Electrical conductivity meter
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An electrical conductivity meter (EC meter) measures the electrical conductivity in a solution. It is commonly used in hydroponics, aquaculture and freshwater systems to monitor the amount of nutrients, salts or impurities in the water.
The common laboratory conductivity meters employ a potentiometric method and four electrodes. Often, the electrodes are cylindrical and arranged concentrically. The electrodes are usually made of platinum metal. An alternating current is applied to the outer pair of the electrodes. The potential between the inner pair is measured. Conductivity could in principle be determined using the distance between the electrodes and their surface area using Ohm's law but generally, for accuracy, a calibration is employed using electrolytes of well-known conductivity.
Industrial conductivity probes often employ an inductive method, which has the advantage that the fluid does not wet the electrical parts of the sensor. Here, two inductively-coupled coils are used. One is the driving coil producing a magnetic field and it is supplied with accurately-known voltage. The other forms a secondary coil of a transformer. The liquid passing through a channel in the sensor forms one turn in the secondary winding of the transformer. The induced current is the output of the sensor.
Another way is to use 4 –electrode conductivity sensors that are made from corrosion resistant materials. Benefit of 4 –Electrode conductivity sensor compared to inductive sensor is scaling compensation and ability to measure low (below 100 µS/cm) conductivities, a feature especially important when measuring near 100% Hydrofluoric acid.
The conductivity of a solution is highly temperature dependent, therefore it is important to either use a temperature compensated instrument, or calibrate the instrument at the same temperature as the solution being measured. Unlike metals, the conductivity of common electrolytes typically increases with increasing temperature.
Over a limited temperature range, the way temperature affects the conductivity of a solution can be modeled linearly using the following formula:
- T is the temperature of the sample,
- Tcal is the calibration temperature,
- σT is the electrical conductivity at the temperature T,
- σTcal is the electrical conductivity at the calibration temperature Tcal,
- α is the temperature compensation slope of the solution.
The temperature compensation slope for most naturally occurring waters is about 2%/°C, however it can range between 1 and 3%/°C. The compensation slope for some common water solutions are listed in the table below.
|Aqueous solution at 25 °C||Concentration (mass percentage)||α (%/°C)|
Conductivity measurement applications
Conductivity measurement is a versatile tool in process control. The measurement is simple, fast and most advanced sensors require only a little maintenance. The measured conductivity reading can be used to make various assumptions on what is happening in the process. In some cases it is possible to develop a model to calculate concentration of the liquid. Concentration of pure liquids can be calculated when the conductivity and temperature is measured. The preset curves for various acids and bases are commercially available. For example it is possible to measure the concentration of high purity hydrofluoric acid using conductivity based concentration measurement [Zhejiang Quhua Fluorchemical, China Valmet Concentration 3300]. Benefit of conductivity and temperature based concentration measurement is the superior speed of inline measurement compared to on-line analyzer.
Conductivity based concentration measurement has limitations. The concentration-conductivity dependence of most acids and bases is not linear. Conductivity based measurement can’t determine on which side of the peak the measurement is and therefore the measurement is only possible on linear section of the curve. Kraft pulp mills use conductivity based concentration measurement to control alkali additions to various stages of the cook. Conductivity measurement won’t determine the specific amount of alkali components but it is a good indication on the amount of effective alkali (NaOH + ½ Na2S as NaOH or Na2O) or active alkali (NaOH + Na2S as NaOH or Na2O) in the cooking liquor. The composition of the liquor vary in different stages of the cook. Therefore it is necessary to develop a specific curve to each measurement point or use commercially available products.
The high pressure and temperature of cooking process combined with high concentration of alkali components puts a heavy strain on conductivity sensors that are installed in process. The scaling on the electrodes need to be taken into account, otherwise conductivity measurement drifts causing increased calibration and maintenance need.
|Wikimedia Commons has media related to Electrical conductivity meters.|
- IEEE paper telling how a four electrode meter works
- A webpage by Horiba telling about temperature compensation
- A book which includes a discussion of temperature compensation
- Milli-Q System High Precision Coaxial Resistivity Cell
- ASTM D1125 - 95(2009) Standard Test Methods for Electrical Conductivity and Resistivity of Water
- ASTM D5682
- DIN 55667
- Maintenance free in-line concentration analyzer for corrosive liquids
- In-line cooking liquor meter for residual alkali and white liquor measurements