Conductivity, resistivity, salinity, and TDS meters are a class of instruments that measure the specific conductance of ions dissolved in solution as a means to analyze water quality. Though each instrument measures a different water quality parameter, all share a strong correlation which allows them to all use conductivity as the measuring principle with resistivity, salinity and TDS values are calculated from conductivity readings.
The specific conductance of a water sample measures its ability to carry an electrical current. This ability is directly related to the concentration of ions in the water. Water without impurities, conducts electricity very poorly. Conductive ions come from dissolved salts and inorganic materials such as alkalis, chlorides, sulfides and carbonate compounds. The more of these ions present in a water sample, the higher the conductivity of the sample. Because of this, we can estimate the degree of impurity based upon the specific conductance of a sample.
Conductivity, resistivity, salinity, and TDS measurements provide a fast, inexpensive and reliable way of measuring water quality and are routinely used in many industrial, agricultural and environmental applications. Since most bodies of water maintain a fairly constant specific conductance that can be used as a baseline for comparison, conductivity (and resistivity, salinity, and TDS) tests can quickly identify changes which indicate deteriorating water quality.
Principle of Measurement
Conductivity, resistivity, salinity, and TDS meters all measure the specific conductance of a water sample in order to determine or calculate a value. The process by which those instruments measure specific conductance consists of measuring the AC resistance of the solution between two electrodes. Conductivity, the inverse of resistivity is determined from the voltage and current values according to Ohm’s law.
Since we know that conductance is dependent upon ions dissolved in the water, the conductivity of a water sample is proportional to its ion concentration, though ionic interactions can alter the linear relationship between conductivity and concentration in some highly concentrated solutions.
Conductivity meters generally use a 2-cell electrode configured into either a dip or flow-through style. The electrode surface is usually platinum, titanium, gold-plated nickel, or graphite. A 4-cell electrode can be used for applications requiring higher accuracy. These electrodes use a reference voltage to compensate for any polarization or fouling of the electrode plates. The reference voltage ensures that measurements indicate actual conductivity independent of electrode condition, resulting in higher accuracy for measuring pure water.
Temperature Compensation
Conductivity measurements are temperature dependent. Increasing temperature causes a decrease in water viscosity allowing more movement of ions in solution. Molecules may also begin to dissociate resulting in an increase in the number of ions. Since increase in movement and number of ions directly results in higher conductivity, then an increase in the solution’s temperature will lead to an increase in its conductivity. The rule of thumb is to expect a 2% increase in conductivity per °C.
All conductivity, resistivity, salinity, and TDS meters have temperature compensation to correct for the effects of temperature. Some meters have adjustable compensation for higher precision across any temperature range. Other meters have fixed temperature compensation referenced to a standards temperature, usually 25°C.
Cell Constant
The geometry of a conductivity sensor, particularly the placement of the electrodes, directly affects the sensitivity and accuracy of measurement. The cell constant (K) describes the precise geometry of the sensor cell. The cell constant is the ratio of the length between electrodes divided by the cross-sectional area of sample between them.
When measuring solutions with lower conductivity, the electrodes can be placed closer together or made smaller resulting in a cell constant of less than one which will yield a better signal. Higher conductivity solutions require higher cell constants to produce a value more easily interpreted by the meter.
Cell constants increase the efficiency of the sensor. It’s a factor that the meter uses to make the standard value agree with the measured value. Conductivity sensors each have a nominal values such as k=1.0. This value may vary somewhat as oxidization, scratches, coating, bending, etc. affect the sensor. Only with calibration is the true value of the cell constant known.
It’s important to choose a cell constant based on the anticipated measuring range:
| Cell Constant |
Optimal Range |
| 0.01 |
less than 1 µS |
| 0.1 |
0.5 to 200 µS |
| 1.0 |
10 to 2,000 µS |
| 10 |
1 to 200 mS |
Calibration
Conductivity meters and cells should be calibrated to a standard solution before using. When selecting a standard, choose one that has the approximate conductivity of the solution to be measured.
