What are Level Transmitters?

Level transmitters are sensors used to measure and monitor the level of liquids or bulk solids in tanks, silos, river water, and tidal environments (sea level).

There are various types of level transmitters, each based on different measurement principles and featuring unique characteristics depending on the manufacturer.

Selecting the appropriate level transmitter requires careful consideration of key conditions such as the physical properties of the material, the measurement environment, installation conditions, and potential disturbances.

This article introduces the operating principles, features, and limitations of each measurement method.

Level Transmitters are used in mass-producing factories, such as material industry, to measure amount stored in silos and tanks that store raw materials and products.

In factories, it is necessary to continuously send raw materials to next process so that production does not stop. Therefore, it is necessary to replenish raw materials before storage capacity in the silo runs out so that production can continue.

It would be extremely inefficient to monitor levels of each of these facilities individually, so level meters are used to monitor and automate (control) storage levels.

There are two main types of sensors for level measurement, "Level Transmitter" and "Level Switch".

The amount stored in container is continuously measured and displayed as ***%, level ***m, etc., and is used as an approximate storage level.

The measured value is converted from 0 to 100% into an instrumentation signal of 4 to 20 mA DC or 1 to 5 V and output, which is used for monitoring and control.

This is a switch designed to detect full or empty conditions by installing the sensor at a predetermined level inside a container.

It is commonly used to control the start and stop of material loading and unloading processes.　When the preset level is reached, the switch outputs an ON/OFF contact signal.

Small containers are often controlled with just a Level Switch, but for medium-sized and large containers, instead of installing multiple Level Switches at the top, middle, and bottom, a Level Transmitter is used to continuously measure the storage level and calculate approximate storage level in real time.

It is often displayed and used.

In some cases, Level Transmitter is also used to check the intermediate storage amount while controlling with a Level Switch.

This also has the advantage of making it easier to notice if there is a problem with a certain sensor.

Since it can be detected early, it can reduce impact on production process and prevent stress on equipment.

There are many types of level meters and level switches depending on the object to be measured and the measurement environment.

Please check the measurement object and environment to be used and select the one that matches.

Also, not everything will match. In such cases, there are many cases where special specifications are available, so please contact the manufacturer in such cases.

Please refer to the information below that will help you select a model.

There are various types of level transmitters available on the market.

You may feel that you understand the characteristics of each type, yet find it difficult to decide which model to select when it comes to actual application.

To help with this, this article explains not only the types and features of level transmitters, but also their limitations.

Level transmitters vary in measurement principles and manufacturer-specific features. Selecting the appropriate device requires careful consideration of material properties, process conditions, installation requirements, and external disturbances.

Measurement methods can be broadly divided into distance-based and direct level measurement types.

This article explains the principles, features, and limitations of each method, focusing on powder, granular, and bulk material applications.

A sensing weight is electrically lowered by a motor until it contacts the material surface. The empty distance is calculated from the measured travel time and the lowering speed. (Distance = Speed ​​x Time)

• Easy to understand, based on a direct physical measurement principle.
• Unaffected by heavy dust or steam generation.
• Suitable for detecting deposits in liquid.

• Requires periodic maintenance due to consumable parts such as the wire rope.
• Not suitable for applications where foreign object contact is prohibited, as the sensor contacts the material.

This is a non-contact level transmitter that measures the empty distance by transmitting an ultrasonic pulse toward the material surface and detecting the time required for the reflected signal to return to the sensor.

• Cost-effective non-contact level measurement.
• High directivity enables reliable measurement in narrow spaces.

• Measurement may be affected in environments with dust or steam.
• Temperature differences or gas generation can change the speed of sound and increase measurement errors.

A radar level transmitter operates by emitting microwave pulses toward the material surface and calculating the empty distance based on the round-trip travel time of the reflected signals.

• Unaffected by temperature changes or gas.
• Reliable measurement even in dusty or steamy environments.

Materials with a low dielectric constant may not be detectable due to weak radar signal reflection.

Example: silica

A non-contact level meter based on FMCW radar technology. Frequency-modulated continuous microwaves are transmitted from the sensor, and the distance is determined by analyzing the frequency difference between the transmitted signal and the signal reflected from the material surface.

