Difference Between Sensors and Transmitters: A Systematic Analysis from Technical Definition to Engineering Application

In industrial automation, process control, and instrumentation systems, “sensor” and “transmitter” are two terms that appear with very high frequency.

In practical communication and application, the two are often used interchangeably and are even considered different names for the same device. However, from the perspectives of technical definition, signal form, and engineering application level, sensors and transmitters are closely related yet essentially different.

Correctly understanding the distinction between them is an important foundation for instrument selection, system design, and fault analysis.

Understanding Sensors and Transmitters from the Perspective of Thermal and Process Instrumentation

Both sensors and transmitters originate from the field of thermal and automation instrumentation.

If the term “sensor” is analyzed literally, its technical connotation becomes clearer:

Sense: perception of the measured physical quantity
Convert: conversion of the perceived physical quantity in terms of energy or form
Transmit: transmission of the converted signal to downstream systems

From a technical logic perspective, perception is the foundation, conversion is the means, and transmission is the purpose.

Definition and Technical Connotation of Sensors

National Standard Definition of Sensors

According to the national standard GB7665—87:

A sensor is a device or apparatus that can sense a specified measurand and convert it into a usable signal according to certain rules, usually consisting of a sensing element and a conversion element.

This definition emphasizes three core characteristics of sensors:

Ability to sense the measured physical quantity
Ability to convert according to a defined rule
Output is a “usable signal,” but not necessarily a standard signal

Basic Structure of Sensors

From a structural and functional perspective, a sensor typically consists of two parts:

Sensing element
Directly contacts the measured medium or object to detect non-electrical physical quantities such as temperature, pressure, level, flow, and gas composition.

Conversion element
Converts the physical changes output by the sensing element into electrical or other forms of signals.

Common sensor output signals include:

2.0 mV/V
3.33 mV/V
Resistance or capacitance variation
Frequency signals, etc.

These signals are characterized by small amplitude and non-standardization, and are usually unsuitable for direct long-distance transmission or direct input into control systems.

Definition and Functional Positioning of Transmitters

Standard Definition of Transmitters

According to national standards and the Encyclopedia of China:

A device whose output is a specified standard signal is called a transmitter.

A transmitter is a sensor that outputs a standard signal.

From this definition, it can be seen that a transmitter is not a concept independent of a sensor, but a further developed product form based on sensors.

Core Functions of Transmitters

The primary tasks of a transmitter are:

Receiving non-standard or weak signals from sensors
Amplifying, linearizing, compensating, and isolating signals
Outputting unified standard signals suitable for long-distance transmission

Commonly used standard industrial output signals include:

4–20 mA
0–5 V
0–10 V
1–5 V
0.5–4.5 V

Among them, the 4–20 mA current signal is the most widely used standard signal in process industries due to its strong anti-interference capability and suitability for long-distance transmission.

Essential Differences Between Sensors and Transmitters

Differences in Functional Level

Sensors focus on the perception and primary conversion of physical quantities
Transmitters focus on signal standardization and engineering transmission

In simple terms:

Sensors solve the problem of “measurement accuracy,”
Transmitters solve the problem of “long-distance transmission and stable utilization.”

Differences in Output Signal Form

Sensor output signals usually have the following characteristics:

Non-standard
Low signal amplitude
Susceptible to interference

Transmitter output signals, on the other hand, feature:

Standardization
High signal strength
Suitability for long-distance transmission

In engineering practice, control systems usually accept transmitter signals rather than raw sensor signals.

Differences in Wiring and Power Supply

Sensors have more diverse power supply and wiring methods:

Two-wire systems
Three-wire systems
Four-wire systems
Active or passive outputs

Transmitters are mainly based on two-wire loop-powered designs, where the power supply and output signal share the same circuit, facilitating system integration and field wiring.

Role Positioning in Instrumentation Systems

In automation systems, both sensors and transmitters belong to primary instruments, responsible for field signal acquisition and conversion.

Secondary instruments (such as indicators, PLCs, DCSs, and recorders) are used for:

Signal display
Control computation
Alarm and interlocking
Data storage and analysis

When sensors and signal conditioning circuits are integrated into one device and directly output standard signals, such devices are usually referred to as integrated transmitters or intelligent transmitters.

Development Trends of Intelligent Transmitters

With the development of electronic technology and digital communication, modern transmitters are no longer simple signal amplification devices.

Products represented by Nexisense intelligent transmitters typically have the following characteristics:

High-precision A/D conversion
Digital compensation and self-diagnosis functions
Configurable parameters
4–20 mA signals with superimposed digital communication

This integrated structure enables devices to possess both the acquisition capability of primary instruments and part of the intelligent processing capability of secondary instruments.

Practical Significance in Engineering Selection

In engineering practice, the choice between sensors and transmitters usually depends on the following factors:

Signal transmission distance
System anti-interference requirements
Control system interface types
Field environment and maintenance conditions

For most industrial applications, directly selecting transmitters with standard outputs is a more stable and reliable solution.

Frequently Asked Questions (FAQ)

Can sensors be directly connected to a PLC?
Generally not recommended. Most sensors output non-standard and weak signals, which must be processed by transmitters or signal conditioning modules before being connected to a PLC.

Do transmitters necessarily include sensors?
Yes. A transmitter is essentially a sensor integrated with signal processing and standardized output circuitry.

Conclusion

Sensors and transmitters are not opposing concepts, but technical units at different functional levels within the same measurement chain.

Sensors are responsible for perception and primary conversion
Transmitters are responsible for standardized output and engineering application

In modern industrial automation systems, the two often exist in an integrated form.

Correctly understanding the differences and relationships between sensors and transmitters not only helps with instrument selection and system design, but also significantly improves system stability and maintainability.

For application scenarios that pursue reliability and engineering efficiency, adopting mature and standardized Nexisense transmitter products is an important foundation for building stable measurement systems.