What is a Secondary Instrument?

Before understanding secondary instruments, let’s clarify what absolute instruments are.

Absolute instruments usually refer to sensors that are used to directly measure parameters of the object being measured, such as pressure, liquid level, flow, and temperature. These sensors convert the measurements into measurable signals or standard signals, which typically include electrical signals, pneumatic signals, or hydraulic signals. These signals are then transmitted to subsequent instruments for further analysis, display, and recording; the latter are referred to as secondary instruments.

The main function of secondary instruments is to receive the output signals from absolute instruments and provide a visual representation of the measured parameters, displayed in digital, analog, or other forms. They also record and analyze the data or adjust process parameters based on preset values.

Absolute instruments are usually installed directly on process pipelines or equipment, or near measurement points where they come into contact with the measured medium. They measure and display process parameters or send parameter signals to secondary instruments.

Secondary instruments, on the other hand, are typically installed further away from the process pipeline or equipment, often on control panels or instrument dashboards, and do not come into direct contact with the measured medium. They receive the output signals from absolute instruments for further processing, displaying, or recording. Secondary instruments generally provide higher accuracy and stability, offering more precise and clearer measurement results and various forms of display and recording. They usually require a power source to operate and receive signals from absolute instruments. As a crucial component of automatic detection devices, secondary instruments are widely used in industrial automation, process control, environmental monitoring, and other fields, playing an important role in improving production efficiency, ensuring product quality, and achieving automated control.

Secondary instruments are one of the components of automatic detection devices, forming a complete measurement system along with absolute instruments. There are many types of secondary instruments, each with specific functions and purposes. Here are some common types of secondary instruments:

Display Instruments:

• Indicating Instruments: For example, indicating instruments in electric unit combination instruments, used to display measured values.
• Recorders: Including paper and paperless recorders for recording and storing measurement data.

Accumulating Instruments:

• Flow Accumulators: Used to measure and accumulate the flow of fluids.
• Energy Accumulators: Used to measure and accumulate energy consumption.

Control Instruments:

• Regulators: Output control signals to adjust process parameters based on the deviation between setpoint and measured values.
• Programmable Logic Controllers (PLC): Used to implement complex control logic and sequential control.

Signal Conversion Instruments:

• Transmitters: Convert measured values into standard signals (such as 4-20 mA or 0-10 V) for remote transmission and processing.
• Converters: Used to convert one type of signal to another, such as voltage/current converters or frequency/voltage converters.

Analytical Instruments:

• Online Analyzers: Used for real-time monitoring of certain components or properties in process media.
• Laboratory Analyzers: Used for precise analysis and measurement in laboratory settings.

Alarm and Safety Instruments:

• Alarm Devices: Emit alarm signals when measured values exceed set limits.
• Safety Instrumented Systems (SIS): Monitor and protect production processes, automatically taking measures when dangerous situations are detected.

Data Acquisition and Monitoring Systems (SCADA):

• SCADA Systems: Used for real-time collection, processing, display, and storage of various data from the field, enabling remote monitoring and control of production processes.

Other Types of Secondary Instruments:

• Calculators: Instruments used for performing mathematical calculations.
• Power Factor Meters: Used to measure the power factor of electrical systems.
• Frequency Meters: Used to measure the frequency of alternating current.
• Synchronous Indicators: Used to indicate the synchronous status of two generators.
• Power Meters: Used to measure power in alternating current circuits.

These secondary instruments play a vital role in industrial automation, process control, environmental monitoring, laboratory analysis, and other fields. Choosing and using the appropriate secondary instruments is crucial for ensuring the stability and safety of production processes.