What Are the Differences Between Pressure Sensors, Pressure Relays, and Pressure Switches

— A Systematic Analysis from Measurement, Control, to Protection

I. Three Core Devices in Industrial Pressure Measurement and Control Systems

In industrial automation and process control, pressure is one of the most common and critical process parameters. Around pressure detection, judgment, and control, three types of devices are frequently mentioned yet often confused: pressure sensors, pressure relays, and pressure switches.

Although all three use pressure as the input variable, they differ significantly in functional positioning, signal form, control logic, and application level. Correctly understanding their essential differences is a key prerequisite for ensuring safe, stable, and efficient system operation.

II. Definition and Core Role of Pressure Sensors

What Is a Pressure Sensor
A pressure sensor is a device or component that senses pressure signals and converts pressure into a usable output signal according to a defined relationship. Its core task is “measurement”, rather than direct control.

In industrial environments, pressure sensors typically convert pressure changes into continuously varying electrical signals for processing, analysis, or control by downstream equipment.

Output Signal Characteristics
Common output forms of pressure sensors include:

Analog signals
Current signals: 4–20 mA
Voltage signals: 0–5 V, 0–10 V

Digital signals
RS485
Modbus RTU
CAN, etc.

These signals are continuous in nature and accurately reflect pressure variation trends over time.

Typical Application Scenarios
Pressure sensors are widely used in:

Water conservancy and hydropower systems
Petroleum, chemical, and oil & gas pipelines
Power and energy systems
Aerospace and defense industries
Smart manufacturing and process automation

Their core value lies in providing precise, real-time, and computable pressure data.

III. Working Principle and Functional Positioning of Pressure Relays

Basic Concept of Pressure Relays
A pressure relay is a hydraulic-to-electrical conversion component that uses pressure signals to directly actuate electrical contacts. In essence, it is a pressure-triggered control device.

When system pressure reaches a preset threshold, the pressure relay will:

Close or open electrical contacts
Send an action signal to the electrical system

Functional Characteristics
Unlike pressure sensors, pressure relays do not output continuously varying signals. Instead, they:

Operate based on threshold judgment
Output clear ON / OFF states
Directly drive actuators

Their main purposes include:

Overpressure or underpressure protection
Sequential action control
Interlocking and safety protection

Application Characteristics
Pressure relays are particularly common in hydraulic systems, such as:

Hydraulic power units
Construction machinery
Automated production lines

In these scenarios, pressure relays emphasize action reliability and deterministic response.

IV. Definition and Application Logic of Pressure Switches

What Is a Pressure Switch
A pressure switch is a device that combines pressure sensing with electrical switching functions. When pressure reaches or deviates from a set value, the switch state changes, thereby opening or closing a circuit.

From a functional perspective, a pressure switch can be regarded as a structurally simplified form of a pressure relay.

Operating Characteristics
Key features of pressure switches include:

Simple structure
Intuitive adjustment
Discrete (binary) output signals
No continuous output

Their control logic is typically:

Pressure reaches setpoint → switch actuates
Pressure returns within hysteresis range → state resets

Typical Application Scenarios
Pressure switches are commonly used for:

Pump start/stop control
Simple pressure alarms
Low-cost control systems

In systems with modest accuracy requirements and simple control logic, pressure switches offer high cost-effectiveness.

V. Core Differences Between the Three

1. Different Functional Positioning
Pressure sensor: continuous measurement and data acquisition
Pressure relay: pressure-triggered electrical control and protection
Pressure switch: simple binary control

2. Different Output Signal Forms
Pressure sensor: analog or digital signals
Pressure relay / pressure switch: switching signals

3. Different Control Levels
Pressure sensor: information layer, serving control systems
Pressure relay / switch: execution or protection layer, directly involved in action control

4. Different System Integration Methods
In modern automation systems, common combinations include:

Pressure sensor + PLC / DCS → precise control
Pressure switch / relay → redundant protection or hardware interlocking

VI. Practical Selection Recommendations in Engineering

In real-world engineering, the choice depends on several key questions:

Is continuous, computable pressure data required?
Is remote transmission and system integration needed?
Is it used only for overpressure or underpressure protection?
What are the control logic complexity and cost requirements?

In general:

Process control systems should prioritize pressure sensors
Safety interlocking and protection should prioritize pressure relays or pressure switches
High-reliability systems may use multiple devices simultaneously to form multi-layer protection

VII. Frequently Asked Questions (FAQ)

Can a pressure sensor replace a pressure switch?
When used with a PLC or controller, similar functions can be achieved. However, for hardware-level safety protection, independent pressure switches or relays are still recommended.

Can pressure relays and pressure switches be used together?
Yes, in some applications, but contact type, actuation logic, and electrical compatibility must be clearly defined.

Do modern systems still need pressure switches?
In cost-sensitive systems or simple systems with extremely high reliability requirements, pressure switches remain irreplaceable.

Conclusion

Although pressure sensors, pressure relays, and pressure switches all use pressure as the input variable, they differ fundamentally in functional positioning, signal form, and system role. Pressure sensors focus on precise measurement and data output, while pressure relays and pressure switches emphasize threshold judgment and action control.

Against the backdrop of continuous upgrades in industrial automation, properly distinguishing and combining these three types of devices is a crucial foundation for building safe, stable, and efficient pressure measurement and control systems. Nexisense remains committed to engineering-driven solutions, providing more reliable and scalable pressure measurement and control products for diverse industries.