Capacitance type : Indirect level measurement

 

Capacitance is the property of a circuit that stores electrons and thus opposes a change in voltage in the circuit. A capacitor is an electrical component that consists of two conductors separated by a dielectricic or insulator. The capacitance value of a capacitor is measured in Farads (F), and the value is determined by the area of the conductors (usually called plates ), the distance between the plates, and the dielectric constant of the insulator between the plates.
 
Generally, one thinks of a capacitor as having two small parallel plates separated by air or another type of dielectric. For capacitance level measuring applications, however, one plate is a probe while the other plate is comprised of the tank of the level vessel ( see figure of capacitance below ). The dielectric is the material in the vessel which will determined the capacitance value when it rises and falls. The liquid in the tank will be measured by the capacitance measurement meter. Capacitance level measurement probes are mostly used in on /off applications for alarms or switches and control functions. For such applications, the probes can be mounted horizontally, perpendicular to the level surface, so that a large plate area is effectively used.
 
Capacitance level measurement devices play an important role in the manufacturing and processing industries, but under some operating parameters they require special consideration.

 
Generally, the capacitance values of a probe should be high compared to the capacitance of the lead wires connecting it to the measuring circuit. The wire capacitance should be swamped by the
probe capacitance variations. Otherwise, instability and erratic operation may result.
 
Some advantages of capacitance level devices are the following :

·         They contain no moving parts
·         Simple and rugged design is possible
·         Design for corrosive application is possible
·         Generally, they are easy to clean
·         Sanitary design is available for the food processing industry.
·         Extreme temperature and pressure requirements are possible by careful design
·         They can be designed for explosion proof service
 
As is true with most measuring systems, certain limitations do exist for capacitance level techniques. The following are some of the most notable :

·    ·      Measurement is subject to error caused by temperature changes affecting the dielectric constant of the material to be measured

·         Coating of the probe by conductive material causes error in measurement. An example of this is the condensation of water vapor in the vapor phase of a vessel, forming water droplets on the probe
·         Usually empirical calibration techniques are used
·         In solids measurement, variations in particle size affect the dielectric constant
Article Source: Level Measurement by N. Asyiddin (wwwpiyushpanchal2007.mynetworksolutions.com

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Conductance type: Indirect level measurement

  

As the name implies, conductivity level switches operate on the principle of electrical conductivity.

Electrodes located within containers at the point of control make or break contact with the conductive material, thus completing an electrical circuit which operates load contacts for pumps and solenoid valves or performs other control or alarm functions.

The schematic of a heavy-duty switch of this type is shown in figure below. When a source of alternating current is connected to the primary coil, a magnetic flux is set up which induces a voltage in the secondary coil. Current flows in the secondary coil however, only when the rising liquid completes the circuit between the two electrodes. Completion of the secondary coil circuit and the resultant current flow sets up a bucking action in the lower bar of the transformer core. This tends to divert lines of magnetic flux to the core legs and sets up an attraction that moves the armature, closing or opening load contacts.

One pair of contacts connect the secondary circuit to ground when liquid contacts the upper electrode and acts as a holding circuit to maintain the relay in its closed position until the liquid falls below the lower electrode. This holding circuit provides control of the load circuit through the bottom contacts of the relay over any desired range in the liquid level, depending upon the distance between the upper and lower electrodes.

The flow of current through the low energy secondary circuit is very small and varies with the voltage of the secondary coil. The secondary coil is selected to operate over the resistance of the liquid being controlled.

The two-electrode system shown operates a pump to maintain a desired level in a vessel -from a simple on-off action where one electrode and the container complete a circuit via the conductive material to multi-electrode assembly where several switch actions occur at various container levels.

Conductivity switches are economical and have no moving parts in contact with the measure material. Early, heavy-duty designs were used primarily on water applications because of the high energy levels which could result in sparking to produce fires or explosions. However, manufacturers now offer solid-state designs, operating at low energy levels, which can be considered intrinsically safe.

Article Source: Level Measurement by N. Asyiddin (wwwpiyushpanchal2007.mynetworksolutions.com)

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