This article gives an overview of how the common level measurement devices work, covering; radar level transmitters, guided radar transmitters, ultrasonic level transmitters and finally capacitance level transmitters.

Click here for more information on the technologies behind level measurement and the Time of Flight principle.

Radar level transmitters

How do radar level transmitters work?

Radar level transmitters work with high-frequency radar pulses which are emitted by an antenna and reflected from the product surface. The time of flight of the reflected radar pulse is directly proportionate to the distance traveled. If the tank geometry is known, the level can be calculated from this variable.

Advantages of radar level transmitters

Very accurate –  ±0.5 mm (0.02 inches)

Installation at top

Non-contact

Accuracy independent of dielectric constant, density and conductivity

No re-setup required when changing liquids

Things to take into account:

Must take into account tank design

Foam can be an issue

Blocking distance

Turbulent surfaces

Minimum dielectric constant

Guided Radar level transmitters

How do Guided Radar level transmitters work

Guided Radar level transmitters work with high-frequency radar pulses which are guided along a probe. As the pluses impact the medium surface, the characteristic impedance changes and part of the emitted pulse is reflected. The time between pulse launching and receiving is measured and analysed by the instrument and constitutes a direct measure for the distance between the process connection and the product surface.

Advantages of Guided Radar level transmitters

Very accurate – ±0.2 mm (0.04 inches)

Works with some foam

Installation at top

Accuracy independent of dielectric constant, density and conductivity

No re-setup required

Things to take into account:

Must take into account tank design

Heavy foam can be an issue

Blocking Distance

Turbulent surfaces

Minimum dielectric constant

Ultrasonic level transmitters

How do Ultrasonic level transmitters work?

Ultrasonic measurement is based on the time-of-flight principle. A sensor emits ultrasonic pulses which the surface of the medium reflects and the sensor detects again. The required time of flight is a measure for the distance travelled in the empty part of the tank. This value is deducted from the overall height of the tank to yield the level.

Advantages of Ultrasonic level transmitters

Non-contact (lowest cost)

Installation at top

Accuracy independent of density changes, dielectric or conductivity

No calibration with medium required

Things to take into account:

Minimum density required

Foam is an issue

Blocking Distance

Turbulent surfaces

Vapours/gas above the liquid

Temperature difference between liquid and sensor

No vacuum (10 psia), no high pressures (44 psia)

Capacitance level transmitters

How do Capacitance level transmitters work?

The principle of capacitive level measurement is based on the change in capacitance of the capacitor due to the change in the level formed by the probe and the container wall. When the probe is in the air, a low capacitance is measured. When the container is filled, the capacitance of the capacitor increases the more the probe is covered. A capacitance probe may be compared to an electric condenser. As the tank is filled, the probe capacity increases. This change is electrically analysed.

Advantages of Capacitance level transmitters

Very cost effective

Established principle

Fast speed of response

Interface measurement possible

High temperatures and pressures possible

Things to take into account:

Many versions

Conductive/insulator

Probe coating – chemical compatibility

Ground reference tube

Non-metallic

Re-calibration with different media

10 pF minimum span

Foam can be an issue

Other resources.