An electrical conductivity meter ( EC meter ) measures the electrical conductivity in a solution . It has many applications in research and engineering, with common use in hydroponics , aquaculture , aquaponics and freshwater systems to monitor the amount of nutrients, salts or impurities in water .
Theory
Common laboratory conductivity meters use a potentiometric method and four electrodes. Often, the electrodes are cylindrical and arranged concentrically [ citation needed ] . Electrodes are usually made of platinum metal. An alternating current is applied to the outer pair of electrodes. The potential between the inner pair is measured. The conductivity can in principle be determined using Ohm’s law using the distance between the electrodes and their surface area, but generally, for accuracy, a calibration using electrolytes of well-known conductivity is employed .
Industrial conductivity probes often use 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. There is a driving coil producing a magnetic field and supplied with a precisely known voltage. The second forms the secondary coil of the transformer. The liquid passing through a channel in the sensor creates a turn in the secondary winding of the transformer. The induced current is the output of the sensor.
Another way is to use four-electrode conductivity sensors which are made of corrosion-resistant materials. An advantage of four-electrode conductivity sensors compared to inductive sensors is scaling compensation [ clarification needed ] and the ability to measure low (below 100 µS/cm) conductivity (a feature particularly important when using approximately 100% hydrofluoric acid). to measure).
temperature dependence
The conductivity of a solution is highly temperature dependent, so it is important that either a temperature compensation instrument be used, or that the instrument be calibrated at the same temperature at which the solution is being measured. Unlike metals, the conductivity of ordinary electrolytes usually increases with increasing temperature.
In a limited temperature range, the way temperature affects the conductivity of a solution can be modeled linearly using the following formula :
\sigma _{T}={\sigma _{{T_{{cal}}}}[1+\alpha (T-T_{{cal}})]}
Where from
T is the temperature of the sample,
T cal is the calibration temperature,
T is the electrical conductivity at temperature T ,
T cal is the electrical conductivity at the calibration temperature Tcal ,
α is the temperature compensation gradient of the solution.
The temperature compensation gradient is about 2%/C° for most natural water samples; Although it can be between 1 and 3%/C°. The compensation gradients for some common water solutions are listed in the table below.
| aqueous solution at 25 ° | Concentration ( mass percentage ) | α (% / C °) |
| HCl | 10 | 1.56 |
| KCL | 10 | 1.88 |
| H 2 SO 4 | 50 | 1.93 |
| Sodium chloride | 10 | 2.14 |
| HF | 1.5 | 7.20 |
| HNO 3 | 31 | 31 |
conductivity measurement applications
Conductivity measurement is a versatile tool in process control. Measurement is simple and fast, and most advanced sensors require only a little maintenance. The measured conductivity readings can be used to make various inferences on what is happening in the process. In some cases it is possible to develop a model for calculating the concentration of a liquid.
The concentration of pure liquids can be calculated by measuring conductivity and temperature. Preset curves for various acids and bases are commercially available. For example, one can measure the concentration of high-purity hydrofluoric acid using a conductivity-based concentration measurement [Zhejiang Kuhua Fluorchemical, China Valmate Concentration 3300]. An advantage of conductivity- and temperature-based concentration measurement is the improved speed of inline measurement compared to an on-line analyzer.
There are limitations to conductivity based concentration measurements. The concentration-conductivity dependence of most acids and bases is not linear. Conductivity-based measurement cannot determine which peak edge the measurement is on, and therefore measurement is only possible on a linear segment of the curve. [ citation needed ] Kraft pulp mills use conductivity-based concentration measurements to control for alkali additions at various stages of the cook. The conductivity measurement will not determine the specific amount of the alkali components, but it will depend on the amount of the effective alkali (NaOH + 1 2 Na 2 S as NaOH or Na 2 O) or the active base (NaOH + Na 2 S ). is a good sign. NaOH or Na 2O) In the cooking wine. The composition of the wine varies between the different stages of the cook. Therefore, it is necessary to develop a specific curve for each measurement point or use commercially available products.
The high pressure and temperature of the cooking process, together with the high concentration of alkali components, exerts enormous pressure on the conductivity sensor installed in the process. Scaling at the electrodes must be taken into account, otherwise the conductivity measurement degrades, necessitating increased calibration and maintenance.
Working principle of conductivity meter
The method of measuring the behavior of the solution to be tested is called the conduct analysis method. Conductance is the reciprocal of resistance, so the measurement of the conducting value is actually converted again by the measurement of the resistance value, that is, the measurement method of conduction should be the same as the measurement method of resistance. However, during the measurement of solution conductance, when current passes through the electrode, ions will discharge at the electrode, causing polarization errors. Therefore, when measuring conductance, use an alternating current with a frequency high enough to prevent the production of electrolytic products. In addition, the electrodes used are plated with platinum black to reduce the overpotential and improve the accuracy of the measurement results.
