what is a capacitive level sensor

1 what is a capacitive level sensor

The capacitive liquid level sensor measures the height of the page by measuring the change of capacitance. It is a metal rod inserted into the container containing the liquid. The metal rod is used as one stage of the capacitor, and the wall of the container is the other stage of the capacitor. The medium between the two electrodes is liquid and air. The dielectric constants of liquid and air are different. If the liquid level changes, the capacitance changes accordingly, so as to measure the liquid level. when the liquid level rises, the total dielectric constant between the two electrodes of the capacitive liquid level gauge increases accordingly, so the capacitance increases. Conversely, when the liquid level drops, the value of ε decreases and the capacitance also decreases.

1.1 Operating principle of capacitive level sensor

A capacitors in made up of two conductive plates which are separated from each other by a dielectric. The storage capability of capacitor defined by the capacitance , C, is directly dependent on the plate areas（A）, their distance apart （d）and the dielectric constant of the material between the plates:

Capacitive probes are used in liquids that are non-conductive and have a high dielectric constant（ε）and can be used for continuous level monitoring. A capacitive probe shown below, consist of an inner rod with an outer shell immersed in the liquid that we intend to measure the level.

The capacitance of the probe is more measured between the inner rod and the outer shell with the aid of a capacitance bridge. In the portion out of the liquid, air serves as the dielectric between the rod and outer shell. In the immersed section, the dielectric is that of the liquid that causes a large capacitive change as the level of liquid changes. If the tank is made of metal it can serve as the outer shell. The capacitance change is directly proportional to the liquid. The dielectric constant of the liquid must be known for this type of measurement.  Most materials has their unique permittivity at a certain temperature. There are someexamples of materials with their properties shown in Figure 1.1.

1.2 Application field

  Level detection in silos, tanks and bunkers, for both limit detection and continuous measurement;

  Conductive, insulating liquid-chemical industry, oil field, water and sewage treatment;

  Conductive and insulating slurry – papermaking, pharmaceutical, water and sewage treatment;

  Powder, ash, powder-power plant, metallurgy, cement;

  Pellets, coal, grain-power plants, metallurgy, grain;

  Interface, two different liquids-oil field, chemical industry

1.1 What are the advantage of capacitive level sensor?

It is suitable for high pressure, high temperature, strong corrosion, easy crystallization, anti-blocking, anti-freezing, and solid powder and granular materials

They have know moving parts, easy to clean

Relatively inexpensive and reliable

Easy to install and can be adapted easily for different size of vessels

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

Simple to use

2 Influencing factors and measures of capacitance

2.1 affect factors

Because the dielectric constant changes with temperature, capacitive sensors are very sensitive to environmental changes such as temperature and humidity, temperature fluctuations during sensing operation have the most significant impact on performance. In addition, many external factors can affect capacitive sensor elements due to edge effects. The human body itself introduces a lot of parasitic capacitance, which can interfere with the measurement. Capacitive level sensor exhibit non-liner behaviors due to edge effects.

2.2 KSLV605 how to improve (solution)

To avoid those influence factors, we designed KSLV605 capacitive level sensor.

 2.2.1 How to avoid temperature affects

In liquid-level sensing applications, temperature compensation is more difficult because we cannot assume that the liquid sensor is not covered with liquid; it may be covered to any level for any length of time. We must, therefore, compensate for temperature variation through the use of algorithms and optimized sensor designs.This approach minimizes the impact due to temperature drift, which affects a number of parameters in a liquid-level sensing system. First is the capacitance to be measured. Drift impacts other system parameters, such as the integrating capacitor and the current used by the capacitive sensing engine.As a result of these variations, the raw count also increases or decreases because of temperature (Fig. 2.1 ). Capacitive sensing circuitry converts the measured capacitance to a digital count, which is known as raw count.

There are two possible ways to overcome the issue:

(1)Fixed temperature compensation capacitor

A temperature compensation capacitor is a sensor that has similar characteristics as the other sensors used for detecting the liquid level. However, it is not placed in direct contact with the liquid. In other words, this sensor must remain unaffected by the liquid level. The raw count of this capacitor is used as a reference for the actual sensors.

Since the temperature compensation sensor and the actual sensor have the same characteristics, the effect of temperature on both of the sensors will be the same. In this way, the impact of temperature on the liquid-level detecting sensors can be nullified by using the temperature compensation sensor as a reference.

It’s worth noting that temperature compensation on the liquid-level detecting sensor using the temperature compensating sensor has to be performed both when the tank is empty and when there is liquid in the tank.