Conductivity
Conductivity measures the ability of a test sample to carry an electrical current. Since that electrical current depends upon dissolved ions to carry it, conductivity can be seen as a measure of dissolved ions present in a test sample. Those ions are impurities that come from dissolved salts and inorganic materials such as alkalis, chlorides, sulfides and carbonate compounds. The more of these ions present in a water sample, the higher the conductivity of the sample. Those dissolved ions are also a major source of pollution making conductivity tests one of the basic water quality measurements
Conductivity is the reciprocal of resistivity. In practice, conductivity is used when resistance values are very low, such as when referring to water ranging from drinking water to sea water, and resistance is used when conductivity values are very low, such as when referring to deionized or reverse-osmosis water.
The relationship between resistance and conductivity is illustrated by the unit of measurement for conductivity, the mho (ohm spelled backwards). Siemens per unit area is a more common measurement for conductivity and is equal to the mho. As conductivity ranges in aqueous solutions are usually small, the measurements are usually made in milliSiemens/cm (mS/cm) and microSiemens/cm (μS/cm).
Resistivity
Resistivity measures water’s opposition to the flow of an electrical current over distance which is directly related to the amount of impurities; usually in the form of dissolved salts, alkalis, chlorides, sulfides and carbonate compounds; in the sample. Water with a high concentration of such impurities will have a low resistivity and vice versa.
Resistivity, measured in ohms per unit area, is the reciprocal of conductivity so if a water sample has a low resistivity it will have high conductivity. In practice, conductivity is used when resistance values are very low, such as when referring to water ranging from drinking water to sea water, and resistance is used when conductivity values are very low, such as when referring to deionized or reverse-osmosis water.
Resistivity is a vital measurement when ultra-pure water is required, such as for use in a growing number of laboratory and industrial processes. In the field, resistivity measurements are used as part of a water quality testing regime. Resistivity measurements can be used to check for contamination from agricultural runoff, landfill leachate, and road salt.
Salinity
Salinity refers to the total concentration of all dissolved salts in water. Since salts form ionic particles when dissolved, salinity is a strong component of conductivity. Though salinity can be measured via a complete chemical analysis, this method is difficult and time consuming. More often, salinity is estimated using algorithms based upon conductivity, which is much easier to measure. Salinity values can be expressed as parts per thousand (ppt) or as practical salinity units (psu) which compares the sample to a salinity standard such as seawater.
Salinity is an important water quality measurement as it affects the basic chemistry of water as well the biological processes that occur within it. Salinity influences the types organisms that can live in a body of water. It also affects dissolved oxygen solubility.
Salinity measurements are common in industries ranging from agriculture, aquaculture, hydroponics, food, pools and spas, wastewater management, and others where it is necessary to constantly monitor the salt level.
Total Dissolved Solids (TDS)
Total dissolved solids (TDS) is the measure of all dissolved particles smaller than 2 microns in a water sample. This includes all inorganic and organic substances in molecular, ionized or micro-granular suspended form. In clean water samples, TDS is approximately equal to salinity while in polluted waters , TDS includes organic solutes as well.
The primary sources of dissolved solids are agricultural and residential runoff, leaching of soil contamination, industrial discharge, and discharge from sewage treatment plants. While TDS is not necessarily a “pollutant”, it is used as a general indication of water quality. High TDS levels indicate hard water which can cause scale buildup in pipes, valves, and filters, reducing performance and adding to system maintenance costs.
TDS can be determined in a couple of ways though, most commonly, it is calculated from conductivity readings as this is a simple, effective method of testing. When calculating total dissolved solids from a conductivity measurement, a TDS factor is used. The TDS factor is empirically determined by the nature of the dissolved solids and the water source. TDS measurements, therefore, measure conductivity and multiply the reading by the TDS factor. Values are usually expressed as parts per million.
Things to Consider When Selecting a Conductivity, Resistivity, Salinity, or TDS Meter:
- Which parameter(s) do you need?
- What is the measuring range?
- How much accuracy is needed?
- Is a 2-cell or 4-cell design preferred?
- Which cell constant best fits your needs?
- Where will the meter be used? Is portability a factor?
- Will the meter be exposed to harsh environmental conditions?
- Which probes and calibration accessories are needed?
If you have any questions regarding conductivity, resistivity, salinity, and TDS meters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
