Capacitive Level Sensors

Capacitive Type Level Sensors

Capacitive type level switches and transmitters are widely used in industrial level measurements.

The capacitor is made up of two plates and isolated by some isolators having a fixed dielectric constant.

But in level measurement we use a variable capacitor, capacitance can be changed by changing the gap between the two plates or by changing the dielectric constant, so dielectric constant changes and proportional to that capacitance also changes.

Capacitive level switches

Capacitive level switches are used in industrial applications for the point-level sensor to detect a high or low level of a vessel or tank.

Working principle of capacitive level switch

In capacitance type level switches, there are two main units:

1. Capacitive probe

2. controller

The capacitive probe is made up of two plates mostly in the shape of two rods. and the material we want to sense acts as a gap between the plates.

As material touches, the sensor probe, the dielectric constant changes and affects the capacitance of the probe. This change in capacitance is sensed by the controller and we can change the contacts of relays by electronic circuits from open to close or vice versa.

Capacitive level switches are used for point-level sensing. We can use these types of level switches to detect high or low levels alarms of hoppers, tanks, and vessels.

Capacitive type of level switches can be used for the level measurement of liquids and solids.

One limitation of the single rod capacitive type level switch is that the hopper or vessel's body should be a conductor for the proper functionality of the sensor probe. Because in this case, we are using the wall of the tank as the earth ground.

Capacitive Level Transmitter

Capacitive level SENSOR is also used for continuous level measurement, in this case, we will measure the head pressure of the medium we are measuring, in short, we will use a dp pressure transmitter to measure the level of the tank or vessels.

Capacitive level sensors are used for the level measurement of liquids.

Working principle of capacitive sensor

It is made up of two parallel plates like a capacitor, The output of a parallel plate capacitor depends on the gap between its movable and fixed plates.

Due to pressure applied on movable plate, it moves so the gap between the plates is changed and capacitance also changes. This change in capacitance is directly proportional to pressure, and that pressure is an indication of level in the tank or vessels.

A capacitive level sensor can be used for direct measurement of the level of liquids by using the following method.

A capacitive type level probe uses a coaxial type cylinder, the capacitance is measured between the inner rod and the outer rod. The total capacitance between the two terminals is the sum of

(i) the capacitance of the insulating bushing,

(ii) capacitance due to solids, liquid vapor and air.

(iii) capacitance due to the liquid or solids.

If the total capacitance measured when the tank is empty is expressed as C, at this capacitance it will show zero.

Then the Reference capacitance C2 of the liquid. C1 is the capacitance of the level of the liquid which is changing. level of the liquid can be expressed as

level = C1/C2.

We can process this change in capacitance by CDC and processor and convert it to a standard signal or digital data for use in PLC and DCS.

Applications of a capacitive level sensor

Capacitive type of level sensors is widely used in all industries including chemical and petrochemical industries.

These types of level sensors are used for a wide range of temperatures ranging from -40°C to 200 °C.

Capacitive type level sensors can work well in the pressure variation ranging from 25 to 60 kg/cm2.

Advantages of a capacitive level sensor

Some advantages of capacitive level measurement are given below:

1. Capacitive type level sensors are useful for a small system.

2. Capacitive level switches have good frequency responses.

3. Capacitive level sensor required small power to operate.

4. Loading effect in capacitance type level sensors is less due to high input impedance.

5. The advantage of capacitance type sensor is that permittivity of the liquid is less sensitive to variation of temperature and can be easily compensated.

6. The sensitivity of capacitance type level sensors is high.