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A Practical Guide to Conductivity Measurement
by Mark Heyda
Units of Measurement
Electrical Conductivity is the ability of a solution to transfer (conduct) electric current. It is the reciprocal of electrical resistivity (ohms). Therefore conductivity is used to measure the concentration of dissolved solids which have been ionized in a polar solution such as water. The unit of measurement commonly used is one millionth of a Siemen per centimeter (micro-Siemens per centimeter or µS/cm). When measuring more concentrated solutions, the units are expressed as milli-Siemens/cm (mS/cm) i.e.- 10-3 S-cm (thousandths of a Siemen). For ease of expression, 1000 µS/cm are equal to 1 mS/cm. Often times conductivity is simply expressed as either micro or milli Siemens. However this unit of measurement is sometimes (incorrectly) referred to as micro-mho's rather than micro-Siemens. The expression "mho" was simply the word ohm spelled backwards.
Several means of conductivity expression have been adopted by various industries as a way of making the units of expression into whole numbers. The water softening industry refers to "grains" of hardness and uses TDS or total dissolved solids as a measurement scale. While TDS is really a gravimetric measurement, because in solution the solids are predominately present in ionic form, they can be approximated with conductivity. The TDS scale uses 2 µS/cm = 1 ppm (part per million as CaCO3). It is also expressed as 1 mg/l TDS. While the method of measurement is the same, some conductivity meters can make the conversion and express the results of a measurement in many different units. This is helpful for users who are accustomed to one particular unit of measurement.
Resistivity versus Conductivity
When the ionic concentration is very low (such as in high purity water), the measured conductivity falls below a value of one micro Siemens per centimeter. In order to express these numbers as whole numbers as opposed to fractions, the resistivity scale is often used. The numbers are exactly the inverse of each other. For example: the reciprocal of 0.10 µS/cm [or 1/(0.10 x 10-6 S/cm)] is then 10 x 106 ohms-cm (10 MΩ-cm). This is also commonly referred to as "mega-ohms". Either unit of measurement can be used to state exactly the same value. Commonly the conductivity scale is more versatile as it can be used for a broader range of measurements.
Because air is soluble in ultra high purity water (18.3 MΩ-cm), the reading will not be stable in an open container.
Temperature plays a role in conductivity. Because ionic activity increases with increasing temperature, conductivity measurements are referenced to 25ºC. The coefficient used to correct for changes in temperature, β is expressed as a percentage per degree Celsius. For most applications, beta has a value of two. In order to establish the true value of beta a solution is measured at the elevated temperature (without temperature compensation). Then the solution is cooled and re-measured. β can then be exactly calculated for that particular solution. Advanced meters allow for custom reference temperatures.
Probe Types and Polarization Errors
The probe used to measure conductivity was originally an amperometric system which had two electrodes spaced one centimeter* 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.
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 preferred metal is platinum because it can withstand higher temperatures and produces a more stable reading. Platinum sensors are also easier to clean. Advanced microprocessor conductivity instruments can vary the voltage applied to the sensor which enables them to extend the range of a potentiometric probe even further. This technique allows advanced meters to be able to measure both high and low conductivities as well as the ultra low 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.
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:
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:
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. These instruments are 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-line probe cleaning, 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. 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. These are good work horses for most applications. However, the third type of controller uses an inductive probe. This probe has many advantages in an industrial setting. Common 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 environment of use.
The above article may not be copied or reproduced without consent of the author. All rights reserved. Mark Heyda 2006
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