• Unaffected by temperature changes or gas.
• Reliable measurement even in dusty or steamy environments.
• High directivity enables measurement in narrow spaces.

Materials with a low dielectric constant may not be detectable due to weak radar signal reflection.

Example: silica

Microwave pulses propagate along a guide rope installed from the tank ceiling. The sensor measures the round-trip time of the reflected signal and converts it into the air distance to the measured object.

• Unlike ultrasonic sensors and non-contact radar level transmitters, this method has virtually no beam angle, making it ideal for installation in narrow spaces.

• Severe material adhesion to the probe can result in erroneous measurements.
• Kinking or breakage of the wire, as well as material buildup on the probe, can generate spurious reflections (noise), increasing the likelihood of false measurements.

Non-contact level meters generally use two measurement principles: time-of-flight and phase difference detection.

• TOF method: Measures the round-trip time of a reflected laser pulse to calculate the distance to the target.
• Phase difference method: Calculates the distance by detecting the phase difference between the transmitted and reflected modulated laser signals.

• High directivity allows measurement in narrow spaces and over distances of several hundred meters.
• TOF method: Uses a Class 1 laser and provides fast response.
• Phase difference method: Delivers high accuracy and resolution.

• In environments with heavy dust or steam, the laser beam may be scattered or attenuated, making reliable measurement difficult.
• Due to its high directivity, if the target surface is smooth or mirror-like and inclined, the reflected signal may be weak and may not return sufficiently to the sensor.
  • Please note that the phase difference detection method often uses a higher laser class and therefore requires appropriate laser safety management in many applications.

A float attached to a stainless-steel tape tracks the liquid level, and the tape length is continuously measured to determine the empty distance. Types include spring-balance, sealed-pipe, and counterweight designs.

• Featuring a simple and robust structure, this method provides high measurement accuracy and long-term durability, making it suitable for liquid level detection in both large and small tanks.

• Prone to material buildup.
• Limited maintainability in installations with dense fire protection equipment.
• Solid accumulation on the float may result in measurement errors.
• Adhesion inside stilling/breakwater pipes can hinder float movement and cause malfunction.

The displacer-type level transmitter uses a displacer with a density greater than that of the process liquid. Variations in liquid level cause proportional changes in buoyant force acting on the suspended displacer, which are mechanically transmitted and converted into a level signal.

• Suitable for measuring liquids with low specific gravity and effective for liquid level measurement in high-pressure environments.

• Limited measurement range (approx. 300–3,000 mm).
• Prone to malfunction if obstructed by deposits or sludge.
• Spring is a wear part.
• Recalibration required when liquid density varies.

A capacitor is formed between a probe electrode suspended from the tank ceiling and the tank wall. When the measured material enters the space between the electrodes, the capacitance changes in proportion to the level. This change is measured and converted into a level measurement.

• Suitable for measurements in narrow spaces.

• As a contact-type sensor, there is a risk of contamination due to foreign material.
• The tank must be emptied or drained to perform zero adjustment.
• Measurement errors may occur when the relative permittivity of the measured medium changes.

A diaphragm detects changes in liquid pressure caused by variations in liquid level and converts them into a level measurement. However, this method must be used under atmospheric pressure conditions inside the tank.

• Easy to install even in places with limited installation space.

• Excessive foam can cause uneven liquid pressure, resulting in reduced measurement accuracy.
• Heavy adhesion or sedimentation of solids on the sensing surface can degrade accuracy.
• Liquid pressure varies with the specific gravity of the liquid, requiring recalibration or adjustment.
• The tank must be drained when performing maintenance.

It consists of two diaphragms: one that measures liquid pressure and another that measures the internal tank pressure. The liquid level is calculated by subtracting the tank internal pressure from the measured liquid pressure.

• Measurement is possible without being affected by pressure changes inside the tank.

• Excessive foam can cause uneven liquid pressure, resulting in reduced measurement accuracy.
• Heavy adhesion or sedimentation of solids can degrade measurement accuracy.
• Liquid pressure varies with the specific gravity of the liquid, requiring adjustment or recalibration.
• The tank must be drained when performing maintenance.