In case permittivity and temperature are unknown, a reference sensor can be used to replace the reference capacitance. Figure2.2 shows an example of a container with two sensor plates and a reference sensor at the bottom. This is to make sure, the reference is measuring the liquid even with a low fill level.

This configuration producesratiometric measurements. This means the ratio between level sensor and reference sensorwill be always the same independent of material properties or temperature. In this configuration, both parameters will affect both sensors and therefor balances itself amongst them.

(2)Software algorithm on baseline

Software algorithms to detect a finger touch with capacitive sensing comprise a reference that is a filtered version of the raw count. This reference keeps track of slow environmental changes in raw count, and it’s typically called a baseline.

The baseline is used to detect the presence of a finger in case of touch detection. With liquid-level sensing, the baseline can be used to track changes in raw count. Temperature drift is often slow; hence, its effect on raw count is also slow.

In such cases, with appropriate values of baseline update parameters, the baseline can be used to compensate for temperature variations in raw count. If the effect of temperature on raw count is very fast, then a temperature compensation capacitor should be used instead.

A secondary effect of temperature is condensation. A liquid that is significantly colder than the ambient air temperature may cause condensation to form on the sensor surface. Condensation may cause a higher capacitance, which, in turn, causes an increased error.

Condensation during low-temperature testing can be reduced by insulating the surface of the sensor. Another approach is to provide a small insulating air gap between the liquid container and the sensor substrate. The air gap should not be larger than 3 mm for the best performance.

2.2.2 How to avoid edge effects

The human body itself can introduce a lot of parasitic capacitances and therefore disturb the measurements.

The high content of water in the human body causes a potential difference between the sensor plates and therefor influences the measurement. This additional parasitic capacitance will be measured and would bring a distortion to the fill level relation, as can be seen in the simulation in Figure 2.4.

One way to reduce external parasitic influences is to build up a shielding around the level sensor, as shown in Figure 13. An additional conductive container, that is not connected to the sensor ground acts as a shield against parasitic capacitances coming from outside.It is important to keep the sensor ground and the container isolated from each other to have the measurement system isolated from the environmental ground.

3. How do you use a capacitive level sensor KSLV605

3.1 installation method

(1) Gas phase and liquid phase installation

Since the products are only different in appearance design and material, but both belong to the external liquid level measurement and control instrument, the installation methods of the two are basically the same, which will be explained together here. In general, the installation is extremely simple and fast, just connect the gas-liquid phase connection flange on the liquid level measurement and control instrument with the gas-liquid phase flange on the equipment, add a gasket in the middle, and fix it with bolts. (Note: The connecting flange of the liquid level monitoring and control instrument has been welded according to the size agreed by the two parties, and does not need to be reconfigured. The user should configure the valve and pipeline by himself) as shown in the figure 3.1.

Note: Before installation, be sure to clean the inner hole of the outlet pipe on the equipment to ensure that the outlet pipe of the equipment is unobstructed and the sealing surface of the flange is intact. At the same time, a valve can be added between the flange of the liquid level measurement and control instrument and the flange of the equipment to facilitate the disassembly and assembly of the instrument during maintenance or replacement.

(2) Installation of boiler type capacitive liquid level meter

Capacitance level gauge is a product specially used for large, medium and small boiler air bag and other types of high temperature liquid level measurement.

It adopts special materials and radio frequency technology, so that the whole machine can run stably for a long time in a high temperature environment. Because it is specially used in high temperature environment, the structure and installation method of liquid level measurement and control instrument are different from other products.

First of all, it is different from other products in that its transmitter is located under the sensor, there is a sealed and heat-dissipating section from the measuring cylinder to the transmitter, and then downward is a 90-degree curved arm to lead the transmitter to the sensor side, which ensures that the transmitter is protected from high temperatures near the gas port. On the other hand, when the high temperature medium transfers heat downward to the transmitter, it first passes through a special heat dissipation section, which will greatly reduce its heat. Leading the transmitter to the lower side of the sensor is mainly to prevent the leakage of the sealing section of the sensor and prevent the medium from spreading down to the transmitter part along the outer wall of the measuring cylinder, causing short circuit or corrosion.

To sum up, the structure of this level meter has obvious advantages, which is why it can run stably for a long time in a high temperature environment. For installation, it should be noted that the transmitter is below, and the distance from the sewage pipe is relatively close, so it cannot be installed in reverse. The installation is as shown in the figure 3.2.

