Traceable StandardsIC Controls manufactures conductivitystandards and performs quality control usingNIST materials. Certificates of traceability toNIST materials are available as P/N A1900333
Where to Perform ConductivityCalibrationsA suitable place to conduct a calibration is at acounter or bench with a sink in an instrument
shop or laboratory. However, IC Controlsprovides kits that are kept small and portableso that they can be taken to installation sites,along with a container of water forcleaning/rinsing and a rag/towel for wiping ordrying. Calibration at the site offers theadvantage of taking into account the wiringfrom the analyzer to the sensor, and correctingfor any errors induced.
When calibrating, ensure there are no air
bubbles inside the cell; air bubbles will causelow conductivity readings. Remove bubbles bytapping the sensor or alternately raising andlowering the sensor to flush them out.
With the conductivity cell centered in thebeaker and no air bubbles in the cell, monitorfor the reading to stabilize and then calibratethe analyzer. Note: the reading may graduallychange while the sensor equilibrates to thestandard temperature. With analogconductivity analyzers the technician mustdecide when the temperature is stable and thenturn the standardize adjuster. With amicroprocessor-based analyzer such as themodel 455, the program acts as an expertthermal equilibrium detector and flashes itsreading until temperature stabilizes. Asomewhat different but steady (non-flashing)reading indicates calibration is complete.
Selecting ConductivitySensorsIn order to ensure integrity of conductivityreadings, several steps are needed to considerthe above factors. First, a survey should bemade of all applications. The user should fill
Guide to Cell Constant Usable RangesCELL CONSTANT DESIGN RANGE LOWEST RANGE HIGH RANGE OVER-RANGE *
cm-1 µS/cm µS/cm µS/cm µS/cm
0.01 0 to 10 0 to 1 0 to 100 0 to 1 000*
0.02 0 to 20 0 to 2 0 to 200 0 to 2 000*
0.1 0 to 100 0 to 10 0 to 1 000 0 to 10 000*
0.2 0 to 200 0 to 20 0 to 2 000 0 to 20 000*
0.5 0 to 500 0 to 50 0 to 5 000 0 to 50 000*
1.0 0 to 1 000 0 to 100 0 to 10 000 0 to 100 000*
2.0 0 to 2 000 0 to 200 0 to 20 000 0 to 200 000*
5.0 0 to 5 000 0 to 500 0 to 50 000 0 to 500 000*
10.0 0 to 10 000 0 to 1 000 0 to 100 000 0 to 1 000 000*
20.0 0 to 20 000 0 to 2 000 0 to 200 000 0 to 1 000 000*
50.0 0 to 50 000 0 to 5 000 0 to 500 000 0 to 1 000 000*
* Note: use over-range with caution. Some sensor designs may limit when used on over-rangeand may not reach the maximum shown.Table 3 Cell constant usable ranges
in Conductivity Application Analysis sheets foreach measurement point, factoring in varyingchemical combinations, conductivity rangesand temperatures. This will allow for aselection of sensor styles and cell constants toallow standardization. At this time IC Controlswould address the following:
1) Sensor recommendation - We will attemptto stay with as few cell constants and stylevariations as possible. For example, theIC Controls model 404-1.0 may be a suitable,economical choice.
2) Analyzer recommendation - TheIC Controls model 455 may be recommendedbecause of its wide range capability, accuracyand automatic compensation flexibility.
3) Cleaning Schedule - At least for foulingapplications, to ensure sensor and cell constantintegrity.
4) Calibration Schedule - To documentaccuracy and ensure repeatability ismaintained.
IC Controls will be happy to answer anyquestions and work with users to ensurereliable conductivity readings. In NorthAmerica we can be reached toll-free at 1-800-265-9161, or internationally contact us at:
