Probe Types and Polarization Errors Amperometric

The probe used to measure conductivity was originally an amperometric system which had two electrodes spaced one centimetre* apart from each other. [* Probes with different electrode spacing allow measurement of various conductivities.]

The amperometric method applies a known potential (voltage, V) to a pair of electrodes and measures the current (I). According to Ohm's law: I=V/R where R is the resistance. The higher the current so obtained, the greater the conductivity. The resistance in this method unfortunately is not constant even though the distance may be fixed. Salt deposition on the electrodes due to electrolysis can vary the resistance. For low to medium levels of conductivity (< 2 mS/cm) this may be sufficient, but for greater accuracy and for higher levels, a different method is required.

3.6.13 Potentiometric

A potentiometric method is based on induction and eliminates the effects of polarization common to the amperometric method. The potentiometric method employs four rings: the outer two rings apply an alternating voltage and induce a current loop in the solution while the inner rings measure the voltage drop induced by the current loop. This measurement is directly dependent upon the conductivity of the solution. A shield around the rings maintains a constant field by fixing the volume of solution around the rings.

Because a potentiometric (4-ring) conductivity sensor is not limited by electrolysis which commonly affects amperometric probes, it can be used to measure a much wider range of conductivities. Practically, stainless steel rings can be used. But, the conductivity of deionized water with one probe.

Inductive or Toroidal

Another method of conductivity measurement uses an inductive probe (sometimes referred to as a toroidal sensor). Typically these are found in industrial process control systems. The sensor looks like a donut (toroid) on a stick. The advantage of this technology is measurement without any electrical contact between the electrode and the process fluid. The probe uses two toroidal transformers which are inductively coupled side by side and encased in a plastic sheath. The controller supplies a high frequency reference voltage to the first toroid or drive coil which generates a strong magnetic field. As the liquid containing conductive ions passes thru the hole of the sensor, it acts as a one turn secondary winding. The passage of this fluid then induces a current proportional to the voltage induced by the magnetic field. The conductance of the one turn winding is measured according to Ohm's law. The conductance is proportional to the specific conductivity of the fluid and a constant factor determined by the geometry and installation of the sensor. The second toroid or receiving coil also is affected by the passage of the fluid in a similar fashion. The liquid passing thru the second toroid also acts as a liquid turn or primary winding in the second toroidal transformer. The current generated by the fluid creates a magnetic field in the second toroid. The induced current from the receiving coil is measured as an output to the instrument. The controller converts the signal from the sensor to specific conductivity of the process liquid. As long as the sensor has a clearance of at least 3 cm the proximity of pipe or container walls will have a negligible effect on the induced current.

Calibration

Most conductivity meters can be calibrated using a standard of a known value. Often a value of 1413 µS/cm is used. Some meters will allow the user to select from a wide range of pre-selected values. Calibration should be performed using a standard which is as close to the solution being measured as possible. More advanced meters will allow calibration at two, three, four or even five points. This results in good accuracy over a wider range of measured values. Some meters will even recognize the value a standard when the probe is immersed during calibration similar to auto buffer recognition in pH meters. This simply is another way a making a conductivity meter easier to use. Temperature is so important in conductivity measurement, it should also be calibrated at least one and preferably two different points.

US Pharmacopoeia and European Pharmacopoeia Standards

USP <645> with Stage 1, 2 and 3 compliance is required for purified water and WFI (water for injection). Only a few resistivity/conductivity meters conform to these requirements. Some of these requirements are:

Resolution of 0.1µS/cm or greater Accuracy at 1.3µS/cm of 0.1µS/cm

Must be able to read with or without automatic temperature compensation Verifiable cell constant +/- 2%

The advanced HI 98188 will easily meet or exceed these criteria.

