How RF Capacitance Level Sensors Work
The capacitance is formed between the level sensing probe and the metallic wall of the vessel which act as two electrodes, and the application media acts as a dielectric medium when sensor is introduced in the vessel. These electrodes are separated by an insulator by suitable media like soild, liquid etc. known as dielectric medium. The dielectric is the main factor in capacitance principle as it's value will be influenced by temperature, moisture content etc. of operating material. These factors should be taken into account while designing the probe.
When level of process material rises the value of dielectric changes which results change in value of capacitance between level sensing probe and the vessel wall. This change can be detected and measured by Radio Frequency technique. These RF signals are interpret by evaluation unit and it convert these signals into relay output signal.
The design of the probe comprises of sense rod and grounding part. The probe can be made of stainless steel which resist stains and corrosion and gives a rigid support to the level sensor. The design of the probe can be vary with tank type and application media. The probe design for various tank type and materials are given below
- Metallic Tank - Generally for metallic tanks, only sensor rod is sufficient without any reference and grounding tube as in this case the metallic tank act as a ground.
- Non-Metallic Tank - In these type of tanks there is always a need of reference electrode. In this case there are two possibilities for providing grounding which are as follows -
- Twin Probe - This arrangement consist of two probes i.e. sense rod and reference rod which provide grounding for material have low pf value.
- Still Well - This arrangement provides a grounding tube over a sense rod which is useful where there is lot of turbulence and material having low dielectric media.
With the support of electronics we differentiate capacitance based level sensors into two categories -
- Point Level Switch - These sensors are used to detect the presence of material at certain point and gives alarm when material is below or over that point. These sensors are known as Point Level Switch. These switches protect the material in the vessel from dry running or overspilling.
- Level Transmitter - The level transmitter is designed to calculate the level at every point and provide a feedback of current level throughtout the process. These sensors are helpful where accuracy is first priority. It has internal temperature compensation feature which give true value of capacitor without influenced by temperature.
Comparison Between Capacitance Level Sensors
|Product Type||Level Switch||Level Transmitter||Level Transmitter||Level transmitter||Level Transmitter|
|Probe Length||100mm to 10m||100mm to 10m||100mm to 10m||100mm to 2000mm||100mm to 10m|
|Application Media||Free-flowing Solid/Liquid||Free-flowing Solid/Liquid||Free-flowing Liquid||Fuel||Free-flowing Solid/Liquid|
|Response Time||2-3 Sec||2-3 Sec||2-3 Sec||8 Sec||2-3 Sec|
|Output||Single-Point, Two-Point, Three-Point Switching and Pump Control Logic||4-20mA, Voltage Analog Output, Digital Output (RS-485), PNP Output||4-20mA||0.2-5V voltage Output, Digital Output (RS-485, RS-232), Fuel Guage Analog Output||4-20mA, Voltage Analog Output, Three Point Relay Output|
- It requires minimum maintenance.
- Ensures long life as it has no moving parts.
- Works well with high temperature.
- It is reliable.
- Build-up on the capacitance probe can cause the capacitance sensor to malfunction. The capacitance level sensor starts giving false alarm when material stick on the probe.
How RF Admittance Level Sensors Work
To remove the limitation of capacitance based level sensors, admittance level sensors are introduced which works on admittance principle.
In this principle, the sense electrode and the vessel wall act as two electrode of a capacitor and a material "shield" is proposed as an insulated electrode that will be added between ground and sense part. The sense and the shield is connected to the same source but they are electrically isolated from each other. Due to this, no RF current is flowing between shield and sense electrode. The main purpose for adding shield is to block the RF current from reaching to the vessel wall via coating of the probe.
The sense probe and the shield are maintained at equi-potential which prevents flow of RF current from probe to the vessel wall while having build-up on the probe and when actual material touches the probe the RF current starts flowing. This design help the electronic circuitary to differentiate between build-up and the real material.
As the level of the vessel increases, the dielectric changes which in turn causes change in admittance. This change in admittance correctly measured by RF technique.
- The specially designed immuno coat probe comprises of sense and shield electrodes that are isloated from each other with the help of insulators.
- The RF source is applied to measure and detect the change in admittance.
- The Evaluation Unit measures the change of admittance accurately by using the Radio Frequency ( R.F.) techniques coupled with the coat-immunizing circuitry.
- The RF signal is transmitted between probe and evaluation unit via coaxial cable to eliminate the effects of changing cable capacitance to ground.
- The processed signal is used to empower relay whose output are available for annunciation / control.
- The PTFE insulation is provided between sense, shield and ground with the support of tapered mechanism which makes it rugged for typical applications.
- The drive current of the active shield is at the same Frequency and Polarity as the probe which prevents the flow of RF current from probe to the ground in case of build-up on the probe.
- It consist of 10 LED Dot Display for sensitivity indication.
- A surge arrestor is also provided which discharge the static charge.
Comparison Between Admittance Level Sensors
|Products||SLA Dot Display||SLA Bar Display||SLA Series||Coat Endure|
|Type||Integral and Remote Model||Remote Model||Integral and Split Model||Compact & Integral|
|Probe Length||150mm to 10m||150mm to 10m||150mm to 10m||65mm to 150mm|
|Application Media||Non-conductive Sticky Media||Non-conductive Sticky Media||Non-conductive Sticky Media||Conductive & Non-conductive Sticky Media|
|Response Time||2-3 Sec||2-3 Sec||2-3 Sec||1-3 Sec|
|Output||Single Point Switching||Single Point Switching||Single-Point, Two-Point, Three-Point Switching and Pump Control Logic||Single Point Switching|
Common Factors in Probe Design for Protection Purpose
- Rod Arrangement - The construction of probe comprises of sense rod made by stainless steel which serves as rigid support to increases strength and also it resist stains and corrosion.
- Standoff - Long standoff is provided to protect the electronics from high temperature.
- Rope Arrangement - For large tanks which required probe length higher than 3m then the rod probe arrangement is not a good idea as it is not comfortable for transportation. For higher probe length, rope probe is better option.
- Mounting Arrangement - As the shape and length of various tanks differs so same mountings can not be used in every tank. It is feasible to use Flanged, Screwed and Tri-clamp mountings suitable according to each tanks.
- Insulation Protection - The probe construction vary according to its application material. Sometimes protection for probe is needed to work properly in particular media. There are 3 insulation prvided for protection which are as follows -
- PTFE Insulation - The furnish product with PTFE coating will be convenient for conductive material as this material can cause short circuit between sense rod probe and the vessel wall.
- Ceramic Insulation - The Level sensor's electronics need protection against high temperature material so it is advantageous to use ceramic insulation where process material heated upto high temperature.