The basic methods in the conductivity measurement of the liquids
In modern industry, the conductivity measurement in the liquids can be extremely crucial due to the different reasons, such as analyzing chemical concentration, assessing possible hazards for marine life, among others.To provide a better explanation, let’s dive into the intricacies of chemical concentration and explore them in more detail. Water in a pure state is not a conductor, and as external contaminants are added, its conductivity is increased. In water filtration and wastewater treatment plants, these elevated levels can pose significant risks for industrial equipment or human life in case of drinkable water processes. As its importance is explained in detail, further methods and principles will be elaborated upon in the following paragraphs.
Conductivity in a liquid occurs due to the directed flow of ions under an applied voltage, and in water, a conductivity increase happens because of the free ions of an added substance which also occurs due to the dissolution. Generally, in scientific contexts, these substances are also called “electrolytes”. Ions can be formed in different ways, and one of them is whenever they give or gain new electrons to or from other atoms. For example, when NaCl (ionic compound bond) is dissolved in water (the reason why water is used for this process is based on its polar molecule characteristics), it is separated to two different ions which are also known as cations (Na+ ,more protons than electrons) and anions (Cl-,more electrons than protons).
The reason for this process is that during the ionic bond formation between Na and Cl atoms, a valence electron of Na atom joins the Cl atom (since Cl has more electronegativity than Na) which causes one electron loss in Na and a gain of that electron in Cl atom.
There are generally two methods to measure conductivity in liquids. The first one is the two probes measurement method. This method involves two separate conductive charged plates, which will act as the positive and negative plates for the external voltage source. The main theoretical formula,which will be commonly used will be derived and written below for clarity:
As we have finally got our main formula, it will be one of the main theoretical approaches to derive the conductivity for the certain applications. Now we can talk about the previously mentioned method. In the simplest terms, we can use a simple current source and voltmeter to determine resistance by measuring the voltage and current. In this case, there will be 2 plates (plates are added for explanatory reasons) connected to the conductors, and they will be separated by the gap in the liquid. It is obvious that the cross sectional area and distance between plates is just a matter of choice, therefore it would be better to be more invested in finding the specific resistivity or conductivity of the liquid.
The picture above perfectly demonstrates our intention. Sometimes manufacturers express the plate geometry as the name of a cell constant, which is the division of the length by the area of the plates.
The main disadvantage of this method is a phenomenon called electroplating. During the current flow, some ions can be attracted to the metal electrodes of the conductors, which can form solid layers on them. Obviously, this can cause significant issues in the conductivity measurement of liquid since these new layers can affect the conductivity on the electrodes. To solve this issue, AC can be used instead of DC to prevent this as much as possible. However, this solution just delays this issue, eventually, it will be inevitable.
Alternatively, this issue can be monitored by adding another voltmeter between the electrodes to observe the difference between the voltage dropped in the liquid and electrodes. If any difference occurs, this will notify us of the possible formation of electroplating in the process.
The second method involves the use of a special electromagnet known as the toroidal inductor to make the electrodeless measurement possible. Before explaining this method, it would be better to analyze the special features of toroidal inductor. A simple toroidal inductor consists of an electromagnetic core and electrical conductors winded over the total closed path of the core. The magnetic field created by the current flow in the conductors will flow through a closed loop path in the core. This will effectively minimize the external leakage of the magnetic field in the system. In this case, Faradey’s law and Ampere’s circuit law can be also used to find specific conductivity. However, they will not be explained in this article (maybe in future articles).
For conductivity measurement, two toroids will be used. One will be energized, which will have its own separate power supply and other will be employed as the measurement component. If the liquid passing through that toroid is conductive, this will effectively create the induced currents on it (they are proportional to the conductivity of the liquid) and the magnetic field of these ions will also create induced currents in the secondary coil (Faraday’s law). These currents can also be considered a type of eddy currents in this application. Obviously, it is applicable for the alternating voltage, and the voltmeter will be connected to the secondary inductor to measure the induced voltage. As near-field electromagnetic induction is used to generate and sense currents for the measurement, any unwanted induced voltage or current due to mutual inductance is prevented as much as possible. In engineering terms, these coils can be also known as the drive and receive coils.
This method has a great advantage of the precaution against corrosive liquids and electroplating. However, they are mostly limited to the relatively high conductivity measurement.
Finally, for more clear visual understanding and credits for main ideas of this small article, these three sources are strongly recommended:
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