“Low dielectric constant liquid measurement, not suitable for radar level gauge”. Inadvertently, we hear this all the time. In fact, when I first entered the industry, I thought so too. After a long time and understanding more, I realized that this statement is a bit one-sided. Let’s discuss it in detail.

what is dielectric constant?

The dielectric constant is the main parameter reflecting the dielectric properties or polarization properties of the piezoelectric smart material dielectric under the action of an electrostatic field. Usually denoted by ε.
Different applications of piezoelectric elements have different requirements for the dielectric constant of piezoelectric smart materials.
When the shape and size of the piezoelectric smart material are certain. The dielectric constant ε is determined by measuring the intrinsic capacitance CP of the piezoelectric smart material.

Influence of Dielectric Constant on Liquid Level Measurement

Commonly used dielectric constant table

gastemperature()Relative dielectric constantliquidtemperature()Relative dielectric constant
water vapor140~1501.00785solid ammonia-904.01
gaseous bromine1801.0128solid acetic acid24.1
helium01paraffin-52.0~2.1
hydrogen01.00026polystyrene2024~2.6
oxygen01.00051wireless porcelain166~6.5
nitrogen01.00058UHF porcelain7~8.5
Argon01.00056barium dioxide106
gaseous mercury4001.00074rubber2~3
Air01hard rubber4.3
hydrogen sulfide01.004Paper2.5
vacuum201dry sand2.5
ether04.33515% water wet sand约9
liquid carbon dioxide201.585wood2~8
methanol2033.7amber2.8
Ethanol16.325.7ice2.8
water vapor1481.5shellac3~4
liquid ammonia-270.816.2Celluloid3.3
liquid helium-2531.058Glass4~11
liquid hydrogen-1821.22Yellow phosphorus4.1
liquid oxygen-1851.465sulfur4.2
liquid nitrogen02.28carbon (diamond)5.5`16.5
liquid chlorine201.9Mica6~8
kerosene202~4granite7~9
turpentine2.2marble8.3
benzene2.283salt6.2
paint3.5Beryllium oxide7.5
glycerin45.8

Dielectric constants of common liquids

The relative permittivity of common solvents is at room temperature and the test frequency is 1KHz.

  • H2O (water) 78.5
  • HCOOH (formic acid) 58.5
  • HCON (CH3) 2 (N,N-dimethylformamide) 36.7
  • CH3OH (methanol) 32.7
  • C2H5OH (ethanol) 24.5
  • CH3COCH3 (acetone) 20.7
  • n-C6H13OH (n-hexanol) 13.3
  • CH3COOH (acetic acid or acetic acid) 6.15
  • C6H6 (benzene) 2.28
  • CCl4 (carbon tetrachloride) 2.24
  • n-C6H14 (n-hexane) 1.88
  • n-C4H10 (No. 4 solvent) 1.78

Extended reading: Non contact level measurement

Influence of Dielectric Constant on Guided Wave Radar Level Meter

From the point of view of process conditions, the dielectric constant of the medium to be measured plays a very important role in the measurement of guided wave radar as a known condition.

According to the electrical properties of the dielectric constant, it is roughly divided into three categories: non-polar substances (εr<2.8), weakly polar substances (2.8≤εr≤3.6), polar substances (εr>3.6) .

For liquefied petroleum gas, petroleum, gasoline or other hydrocarbons in petrochemical plants, and petrochemical products, 1.4≤εr≤4.0.
Alcohol, organic solvent, oil-water mixture, etc., 4.0≤εr≤10.0.
Conductive liquids, such as water-based solutions, dilute acids and bases, εr>10.0.

The Guided Wave Radar Level Meter can measure gas/liquid, gas/solid, and liquid/liquid two-phase interfaces.

Guided wave radar level transmitter

Ideal for measuring liquid/liquid interfaces, such as oil-water interfaces.

Extended reading: Oil-water interface measurement-Magtech magnetic level indicator

The Guided Wave Radar Level Meter emits low-energy pulsed microwaves that are sent down the probe at the speed of light. At the interface between the probe and the medium, a considerable proportion of microwave energy is reflected back to the transmitter through the probe to receive the first echo signal.

A certain percentage of the pulses will continue down the probe, traversing the upper low dielectric constant medium until it is reflected on the lower medium surface and the transmitter receives the second echo.

To measure the boundary, the minimum thickness of the upper medium is 10-20 cm, so that the echoes of the two liquids can be distinguished. The maximum thickness of the upper dielectric depends on its dielectric constant.

The dielectric constant of the liquid in the upper layer is relatively low, and the difference between the dielectric constants of the upper layer and the lower layer liquid is required to be greater than 10, and the upper layer medium εr<3, the lower layer medium εr>20. Only in this way can the obvious reflection effect be obtained. Both level and interface can be measured simultaneously.

The smaller the dielectric constant of the medium, the greater the amplitude of the reflected signal. For the medium with low dielectric constant (1.2≤εr≤2.5), the guided wave radar level gauge can be selected for powdery or volatile medium.

Some substances hinder or absorb electromagnetic waves in the gas phase, and there are substances in the gas phase that weaken electromagnetic waves. For example, dust, powder (graphite, iron alloy, etc.) with high conductivity, or volatile media. For example, the dielectric constant of liquid ammonia is at room temperature 25 ℃. It is 14.9, which is a conductive medium and can be measured effectively.

