Alvaro Realpe, P.h.DProcess control

Universidad de CartagenaFaculty of Engineering

Chemical Engineering Program

Capacitance Level

Measurement Jorlan Ochoa

Gabriel Florez

Marlon Tuñon

Jesser Pajaro

Jossue Laitano

Catherine Centanaro

• The level is a very important process variable since it is linked to the operation of equipment, inventory, etc.• Interface liquid gas or solid-gas (Also: liquid-liquid and liquid solid).• The level affects the quantity delivered, pressure and rate of flow in

and out of the container.• Substances: water, solvents, chemicals, liquids, slurries, granular

materials, and powders. • Can be inside a container or can be in its natural form (e.g. a river or a

lake).

MEASURING LEVEL

Technology to measure

level

In Indirect methods

The level is converted in a measurable signal using a suitable transducer. A corrective factor must be used in recalibrating the instrument.

• Hydrostatic head methods• Load cell• Capacitance• Conductivity

• Point-level sensors This is essential to avoid overflow or emptying of tanks and to protect pumps from dry run.

• Continuous level sensors

Capacitive Level Sensor

• It is used to determine the capacitance of a capacitor of given potential.• They apply throughout the industry (in liquid, solid)• Withstand high pressures, high temperatures and caustic materials.

Items detected:• Solid wood, ceramics, glass, stacks of paper, plastic, stone, rubber, ice, non-ferrous materials, and plant materials.• Liquids: water, oil, adhesive and paint.• Granules: plastic granulates, seeds, food, and salt.• Powder: inks, soap powder, sand, cement, fertilizer, sugar, flour and coffee.

Capacitor principle

• Electric charge can be accumulated and temporarily stored in a capacitor.

• The capacitors are not as useful for storing long-term energy but the energy stored unloaded much faster.

• The ability of a capacitor to accumulate an electric charge is called capacitance.

WORKING PRINCIPLE: Capacitive Liquid Level Sensor

• The capacitive level gauge measures the capacitance of the capacitor formed by the electrode immersed in the liquid and the tank walls.

• Set capacity depends linearly on the liquid level, as the container is filled, the capacity increases.

• The capacity is determined by its surface area, the distance between them, and the type and dielectric properties of the product being measured.

PartsC1: CapacitanceC2: Capacitance steamC3: Capacitance liquidA: Effective resistance between the wall and sensor

1. Head2. Sonda3. External Plate4. Inner plates5. Covering

Dielectric constants

• The sensors can be designed to sense material with dielectric constants as low as 1.1 (coke and fly ash) and as high as 88 (water) or more.

MeasurementMeasurement is made by applying an RF signal between the conductive probe and the vessel wall.

Level Measurement can be divided into three categories:

• Measurement of non-conductive material

• Measurement of conductive material

• Non-contact measurement

• Non-conducting material:For measuring level of non conducting liquids, bare probe arrangement is used as liquid resistance is sufficiently high to make it dielectric.

• Conducting Material:In conducting liquids, the probe plates are insulated using thin coating of glass or plastic to avoid short circuiting. The conductive material acts as the ground plate of the capacitor.

• Proximity measurements (Non-contact type measurements): In Proximity level measurement is the area of the capacitance plates is fixed, but distance between plates varies.

Permittivity

𝜺𝟎=𝟖 ,𝟖𝟓𝒙𝟏𝟎−𝟏𝟐𝑭 /𝒎

It is an electrical property the medium in which are located the two charges.

Capacitance

A simple capacitor consists of two electrode plate separated by a small thickness of an insulator such as solid, liquid, gas, or vacuum. This insulator is also called as dielectric.• Value of C depends on dielectric used, area of the plate and also distance between the plates.

• Where:• C = capacitance in picofarads (pF)• E = a constant known as the absolute permittivity of free space/ Dielectric Constant• ER= relative dielectric constant of the insulating materialA = Capacitor plaques area• d = distance between the conductors /plaquesQ= Stored Electric loadV= Potential Diference

Capacitance Sensors for Conducting Liquids• A cylindrical metal rod has a uniform dielectric coating on it (teflon,

PVC, etc.). The liquid surronds the dielectric but does not come in to contact with the metal rod.

Capacitance Sensors for Non-Conducting Liquids

Capacitive Liquid Level SensorThe inner cylindrical probe (R1) is formed by a series of rectilinear charges, each of them, with a radial electric field at a distance “r” from the rectilinear charge:

Capacitance equation for the concentrical cylindric sensor probes.

E: Radial eletric field.r: Distance between the charges.ε2: Dieletrict constant of air.λ: Linear charge density.

Capacitive Liquid Level SensorTo calculate the capacitance, it takes the sensor as two capacitances in parallel: part one is without liquid and the other that is filled of it.

𝐶𝑡𝑜𝑡𝑎𝑙=𝐶𝑣𝑎𝑐𝑖𝑜+𝐶𝑙𝑖𝑞𝑢𝑖𝑑𝑜=2𝜋 (𝐿−h) 𝜀2

ln(𝑅2𝑅1 )+2𝜋 h𝜀1

ln (𝑅 2𝑅1 )¿2𝜋 [𝜀2 (𝐿−h )+h𝜀1 ]

ln(𝑅2𝑅1 )

Wherein: Ctotal: Total capacitance of the sensor. Cvacio: Capacitance of the air-core sensor.Clíquido: Capacitance of the water-core sensor.L: Height sensor. h: Height liquid in the sensor.

h=[ ln(𝑅2𝑅1 )∗𝐶𝑡𝑜𝑡𝑎𝑙

2𝜋−𝜀2 𝐿]( 1

𝜀1−𝜀2 ¿¿)

Accuracy

• The accuracy of the transducers capacity is ± 1%.• Due to the influence of temperature, moisture content, static charge,

composition, and coating conductivity electrodes level indication, the capacitance system is not used in liquids in tanks inventoried. The capacitance meter must be calibrated in the field to zero and 100% of the signal.

Maximum pressure : (80-250) kg/cm² Maximum fluid temperature: (200 - 400) °C

Advantages Disadvantages

The sensor element is simple and no moving parts It can cause change error of the dielectric constant with temperature flow

The sensor element is easy to clean, Minimum maintenance required

Viscous liquids drivers, covering the sensor element can cause erroneous or completely false readings.

Relatively inexpensive Air bubbles or foam in the liquid or foam at the free surface produce erroneous readings

Versatile You can not measure interface level in conductive liquids.

Resistant to corrosion with the specific test tube. Light density materials under 20 lb/ft3 and materials with particle sizes exceeding ½ in. in diameter can be a problem due to their very low

dielectric constants (caused by the large amount of air space between particles)

Easy to install and can be adapted easily for different size of vessels, adjustable to any kind of recipient

Normally they require field calibration

Good range of measurement, from few cm to about 100 m

Compared with other level

measurement instruments

Equipment (laboratory)

THANK YOU!!