3.2 Calibration

Although the analog adjustment has been made before the product leaves the factory, in order to allow the user to further experience the performance of our product before use, it is recommended that the user perform a simple verification. You can remove the whole set of instruments for calibration. (But do not disassemble the parts of our products)

The calibration of the external liquid level measurement and control instrument is shown in the figure 3.3:

The verification steps are as follows:

1) Prepare a transparent water pipe, mark it with a scale or fix it with a ruler, so that the actual liquid level can be observed and calibrated during calibration. In addition, prepare an ammeter (DC) with a precision of more than three digits, several rubber stoppers, and enough test medium (which can be replaced by water).

2) Connect one end of the transparent water pipe to the liquid phase port of the liquid level measurement and control instrument, block the sewage outlet, and keep the gas phase port unobstructed. And connect the ammeter in series as shown in Figure 3.5), and then turn on the power after confirming that the wiring is correct.

3) Add the medium from the upper end of the transparent tube, the medium flows through the liquid phase tube into the liquid level measurement and control instrument, and the liquid level is added to several points with different heights, because at this time the liquid level in the transparent tube is measured with the liquid level measurement and control instrument. The liquid level in the meter cylinder is exactly in line. At this time, read the value of the ammeter, and then compare the height ratio corresponding to the output standard 4-20mA signal with the collected current value to check the accuracy of the level meter (Note: In order to It is easy to calculate. Generally, several points are taken at 0%, 25%, 50%, 75% and 100% respectively, and the corresponding currents are 4mA, 8mA, 12mA, 16mA and 20mA respectively. The range should correspond to the center of liquid phase and gas phase respectively).

3.3 Wiring

KSLV606 (display model) has two ways of wiring: one is RS485, the other is 4-20mA.

(1) RS485

(2) 4-20mA

After installation, when using it for the first time, make sure to open the gas phase valve first, and then open the liquid phase valve to ensure that the liquid level will not fluctuate violently, causing measurement errors.
In addition, it should be ensured that the connecting cable joints are in good contact and anti-corrosion. In long-term use, pay attention to regular sewage discharge, so as to avoid the accumulation of dirt and affect the performance of the instrument. Taking common copper liquid, C-carbon liquid, Baohe hot water tower, sewage pool and other dirty media as an example, it should be guaranteed to discharge 1 to 2 times a week, while the cleaner medium should be discharged 1 to 2 times a month.
The transmitter housing should be tightly wrapped to prevent water, corrosive medium or gas from entering, and it is forbidden to collide with external force and dismantle it by non-professionals.
There are three common wiring methods for transmitters: as shown in (a) (b) (c)

3. Troubles shooting

1. During use, if the digital display indicates zero, use the 0-200mA range of the DC ammeter, and when the measured current is also 0, the possible faults are:
   a. Is the 24V power supply normal?
   b. The transmitter may be short-circuited
   c. The transmitter has quality problems;
   If the current measured by the DC ammeter is less than 4mA, the possible faults:
   a. The actual liquid level is below the liquid phase port
   b. The current adjustment value of the transmitter is too low
   c. The transmitter has quality problems; if the measured current is consistent with the actual liquid level, there is a problem with the digital display;

Possible failures if the current exceeds 25mA:
a. There is a short circuit in the transmitter circuit
b. The current adjustment value is too high.

2. When the digital display is full, use the 0-200mA range of the DC ammeter. When the measured current is 20mA, there may be a fault:
   a. The current adjustment is too high
   b. There is a short circuit in the transmitter
   c. The actual liquid level is full;
   If the measured current is less than 20mA, the digital display is faulty.
3. The digital display jumps violently. Using the 0-200mA range of the DC ammeter, the measured current fluctuates too much and may malfunction:
   a. Actual liquid level fluctuation
   b, poor line contact
   c. The transmitter has quality problems; if the measured current is stable, the display meter may be faulty
4. There is no change in the digital display. Use the 0-200mA range of the DC ammeter. When the measured current changes normally, it may indicate that the instrument is faulty; if the measured current does not change, there may be a fault:
   a. Transmitter failure
   b. The gas-liquid phase pipe is blocked
   c. There is an open circuit between the sensor and the transmitter and must be reconnected
5. The digital display changes slowly than the actual liquid level. When using the ammeter 0-200mA, the measured current changes slowly, and there may be a fault:
   a. The inner pole of the sensor sticks to impurities, use 25% hydrochloric acid (sulfuric acid) to soak the pole
   b. The gas phase pipe is half blocked, please open the valve, and test it, clear it.
6. When the digital display is high or low, adjust the range or zero knob in the transmitter to adjust.

Note: To measure whether there is 24V power supply in the circuit, please use the positive and negative test leads of the voltmeter to connect the positive and negative of the 24V power line before measuring.
Handling of the existence of short circuit: please check the external circuit and the transmitter circuit and eliminate it.