Information on Conductivity Measurement电导率测量信息 英文版.pdf

JInformation onConductivityMeasurement Analytical Measurement1Basics.71.1General.71.2Methods of measuring conductivity.81.2.1 General.81.2.2 The cell constant.101.2.3 AC voltage.111.2.4 Measurement principles.111.2.5 Inductive measuring cell.142Measurement.152.1Arrangement of a process installation.152.1.1 Measuring cells.162.1.2 Fittings.162.1.3 Transmitter/controller.172.1.4 Instrumentation:cable material/connecting cable.192.2Commissioning the measurement system.192.2.1 Measurement location.192.2.2 Calibration/adjustment.192.2.3 Measurement conditions.212.2.4 Reference solutions.263Quality assurance.273.1Documentation.283.2Maintenance.293.3Problems/actions.323.4Cleaning.343.5On-site test options.343.6Storage of the measuring cell.354Applications.364.1Wastewater treatment plants.374.2Galvanizing plants.374.3Beverage bottling plants.384.4Power stations.394.5Pharmaceuticals.424.6Full desalination plants.444.7Concentration measurements.455Closing remarks.486Legal aspects.497Source information.508Appendix.518.1Sample test certificate for the measuring cell.518.2Sample calibration certificate for a transmitter.525Conductivity measurement is an easily performed measurement techniquefor determining and monitoring the total salt concentration in water.It is en-countered in many areas of industrial and environmental analysis.Whetherit concerns cleaning the filling lines in a dairy,or protection of the cooling wa-ter system in a power station,the correct procedures always depend on theconductivity value.This technical publication presents the basic chemical relationships and typ-ical applications in a general,easily understood form.In addition,informa-tion will be given on the current state of technology with regard totransmitters/controllers and sensors for this process variable.Our aim is to ensure that the“Information on conductivity measurement”isalways kept fully up to date,and therefore invite our readers to provide feed-back and share experience and knowledge.Any suggestions or contribu-tions to the discussion will be most welcome.Reinhard MannsDr.Peter JohnDipl.-Ing.(FH)Development ManagerJUMO GmbH&Co.KG,Fulda,GermanyReproduction permitted with source acknowledgement!3rd edition,Fulda,February 2004Part No.:00411341Book No.:FAS 624Printed:02.0471Basics1.1GeneralThe conductance or conductivity1()2 expresses how well a material con-ducts an electric current.With metals,it is the movement of electrons thatcauses the current flow.In aqueous solutions,ions take over the chargetransport.Ions are formed when salts,acids,or alkalis dissolve.The moreions are present in the liquid,the better it conducts the current.Fig.1:Salts dissociate into positive and negatively charged ionsThis relationship between the ion concentration and the ability to conductthe electric current makes the conductivity an interesting process variable inwater analysis.It is especially suited for determining the concentration ofdissolved salts.The result of a conductivity measurement is not quoted in mg/liter or per-cent,but as a conductivity value in S/m(siemens per meter).In practice,thesmaller units S/cm and mS/cm are commonly used.Table 1:Conversion of conductivity unitsThe following table illustrates the relationship between the salt concentrationand the conductivity.S/mS/cmmS/cmS/cmS/m10.011010,000S/cm10011,0001,000,000mS/cm0.10.00111,000S/cm0.000 10.000 0010.0011NaClNa+Cl-1.To be precise,this refers to the specific electrical conductance2.The former symbols for electrical conductance were (kappa)or even (chi).Since 1993the symbol used is (gamma),in accordance with European standardization.8Table 2:Examples of conductivity values1.2Methods of measuring conductivity1.2.1GeneralThe basic principle of conductivity measurement is the same with all meth-ods:the instrument generates an electric voltage across the measured solu-tion.An electric current flows whose value depends on the conductivity.Depending on the method or application,the instrument either maintains thevoltage signal constant and records the change in electric current,or main-tains the current value constant and evaluates the voltage change.Fig.2:Diagram of a conductivity measuring cellWater or aqueous solutionConductivity rangeat 25CSalt concentrationHigh-purity water0.055S/cm0mg/lFully-desalinated water0.055 to 2S/cm0 to 1mg/lRainwater10 to 50S/cm5 to 20mg/lGround,surface and drinking water50 to 1000S/cm20 to 50mg/lSea water20 to 60mS/cm10 to 40g/lSaline solution77 to 250mS/cm50 to 250g/l9Both measurement principles are based on Ohms law:R:electrical resistanceI:electric currentU:electric voltage,or rearranged for the conductance:electrical conductanceI:electric currentU:electric voltagek:cell constantAt constant voltage,the current increases proportionally with the conduc-tance.At constant current,the voltage decreases with increasing conduc-tance.It is clear from Ohms law that conductivity measurements reallyconcern resistance measurements.The