Portable and Bench Meters

Instruments which measure conductivity are available as portable field instruments which are hopefully waterproof since they are to be used in wet environments. Depending upon the model, the meters can:

Used in various ranges of conductivities Incorporate a temperature sensor in the probe

Auto-range to automatically express the results in the proper units

Allow data logging of measurements with computer output port and data capture software

Rechargeable batteries

Offer automatic calibration from the keypad

Include a potentiometric (4-ring) conductivity sensor

Laboratory bench meters normally will have all of the features available in the portable meters. Additionally, they often can express measurements in micro Siemens, milli Siemens, mega Ohms, TDS: ppm or ppt, and salinity in PS, % or ppt. Look for features such as automatic time interval logging and log on demand plus automatic standard recognition during calibration. GLP (good laboratory practice) features allow the user to store and retrieve data regarding the status of the system. And for those on a tight budget, some conductivity bench meters will even include a pH meter with two separate electrodes. These combination meters share the same display.

Process Conductivity/TDS Controllers

For continuous measurement systems, a controller is used.

typically panel mounted and offer a host of excellent features including but not limited to: auto-ranging, control output relay(s), analog recorder output, in

diagnostic features and even a computer digital output with SMS (Short Messaging Service) or modem capabilities. Process controllers can also be divided into three general types depending upon the type of probe they employ.

applications where cost is a consideration.

temperature compensated probe similar to those used with bench or portable meters.

are good work horses for most applications.

inductive probe. This probe has many advantages in an industrial setting.

problems like contamination or polarization factors are eliminated because the sensor is has no electrodes in contact with the process f

select a controller and a compatible sensor suitable to the type of fluid and physical environment of use.

The Principle of Conductivity measurement When the electrical resistivity of a co

conductivity can be obtained by Two electrodes measurement. On the other hand, Four electrodes measurement with guard electrodes is applied when that of conductive polymer sheet is low (R<10KΩ). This method is ef

or its contact point with the sample. Actual layout of four electrodes measurement is shown in Figure1. Constant current [

sample via the two electrodes for current [ the other two electrodes for potential

difference detector with high input impedance. In this measurement, the [UP1P2] = [P1P2], potential difference [

ranging, control output relay(s), analog recorder output, in-line probe c

diagnostic features and even a computer digital output with SMS (Short Messaging Service) Process controllers can also be divided into three general types depending upon the type of probe they employ. The first uses an amperometric probe for applications where cost is a consideration. The second uses a standard potentiometric type temperature compensated probe similar to those used with bench or portable meters.

are good work horses for most applications. However, the third type of controller uses an This probe has many advantages in an industrial setting.

problems like contamination or polarization factors are eliminated because the sensor is has no electrodes in contact with the process fluid. Depending upon the application, users should select a controller and a compatible sensor suitable to the type of fluid and physical

The Principle of Conductivity measurement

When the electrical resistivity of a conductive polymer sheet is high(R>10K

conductivity can be obtained by Two electrodes measurement. On the other hand, Four electrodes measurement with guard electrodes is applied when that of conductive polymer ). This method is effective to eliminate fault resistivity of the lead wire or its contact point with the sample. Actual layout of four electrodes measurement is shown in Figure1. Constant current [I] from the constant-current power supply [S

two electrodes for current [C1], [C2]. Its potential difference [ potential-difference [P1], [P2] is measured by POT difference detector with high input impedance. In this measurement, the impedance [

], potential difference [UP1P2] and constant current [I] are linked by the

,P₂: electrodes for potential difference

These instruments are typically panel mounted and offer a host of excellent features including but not limited line probe cleaning, diagnostic features and even a computer digital output with SMS (Short Messaging Service) Process controllers can also be divided into three general types erometric probe for The second uses a standard potentiometric type temperature compensated probe similar to those used with bench or portable meters. These the third type of controller uses an This probe has many advantages in an industrial setting. Common problems like contamination or polarization factors are eliminated because the sensor is has Depending upon the application, users should select a controller and a compatible sensor suitable to the type of fluid and physical

nductive polymer sheet is high(R>10KΩ), its conductivity can be obtained by Two electrodes measurement. On the other hand, Four electrodes measurement with guard electrodes is applied when that of conductive polymer fective to eliminate fault resistivity of the lead wire or its contact point with the sample. Actual layout of four electrodes measurement is shown S] runs through the ]. Its potential difference [UP1P2] between ] is measured by POT - the potential impedance [Z_P1P2] of ] are linked by the . Therefore, the conductivity can be calculated from the formula

Fig. 3.6.8 Conductivity Measurement