Level Measurement

What are the electrical methods of level measurement?

Some of the level sensing devices operate by sensing or detecting the differences in the electrical properties, which is created by the interference of the process materials. These properties are capacitance, conductivity, and resistance.

Level measurement by conductivity

The electrical characteristics of the fluids such as dielectric and conductivity are widely used to do the level measurement in a process.

In this type of level measurement, two electrodes are placed in a tank and one of the electrodes is small in length so it won’t be extended to the maximum level while the other electrode is of good length and can be extended to a maximum length. The tank will be grounded and it would act as a common ground. Mostly a stilling well is also used so that the interface is not disturbed and also can prevent the false measurement. So as we can see in the above image the electrode terminals are connected to the relays, and this would transmit the signal to the display or to a control device. If the process level came in contact with any part of the electrode that extends to the minimum level then there could form a conductive path from the grounded tank to the electrode. If the level falls below this electrode, then the path will be interrupted and the gas or vapor would be non-conductive.

This level conduction then would activate a device that would energize a control device so that the pump could be operated, then the system level would automatically be adjusted by the feed controller or other processing equipment. In case if the level rises to a point in which both the electrodes could come in contact with the conductive material then the current will flow through both the electrodes and tank. A relay would be connected to the electrode that is positioned at the maximum level, it will detect the high level and transmit a signal to an alarm or a control device so that it can correct the condition.

What are the advantages of conductive level measurement?

  • It is not expensive and its design is very simple
  • It is reliable
  • Less maintenance
  • The only moving part is the relay
  • The activation is done from a wide range of conductivities
  • It has no moving parts so it is really suitable for indicating the level for many water-based materials

What are the disadvantages of conductive level measurement?

  • The process substance must be conductive
  • We can only do the point detection measurement
  • There is a chance for sparkling so it is not suitable for flammable substances
  • There is a chance for malfunction if the conductive buildup occurs on the electrode holder

What are the applications of conductive level measurement?

In this type of level measurement, the ability of the process material to conduct the electric current is used to measure the level. So basically this method is mostly used for point measurement of liquid interfaces of relatively high conductivity. So we can check the level of water-based solutions such as brine solutions, acids, caustic solutions, and certain types of beverages.

Capacitive level measurement

The operation of this type of level measurement is based on the change of capacitance. The electric device has an ability to store energy and this is called capacitance, this type of measurement utilizes the advantage of dielectric constant. The working of a capacitive level transmitter is similar to that of a capacitor, the probe has a metal center rod which would act as a plate of the capacitor and the vessel wall would act as the second plate. If the process material that is to be measured is non-conductive then it acts as the dielectric insulator between the two plates. If the process material is conductive then the probe insulation is the dielectric material in the capacitor. If the process material comes in contact with the probe then there would be a value reading if more products come in contact with the probe then the value reading will be high. This value reading or pF reading is the measure of capacitance and it is proportional to the product, or it shows the product height.

What are the advantages of capacitive level measurement?

  • It can operate at extreme pressure and temperature conditions
  • It is very reliable and economical
  • It has less moving parts and less maintenance is needed
  • Installation is very easy, and it can be used for different types of vessel
  • It has a good measurement range

What are the disadvantages of capacitive level measurement?

  • It is intrusive
  • It won’t work well with a varying dielectric material
  • Certain process materials such as acids and caustics which won’t appear to coat the sensing element are highly conductive so error measurements could happen because of the thin film they leave

What are the applications of capacitive level measurement?

  • It is used in pharmaceutical industries to control the slurry level
  • It is used in the food and packing industries
  • It is used in chemical industries to check the level of the substances
  • It can be seen in paints and pigment industries to check the level of the fluids
  • It can be used with corrosive materials like hydrofluoric acid
  • It can be used with liquid metals at a very high temperature
  • It can be used with very high-pressure industrial processes

Resistance type level measurement

This type of level measurement utilizes the electrical relationship between the resistance and the current flow in order to measure the level. This sensor measures the level by changing its resistance when the level changes. Then the measured resistance would be converted into an electrical signal which would be proportional to the level of the liquid in the tank. The below image is of the resistance tape level sensor and this consists of a metal base strip and a wire winding. When the sensor is placed in a liquid, the pressure will cause the winding to contact the base strip, this contact would create a change in resistance of the wire. It has an internal transmitter that can convert the resistance measurement to a 4-20ma output.

What are the advantages of resistive type level measurement?

  • When compared to other electrical level sensors the resistance type level detectors only need less maintenance.

What are the applications of resistance type level detectors?

  • It can be used for liquid gas interfaces and also for slurries and solids
  • This type of level measurement can be used to measure raw sewage, sludge, and gasoline levels

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