conductance value I/U is obtainedfrom the reciprocal of the resistance.RUI-?IU-k?101.2.2The cell constantBoth the resistance and the conductivity depend on the dimensions of theelectrical conductor.The length and surface area of the conductor deter-mine the cell constant.Fig.3:Schematic representation of the active areaWith a short length of conductor the electrodes are close together.The smallerthe distance between the electrodes,the lower is the resistance of the mea-sured solution.The influence of the electrodes on the ions increases.A large conductor surface area is synonymous with large electrode surface ar-eas.The bigger the surface area of the electrodes,the smaller is the resistanceof the measured solution.As the surface area increases,more and more ionscome within the range of influence of the electrodes.The surface area of the electrical conductor is normally larger than the electrodesurface area.The electrodes not only affect ions that are directly between theelectrode surfaces,but also those in the boundary fields.The influence of theelectrode boundary fields on the ions decreases with the distance.The bound-ary fields can be limited by the construction of the cell or the location,e.g.pipeinternal diameters.These influences are taken into account in the cell constant:klA-?11k:cell constantl:length of the conductorA:surface area of the conductorThe precise value of the cell constant is obtained from a calibration with a ref-erence solution.1.2.3AC voltageThe electric current between the electrodes depends on the movement of theions in the measured solution.During measurement,the ions move towardsthe electrode that is oppositely charged at the time.Each ion that reaches oneof the electrode surfaces balances out a part of the voltage between the elec-trodes.As it is no longer mobile,it blocks the current flow.This effect(polar-ization)can be counteracted by an AC voltage.Because of the constantreversal of polarity,only a small quantity of ions reach the electrodes,and thenonly for a short period.The more ions the solution contains,that is the higherthe conductivity,the higher must be the frequency that the instrument uses toprevent polarization of the electrodes.Modern instruments(e.g.JUMO dTRANS Lf 01)match the measurement fre-quency to the range in order to achieve optimal measurement results.1.2.4Measurement principles!Conductive principle2-electrode measuring cells4-electrode measuring cells!Inductive principle2-electrode measuring cellsThis is the simplest design for a conductivity measuring cell.The 2-electrodemeasuring cell is perfectly adequate for general industrial measurement.This cell consists of two electrodes and a housing that fixes the two elec-trodes.A constant AC voltage is applied between the two electrodes.Thecurrent flowing through the measured solution is the measurement signal.With this type of cell,the cell constant and the nature of the electrode sur-face depend on the purpose the cell is to be used for.12Fig.4:Diagram of a 2-electrode measuring cellA small cell constant means a large measurement signal.This effect is desir-able for small conductivity values.At higher measured values it can overloadthe instrument.The optimal cell constant is thus all a question of the range.Table 3:Cell constants for various conductivity ranges withapplication examplesWith measuring cells with a constant k 0.1cm-1 the electrode surfaces aresmooth,to improve the adjustment behavior at low conductivity values.ApplicationexamplesConductivity rangeCellconstantDistillate,condensate,high-purity,fully-desalinatedwater 10S/cmk 0.1cm-1Ground,surface and drinking water 10mS/cmk 10cm-113Measuring cells with a constant k 0.1 have rough electrode surfaces to re-duce the tendency to polarize at higher conductivity values.4-electrode measuring cellsThe measuring cells contain two pairs of electrodes.One pair measures thecurrent,the other measures the voltage applied across the measured solu-tion.Fig.5:Diagram of a 4-electrode measuring cellThe advantage of 4-electrode measuring cells is their insensitivity to interfer-ing resistances due to long connecting cables,contaminants,or due to po-larization,for example.These effects lead to low readings as they reduce thevoltage that the electrodes apply to the measured solution.The second pairof electrodes determines the voltage across the measured solution.The in-strument can take account of an interfering resistance by means of an elec-tronic adjustment based on the measured current/voltage values of the twoelectrode pairs.14The conductivity meter calculates the conductivity value in accordance withthe equation quoted earlier:An additional resistance,a layer of dirt,for example,always reduces the cur-rent and voltage in the same proportion.As the instrument measures bothparameters,it can calculate the conductivity value,within its specification.4-electrode measuring cells are used mainly in precision laboratory instru-ments.1.2.5Inductive measuring cellFig.6:Diagram of an inductive measuring cell(1)PVDF or PEEK body(4)Measured solution(2)T-shaped flow-throughchannel(5)Receiver coil(3)Liquid loop subjected tomeasuring current6)Exciter coilIU-k?