Electrolytic conductivity (EC) is a measurement made in which electrical charges on particles in a medium are moved under the influence of a potential difference. EC is a measure of concentration, not for a specific ion type. An ion is a charged particle present in the solution that contributes to the current flow. Ions are formed when a salt such as sodium chloride is dissolved in water to form electrically charged particles. A simplified example is sodium chloride that separates into Na+ and Cl-. However, the measurement is affected by many things such as the type of ionic compound(s) dissolved in the water, the ions mobility, the solution viscosity, temperature and concentration.
Electrical Conductivity uses units of Siemens/cm (S/cm, mS/cm, μS/cm, dS/m). For example, ultrapure water is said to have a conductivity of 0.055μS/cm at 25°C.
Conductivity versus Resistivity
Although conductivity and resistivity are reciprocal units that may be converted easily, the practice is to use resistivity readings for very low electrolyte concentrations or trace contaminants (i.e. ultrapure water) and conductivity for expressing meaningful salt levels (i.e. seawater, water treatment, electroplating baths, acid concentrations). Electrode style and measurement techniques also contribute to making conductivity or resistivity measurements reliably.
Conductivity versus Total Dissolved Solids
Total Dissolved Solids (TDS), is a method used to determine solid content in a solution. To determine TDS, the solution whose volume is known is evaporated and the residue weighed. A conductivity measurement is commonly used to estimate TDS based on the assumption that solids are predominately ionic in nature and the relationship between the dissolved ions and conductivity is known. TDS uses units of mg/L (ppm), or g/L. On some meters the user can input the TDS factor for the conversion. On more basic units the factor is automatically set to 0.50. A typical TDS factor for strong ionic solutions is 0.5, while for weak ionic solutions (e.g. fertilizers) is 0.7.
TDS = EC25 x factor
For example: 100μS/cm conductivity is a TDS of 50ppm when the factor is 0.5.
Conductivity measurements can be used for determining salinity as it relates to general oceanographic use. Three measurement scales are used, and depending on the sophistication of the meter, are available for salinity measurement in Seawater. The three scales are Practical Salinity Scale (PSU), 1978, Percent Scale (%), and Natural Seawater Scale (g/L) 1966.
Hanna Instruments manufactures three types of conductivity probes. The simplest design is a 2-Electrode Probe that uses an amperometric approach to make the measurement; a known AC voltage is applied at a specific frequency between a pair of electrodes in solution.The current produced is measured and reported in conductivity units referenced to a calibrated standard. Electrodes are made of graphite or metal. Fouling due to mineral deposits and polarization at high concentrations are drawbacks of this technology. Two electrode probes are best used in clean water applications when conductivities remain less than 5 mS/cm.
Four electrode conductivity (four-ring conductivity) uses a potentiometric approach to make the measurement; an alternating current is applied to the outer two “drive” electrodes to induce a current in the solution. The voltage is measured between the inner pair of electrodes in solution. The voltage is proportional to the conductivity. This technology extends the linear range of measurement over three decades. Electrodes are made of graphite, stainless steel or Platinum. Polarization effects are reduced.
The third type of conductivity probe manufactured by Hanna is often found in industrial processes connected to a controller. An Inductive, Electrodeless or Toroidal conductivity probe uses two or more toroidal transformers which are inductively coupled side by side and encased in an inert plastic sheath. By applying a high frequency voltage to the drive toroid, a magnetic field develops that induces a current in the surrounding solution. A receiver toroid on the other side of the sensor measures the strength of the induced current. The strength depends on the conductivity of the solution. The benefits of this technology are no polarization effects, choice of material encapsulation can produce chemical resistant and relative immunity to fouling, and solutions are not needed for calibration.
Below are the most common products used for measuring conductivity.
Benchtop meters are available with probes that use the four-ring technology. The benchtop meters include those with manual calibration and manual selection of ranges and versions with automatic calibration and automatic range selection. Meters are also available to perform the USP <645> method including a multiparameter pH/EC version that is useful for all three stages of verification.
Portable meters are available with probes that use either the two pin (amperometric) or four-ring (potentiometric) technology. The portable meters include those with manual calibration and manual selection of ranges and versions with automatic calibration and automatic range selection. Meters are available as single parameter or multiparameter versions. Single parameter portables with the USP <645> method pre-programmed are also available.
