Liquid Sensors: Definition and Function 

Liquid Sensors (also known as Fluid Sensors) are core devices in industrial automation, playing a vital role in monitoring, measuring in tanks, pipes, and production systems. This article provides a comprehensive overview of the definition, classification, applications, and how to choose the most suitable fluid measurement sensor.

What Are Liquid Sensors? How Do They Work?

Definition of Liquid Sensors

A Liquid Sensor is an electronic designed to detect the presence or measure a specific physical parameter of a liquid. It converts this physical information (such as level, pressure, flow, or temperature) into an electrical signal (analog or digital) for processing by a control system (PLC, microcontroller).

Common Functions of Liquid Sensors

Depending on the type of sensor, they perform the following main functions:

1. Level Measurement (Level): Measures continuously (where the liquid level currently is) or point level (whether the liquid has reached a specific detection point).
2. Flow Measurement (Flow): Measures the flow rate or the total volume of liquid passing through a pipe over a period of time.
3. Pressure Measurement (Pressure): Measures the force exerted by the liquid on a unit area.
4. Detection (Detection): Determines the presence, absence, or leakage of the liquid.

Why Use Liquid Sensors Instead of Manual Methods?

In the modern industrial environment, the use of Liquid Sensors is indispensable. They replace manual methods due to:

• Accuracy and Consistency: Eliminating human error and providing continuous, real-time data.
• Safety: Allowing monitoring of hazardous, corrosive, or high-temperature liquids without direct contact.
• Automation: They form the foundation for building automated control systems such as pumps, valves, and automated filling/dosing lines.

Common Types of Liquid Sensors in Use

By Measurement Purpose

This is the most basic classification based on the parameter requiring control:

Sensor Type	Primary Function
Liquid Level Sensors	Continuous measurement or warning for full/empty levels (liquid level switch).
Liquid Flow Sensors	Measures the volume or speed of fluid flowing through a pipe.
Liquid Pressure Sensors	Measures static or dynamic liquid pressure in a closed system.
Liquid Presence / Leak Detection Sensors	Primarily used for safety warnings or overflow control.
Dosing / Filling Sensors	Combines level or flow measurement to dispense a precise volume of liquid.

By Measurement Technology

The technology determines how the sensor interacts with the liquid and its environment:

Non-Contact Liquid Sensors (Non-Contact)

This technology trend is preferred for its durability and high hygiene standards, non-contact with the media, and suitability for the food and pharmaceutical industries:

• Ultrasonic Liquid Level Sensors: Use ultrasonic waves to calculate the distance from the sensor to the liquid surface. High accuracy, unaffected by viscosity.
• Capacitive Sensors: Capacitive liquid level sensors can operate in two ways:
  • Contact: The probe is immersed in the liquid.
  • Non-Contact: The non-contact capacitive liquid sensor is adhered to the outside of a non-metallic tank wall to detect the point level. (Examples: Models like XKC Y26 or non-contact liquid level sensor Y26).
• Photoelectric Liquid Detection Sensors: Use a light beam to detect changes in refraction when liquid passes through (often used in small pipes or for point level detection).

Direct Contact Liquid Sensors

These sensors require direct contact with the liquid environment being measured. They rely on simple and intuitive physical principles and are widely applied in non-aggressive environments.

• Principle: Requires physical interaction between the sensing element and the liquid (immersion, contact, or exposure to pressure).
• Advantages: High reliability, simple installation, often lower cost (especially float sensors).
• Disadvantages: Susceptible to corrosion by chemicals, accumulation of residues, and risk of contamination to the measurement environment (e.g., in the food/pharmaceutical industry).

Typical Types:

• Float Switches: Use the buoyancy of a float to trigger a mechanical switch, typically used for simple point level indication.
• Submersible Pressure Transducers: Measure the hydrostatic pressure of the liquid column to determine level (continuous measurement).
• Temperature Sensors: Measure the liquid’s temperature directly.
• Conductive Probes: Use immersed electrodes to detect point level based on the liquid’s electrical conductivity.

Typical Liquid Sensor Product Lines

Specialized Liquid Sensor Solutions from Baumer: 

For industries demanding stringent hygiene (Food, Pharma, Beverage) and high analytical precision, Baumer’s solutions are highly trusted. Baumer Liquid Sensors go beyond just level and flow measurement, focusing on deeper liquid analysis:

• Conductivity Measurement: Crucial for CIP (Cleaning In Place) processes to distinguish between detergent and rinse water.
• Air and Gas Bubbles Detection: Prevents pump damage and ensures filling quality and product purity. (Example: Baumer analytical liquid sensor – Sensors for conductivity measurements and detection of air and gas bubbles).
• Turbidity and Concentration Measurement: Supports quality control of the final product.

These sensors often come with hygiene certifications (like EHEDG) and are designed for direct installation in pipes, optimized for liquid detection in pipes and high-pressure environments.