(1)(2)(3)(4)(5)(6)15In this cell,the electrodes are replaced by two coils.One of the coils is theexciter coil.An AC current flows in this coil,producing a magnetic field in thevicinity of the coil.The measured solution is in the core of the coil.The cur-rent flow required for the measurement is induced in the measured solutionby the magnetic field of the coil.The current flowing in the measured solution produces its own magneticfield.This magnetic field induces an AC current in the second coil(receivercoil)with the appropriate voltage.The voltage in the receiver coil is directly dependent on the current flowingin the measured medium,that is on the conductivity.Because a magnetic field can even act through a plastic or teflon pipe,nodirect contact between the coils and the measured medium is required.Theadvantages of this non-contact measurement technique are obvious.Mea-surement of aggressive media such as acids or alkalis can be made withoutany problems.High conductivities cannot cause polarization effects andconsequent low readings.2MeasurementThe optimal conductivity meter depends on the particular application.Benchinstruments are used in the laboratory,hand-held instruments on site,andtransmitters for continuous process measurement.Continuous measurement is essential for all applications where a full pictureof the salt content of the water is required.2.1Arrangement of a process installationThe term measurement system includes the full set of instruments andequipment used for conductivity measurement,consisting of:!conductivity sensor(measuring cell)!immersion or flow-through fitting!transmitter(instrument)!connecting cable162.1.1Measuring cells2-electrode and 4-electrode measuring cells consist of a housing and theelectrodes.The important considerations for a particular application are thecell constant and the nature of the electrode surfaces.Conductivity:10S/cm:2-electrode cell k 0.1/cm,smooth electrode surfaces 10S/cm to 10mS/cm:inductive,2-electrode or 4-electrode cellk 1/cm,rough electrode surfaces 10mS/cm:inductive or 4-electrode cell k =1/cm,rough electrodesFig.7:2-electrode cells with cell constant k=0.01(top)and k=0.1(bottom)2.1.2FittingsFittings are used for holding and protecting the measuring cell.Immersionfittings permit measurements not only at the surface of the liquid but alsodeep inside it.A wide range of mounting elements and accessories permitmounting on almost all containers.The immersion fittings are normally man-ufactured from polypropylene(PP)and are supplied in immersion lengths upto 2000mm.However,other materials(e.g.V4A)are also available for specialpurposes.Flow-through fittings permit measurement directly in the liquidflow lines or in the bypass of these lines.17It is essential that all fittings are situated in an easily accessible position,topermit regular servicing and inspection.It should be possible to change thesensor at any time without undue effort.When mounting the sensor,it is important to ensure that the boundary field(see Chapter 1.2.2“The cell constant”on page 10)is not interfered with.Thismeans that a minimum clearance must always be maintained between thesensor and the bounding surface(pipe wall,container side,or the like).2.1.3Transmitter/controllerThe transmitter has the job of conditioning the signal of the measuring cell.Here the signal from the measuring cell is converted to a standard signal(e.g.4 20mA current signal)and can then be relayed to a downstreamPLC or dosing unit.The indication and control of the conductivity then takesplace here.If an on-site indication is required,suitable panel-mounting in-struments must be installed.Appropriate surface-mounting enclosures orspecial instruments with cases suitable for site mounting can be supplied forlocal installation.Most of the instruments available nowadays are micropro-cessor instruments that can be individually adjusted to the particular mea-surement loop.This adjustment includes,on the one hand,the calibration ofthe cell constant,and,on the other hand,the calibration of the temperaturecoefficients(see also Chapter 2.2.2).Limit controllers are the type mainly used for conductivity control.Fig.8:JUMO dTRANS Lf01(transmitter/controller for 2-electrode measuring cells)18Fig.9:JUMO 202520(transmitter/controller for 2-electrode measuring cells)Fig.10:JUMO CTI-920(inductive conductivity measurement system)192.1.4Instrumentation:cable material/connecting cableThanks to the modern,precise measurement technology of present-day transmitters,there are now only a few points to consider whenselecting cable material,such as:!A shielded control cable type should be used for the cable material.!The cable should always be installed as a continuous run,i.e.terminalboxes,supplementary cable extensions or intermediate connectors mustbe avoided.!Do not run the connecting