The advanced Guided Wave Radar Level Meter is especially suitable for measuring a variety of powders. And liquids with inclined liquid levels caused by vortices.

Because the reflected wave does not depend on a “flat” surface to reflect the echo, microwave safe transmission makes the medium surface condition less affected. Disturbed liquid surface or foam formation, as well as different curved surfaces or furnace outlet will not affect the measurement. Therefore the Guided Wave Radar Level Meter also has a good measurement effect in turbulent situations.

From the perspective of measurement range, the medium with higher dielectric constant has better reflection performance and is suitable for longer measurement range. Guided Wave Radar Level Meter adopts two-wire loop power supply, 24V (DC) or 220V (AC) liquid level transmitter, low energy consumption, and meets the control level process requirements.

Extended Reading: Low Cost Float Level Sensor for Tank Liquid Volume Monitoring

Influence of Dielectric Constant on Radar Level Meter

The Radar Level Meter adopts the principle of transmit-reflection-receive. The antenna of the Radar Level Meter emits electromagnetic waves. These electromagnetic waves are reflected by the surface of the measured object and then received by the antenna. The time from emission to reception of electromagnetic waves is proportional to the distance to the liquid surface.

Non-Contacting Radar Level Measurement

In the measurement of Radar Level Meter, the measured medium needs to provide sufficient radar reflection signal. In general, the higher the dielectric constant, the stronger the reflected signal. The farther away the target is, the greater the intensity of the reflection is required. In order to have sufficient signal back to the radar transmitter.

When the radar wave reaches the liquid level surface and is reflected. The radar wave will be absorbed and attenuated. When the attenuation is too much, the radar level gauge will not receive enough signal, resulting in inaccurate measurement. This is the dielectric constant of the measured medium. Effects of radar level gauge measurements.

From this point of view, the dielectric constant has an effect on the Radar Level Meter measurement. However, this problem has been well resolved.

The Radar Level Meter can also perform interface measurements, especially on oil-water interfaces.

Principle: When measuring the interface, the part of the pulse that is not reflected on the surface of the upper product will continue down to the surface of the lower product, and then be reflected back.

The requirements for the dielectric constant when the Radar Level Meter measures the interface are as follows:

(1) The dielectric constant of the upper product must be known and must not be changed;
(2) The difference between the dielectric constants of the two products must be greater than 6;
(3) Mainly used in the interface between oil-water liquid and water/water-like liquid with lower dielectric constant of the upper layer (<3) and higher dielectric constant of the lower product (>20).

Frequently
Asked
Questions

Water is extremely unique in that it has a high dielectric constant (=80). A high dielectric constant suggests that the solvent (in this case, water) has the ability to screen charges. This means that water molecules will surround both anions and cations in a solution, and ultimately, diminish the attraction of the two charges. 

For example, if you place a cation (such as sodium) in water, the positive charge (+) of sodium is able to interact with anions and other molecules. However, because water has such a high dielectric constant, there is a decreasing effect on the interaction between sodium and its counter-ion. This is primarily because the charges interact with the electric dipole of water more than with each other, decreasing the overall interaction. In comparison to water, the interaction between anions and cations is much greater when placed in solvents with lower dielectric constants

Quote from: Wikibooks Structural Biochemistry/Unique Properties/High Dielectric Constant

The dielectric constant of polar liquid dielectrics decreases with increasing voltage frequency. This is because the carrier mobility is enhanced, making it easier to conduct electricity. such as electrolytic capacitors.

So far there is no sensor that can measure the volume of liquid. Because there are various shapes of liquid containers, such as cylinders, spheres, rectangles, cylinders and squares with hemispherical ends.

The height of the liquid in these containers can be detected by a variety of sensors. According to the shape of the container, the volume and weight of the liquid in the container can be calculated.

For rectangles, cylinders or squares, the volume and weight of the liquid can be calculated directly by multiplying the floor area of the container by the height of the liquid. And the corresponding relationship is linear. The hemispherical cylinder and sphere at both ends can be calculated by definite integral. The law of curvature change is the volume formula of the function. Calculate the volume and weight of the liquid according to the height of the liquid.

Read more about: Monitoring liquid volume solutions

Radar level instruments measure the distance from the transmitter (located at some high point) to the surface of a process material located farther below in much the same way as ultrasonic transmitters – by measuring the time-of-flight of a traveling wave.

A guided wave radar level transmitter is also called a wave-guided radar level sensor. They can measure both levels and the interface between two media. Through probe rod or cable, complete level contact types continuous level measurement. Such as Coaxial Probe Guided Wave Radar Level Transmitter. This is different from ultrasonic level measurement, which is a non-contact measurement. A guided wave radar level transmitter is often used for tank level measurement. Including liquid and solid. Output 4~20mA/HART, so as to measure and control the level during production.

Extended Reading: Guided-wave radar (GWR) level transmitter working principle

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Sino-Inst offers over 50 level sensors for level measurement. About 30% of these are Radar level meters, 30% of these are guided wave radar level sensor, 40% is other tank level sensors.

A wide variety of liquid level meters options are available to you, such as free samples, paid samples.

Sino-Inst is a globally recognized supplier and manufacturer of liquid level measurement instrumentation, located in China.

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