Portable Meters with Potentiometric Probe
The following portable meters use the potentiometric (four ring) version of probes. This technology allows for a wide range of measurement using a single probe. Portable meters available include versions with manual calibration and manual selection of ranges and versions with automatic calibration and automatic range selection. Portables are available with multiple measuring modes including EC, TDS, salinity, and resistivity.
Portable Meters with Amperometric Probe
The following portable meters use an amperometric (two-electrode) version of probes. With this technology the ranges are limited to either a low or high range version.
Multiparameter Portable Meters
Multiparameter portable meters are available with either four-pole or two-pole technology. The multiparameter meter includes application specific meters for agriculture, hydroponics and water treatment.
A variety of testers are available that use an amperometric two-electrode design. Versions of these meters include those for the measurement of high purity water and for water purified by reverse osmosis. Testers are available to display results in either conductivity or total dissolved solids. Some testers offer the option to switch modes. Many of the testers use graphite sensors for the electrode to reduce polarization effects and have an exposed temperature probe for fast response when compensating for temperature variations.
Application Specific Testers
Application specific testers include versions for direct soil conductivity measurements and for salinity readings in aquariums.
Multiparameter testers can measure pH, EC, TDS, and temperature. The multiparameter testers have a replaceable pH electrode and EC/TDS electrodes made of graphite to reduce polarization effects. These tester have an exposed temperature probe for fast response when compensating for temperature variations that dramatically affect EC and TDS readings.
Conductivity Probes and Sensors
The following are replacement probes and sensors for benchtops, portable, and testers that can measure conductivity and total dissolved solids.
Monitors and Controllers
Monitors and controllers are available for the continuous monitoring of solutions. Monitors display results in real time while controllers offer the ability to use a relay to activate a solenoid valve, dosing pump, or other device to make adjustments to a process. This is useful for many applications including maintaining a fertilizer concentration of irrigation water in a greenhouse, the opening of a valve to reduce conductivity in rinse water or a blow down with a boiler. Some controllers offer analog and digital outputs for connection to a remote device such as SCADA, PLC or other compatible devices.
Controllers and Transmitters
Controllers are available in wall or panel mounted versions. Types of control for the controllers include simple on/off and PID (Proportional, Integral, and Derivative functions). Controllers offer analog and digital outputs. There are also transmitter only versions that have 4-20 mA analog output.
Mini controller are inexpensive options for monitoring and control. These controllers offer simple on/off control and can be ordered with relay activation either above or below the set-point. The mini controllers use an amperometric two electrode design and have ranges for monitoring very low EC/TDS to solutions with EC up to 20 mS/cm.
Monitors are continuous reading devices used to monitor a process. They are available as single parameter and multi parameter models. The monitors use an amperometric two electrode design and have ranges for monitoring very low EC/TDS to solutions with EC up to 20 mS/cm
The following category is for the probes used with the monitors and controllers. These probes include both amperometric two electrode design along with those that use the potentiometric four electrode design. The potentiometric design includes models for submersion and in-line mounting. Some models are made for high temperature and pressure.
Hanna conductivity, TDS, and salinity calibration solutions are prepared against a NIST traceable potassium chloride solutions. Hanna solutions have the lot number and expiration date clearly marked on the label, and are air tight with a tamper-proof seal to ensure the quality of the solution. Hanna’s line of calibration solutions have been specially formulated to have an expiration of 5 years from the date of manufacture for an unopened bottle. Solutions are available in a variety of sizes of bottles and sachets covering a wide range. Solutions can be purchased with or without a Certificate of Analysis (COA) documenting traceability.
Conductivity Solutions in Bottles
Conductivity Solutions in Bottles with COA
Conductivity Solutions in Single Use Sachets
TDS Solutions in Bottles
TDS Solutions in Sachets
Specialty solutions include a salinity standard used with the calibration of the marine tester and quick-cal solution used with multiparameter testers and portables that are programmed with the quick calibration mode. The quick-cal solutions is both a pH buffer and EC standard.
Accessories include shockproof rubber boots used with portable meters to offer additional protection against accidental drops of the meter. The shockproof rubber boots offer increase the tactile feel of the meter giving it a non-slip grip. Other accessories included replacement items supplied with some of the multiparameter meters.