Other Popular and Industrial Brands: 

In the general and high-end industrial segments, besides Baumer, the market features many reputable suppliers, each with unique strengths:

• Omron (Japan): Preferred in basic automation.
  • Strengths: High stability and commonality, easy to integrate (strong in basic Omron liquid level sensors and point level switches).
  • Weaknesses: Less focus on complex, specialized process measurement solutions.
• Endress+Hauser (Switzerland/Germany): Experts in process measurement.
  • Strengths: Highly specialized in level (Radar, Ultrasonic) and flow measurement. Extremely high-quality and reliable products.
  • Weaknesses: High cost, typically used for large projects requiring strict standards.
• Siemens (Germany): Provides comprehensive automation solutions.
  • Strengths: Measurement product line (level, pressure) has high accuracy and optimal integration with the Siemens automation ecosystem.
  • Weaknesses: High initial investment cost.
• ABB (Switzerland/Sweden): Large-scale industrial solution provider.
  • Strengths: Broad product range, strong in flow measurement and process analytical solutions.
  • Weaknesses: Focused on large-scale projects, less flexible for smaller applications.
• XKC (Representative of the mass market segment, primarily from China):
  • Strengths: Extremely competitive pricing, an economical choice for basic applications (e.g., XKC-Y26 NPN/PNP for non-contact point level detection).
  • Weaknesses: Variable durability and stability, often lacking high-level industrial certifications.

Applications of Liquid Sensors in Practice

Liquid Sensors are the backbone of many industries:

1. Filling and Packaging Industry: Uses liquid dosing sensors and flow sensors to ensure precise volume in every bottle or container.
2. Tank, Silo, and Container Systems: Use level sensors (ultrasonic, radar, float) to monitor inventory and prevent overfilling.
3. Food and Beverage Production Lines: Requires non-contact sensors or Stainless Steel 316L materials to ensure hygiene (CIP/SIP standards).
4. Water Treatment / Chemical Processing in Factories: Uses sensors to measure level, chemical flow, and monitor water quality sensors (pH, ORP).
5. Liquid Leak Safety: Deploys liquid leak detection sensors in sensitive areas and chemical storage facilities for early warning, preventing environmental damage and asset loss.

How to Choose the Right Liquid Sensor (Checklist)

To select the correct device, you need to clearly define the following factors:

1. Based on Liquid Type:
   • Corrosive (Acids/Bases): Choose chemical-resistant sensor materials (PVDF, PTFE, Ceramic).
   • Viscous/Thick: The sensor must be self-cleaning or use technology unaffected by residue buildup (Ultrasonic, Radar).
   • Clean/Beverage: Prioritize non-contact liquid sensors for absolute hygiene.
2. Based on Measurement Method:
   • Do you need Continuous Level measurement or just Point Level Detection? (Continuous needs Radar/Ultrasonic; Point needs Capacitive/Photoelectric).
3. Installation Conditions:
   • High Pressure/Temperature: Requires specialized sensors with wide tolerance ranges.
   • Vibrating Environment: Avoid mechanical float sensors.
   • Metal/Non-metal Tank: Affects the choice of Capacitive technology.
4. Signal Requirements:
   • Do you need a Digital signal (On/Off – like NPN, PNP, Relay) or an Analog signal (Continuous Measurement – like 4-20mA, 0-10V)?

Frequently Asked Questions About Liquid Sensors (FAQ)

Is there a difference between a Liquid Sensor and a Liquid Level Sensor?

Yes. A Liquid Sensor is a general term encompassing all types of parameter measurement (level, pressure, flow, temperature). A Liquid Level Sensor is a specific subset, focusing only on measuring the height of the liquid.

When should Non-Contact Sensors be used?

They should be used when the liquid is a corrosive chemical, food, beverage (requiring hygiene), or highly viscous, which easily adheres to and causes errors in contact sensors.

Are Leak Detection Sensors necessary?

They are essential, especially in areas storing oil, chemicals, or in data centers. Leak detection sensors help protect the environment, electronic equipment, and minimize property damage when incidents occur.

Can one sensor be used for multiple types of liquids?

It depends on the type. Ultrasonic and radar sensors can often be used for various liquids, but capacitive sensors cannot, as they highly depend on the liquid’s dielectric constant (electrical resistance) for accurate operation.

Conclusion

The Liquid Sensor is the heart of any fluid control system, ensuring accuracy, safety, and efficiency in every industrial process. Selection must begin by clearly defining the measurement need (level, flow, pressure) and the characteristics of the liquid. Investing in the right technology from the start will yield maximum operational efficiency.

Servo Dynamics Engineering: Authorized Distributor of Baumer in Vietnam

Servo Dynamics Engineering is the official authorized distributor of Baumer – the leading Swiss sensor technology group – in the Vietnamese market.

If you are looking for industrial liquid sensor solutions, especially high-end, precise products that meet international standards (like Baumer sensors), we are committed to providing:

• Genuine, Swiss-quality products with full hygiene certifications (EHEDG) for the Food/Pharmaceutical industry.
• Competitive pricing due to direct import, eliminating intermediaries.
• In-depth technical support and official warranty from a team of Baumer-trained engineers.

Contact Servo Dynamics Engineering today to receive consultation on the most suitable Baumer measurement solution for your system!