What are the types of automation instruments?

Many people new to instrumentation feel overwhelmed when they first see the multitude of instruments on-site.

With names like pressure gauges, thermometers, flow meters, level meters, control valves, transmitters, PLCs, DCSs, alarms, and analyzers, it is easy to get confused about how to categorize them.

In reality, automation instrumentation is not that complex. From an on-site perspective, it primarily revolves around a single objective: measuring process parameters, transmitting them, displaying them, and controlling them—while also providing alarms and equipment protection when necessary.

Based on function, automation instruments can generally be divided into six major categories:

Field measurement instruments, field control instruments, field actuators, display and recording instruments, regulating/control instruments, and specialized measurement instruments.

Automation instruments are not merely devices for "reading values."

In a complete control loop, measurement instruments collect field parameters, the controller performs analysis and calculations, the actuator executes actions, and display/recording instruments present data to operators and store it for the system.

Take a temperature control loop, for example: a thermocouple or resistance thermometer measures the temperature; a temperature transmitter sends the signal to a PLC or DCS; the controller calculates the required response and outputs a signal; and finally, a control valve adjusts the flow of steam, hot water, or cooling water.

Therefore, instruments do not exist in isolation. When troubleshooting on-site, one cannot simply focus on a single gauge; the instrument must be analyzed within the context of the entire control loop.

Field measurement instruments serve as the data source for automation systems. If measurements are inaccurate, subsequent display, recording, and control functions will all be compromised.

Common field measurement instruments include those for measuring liquid level, pressure, temperature, and flow, as well as gas and liquid analysis instruments.

Level instruments are used to measure liquid levels, material levels, or interface positions in storage tanks, towers, reactors, and silos. Common types include magnetic flap level gauges, float-type level gauges, radar level meters, ultrasonic level meters, capacitive level meters, differential pressure level meters, and level switches.

Pressure instruments are used to measure pressure, differential pressure, or vacuum levels in pipelines, vessels, and equipment. Common types include pressure gauges, pressure transmitters, differential pressure transmitters, pressure sensors, and pressure switches. Pressure gauges are primarily used for on-site readings, pressure transmitters for remote signal transmission, and pressure switches for alarms and interlocking functions.

Temperature instruments measure the temperature of a medium or the surface of equipment. Common types include bimetallic thermometers, glass thermometers, thermocouples, resistance temperature detectors (RTDs), temperature transmitters, temperature switches, and infrared thermometers. Thermocouples are suitable for high-temperature applications, while RTDs are better suited for low-to-medium temperature measurements.

Flow instruments measure the flow rate of gases, liquids, or steam. Common types include electromagnetic, vortex, turbine, ultrasonic, mass, orifice plate, and variable area (rotameter) flow meters. Selection requires careful verification of the medium, flow range, pressure and temperature, viscosity, conductivity, and straight-pipe run requirements.

Gas analysis instruments detect gas composition and concentration. Common types on the market include oxygen analyzers, combustible gas detectors, toxic gas detectors, infrared gas analyzers, thermal conductivity gas analyzers, laser gas analyzers, and gas chromatographs.

Liquid analysis instruments measure parameters such as pH, conductivity, turbidity, concentration, density, viscosity, dissolved oxygen, COD, and ammonia nitrogen. These instruments require relatively high maintenance; cleaning, calibration, and inspection of sampling lines are essential tasks.

Field control instruments are responsible for on-site signal processing, signal conversion, and local control.

Common types include pneumatic, electric, and hydraulic control instruments.

Pneumatic control instruments use compressed air as their power source; they have relatively simple structures and remain valuable in explosion-proof applications. Electric control instruments use electrical signals, making them easier to integrate with PLCs, DCSs, and supervisory computers. Hydraulic control instruments deliver high output force and are typically used in heavy-load or high-thrust applications.

Key factors when selecting these instruments include input/output signals, control accuracy, response speed, power supply conditions, explosion-proof ratings, the field environment, and system interfaces.

Field actuators are the components in a control loop that actually perform the physical action.

After the control system issues a command, the actuator executes it by adjusting valves, dampers, sluice gates, or other mechanical structures to alter process conditions such as flow rate, pressure, liquid level, or temperature. Common actuating devices include pneumatic, electric, and hydraulic control valves; shut-off valves; butterfly valves; ball valves; gate valves; actuators; valve positioners; and smart valve positioners.

During field maintenance, one must look beyond whether the actuator can simply open and close. It is also necessary to check for smooth operation, accurate positioning, normal feedback signals, appropriate response times, and whether the fail-safe position meets process requirements.

Many control fluctuations stem not from controller parameter settings, but from issues such as valve sticking, inaccurate positioners, unstable air supply pressure, or sluggish actuator response.

Display and recording instruments show field parameters, equipment status, and alarm information, while also storing critical data.

Common examples include digital displays, analog displays, display-controllers, multi-point scanners, paper-based recorders, paperless recorders, touch-screen HMIs, operator station terminals, and audible/visual alarm devices.

Digital displays are suitable for single-point parameter monitoring. Display-controllers can incorporate alarm and basic control functions. Paperless recorders are ideal for tracking trends in parameters such as temperature, pressure, flow, and level. HMIs and operator stations are used for visual interfaces, parameter configuration, alarm monitoring, and equipment operation.

Key selection criteria include input signals, display methods, recording intervals, storage capacity, communication interfaces, alarm outputs, and power-loss protection.

Regulation and control instruments serve as the "brain" of the automation system, handling automatic regulation, logic control, sequence control, and safety protection.

Common field devices include PID controllers, single-loop and multi-loop regulators, PLCs, DCSs, FCSs, ESDs, SISs, and industrial computers.

PID controllers are typically used for regulating continuous parameters like temperature, pressure, flow, and level. PLCs are commonly employed for equipment start/stop operations, sequence control, interlocking protection, and discrete (on/off) signal processing. DCSs are suitable for the centralized monitoring and distributed control of large-scale continuous production units. SISs and ESDs are primarily used for safety interlocking and emergency shutdowns; they should not be treated merely as standard control systems.

Selection factors include the controlled process, I/O point count, control strategy, communication protocols, redundancy requirements, safety integrity levels (SIL), alarm management, and long-term maintenance capabilities.

Specialized measuring instruments are primarily used for detecting unique parameters, monitoring equipment health, and performing field calibration. Common examples include shaft vibration monitors, shaft displacement monitors, tachometers, weighing instruments, thickness gauges, signal calibrators, flame detectors, online moisture analyzers, online density meters, and equipment condition monitoring instruments.

Instruments for measuring shaft vibration, shaft displacement, and rotational speed are typically used on rotating equipment such as compressors, steam turbines, fans, and pumps. Weighing instruments are used in silos, belt scales, and batching or packaging systems. Thickness gauges are used to measure the thickness of plates, pipes, and vessel walls, as well as coating thickness. Flame detectors are commonly used for combustion protection in boilers, heating furnaces, and incinerators.

Selecting these instruments requires considering factors such as equipment type, the parameter being measured, installation location, response speed, output signals, and protection requirements.

When selecting automation instruments, one should not focus solely on the model number and price.

First, identify the parameter to be measured; then, determine the measurement range, accuracy, materials of construction, installation method, power supply, and output signal.

If the medium is corrosive, the wetted parts must be made of the appropriate material. For high-temperature media, consider heat dissipation, condensation, isolation, and thermowells. If the medium is prone to crystallization or clogging, or contains impurities, consider using diaphragms, purging, flushing, or sample pre-treatment. In areas with significant vibration, consider bracket mounting and installation location.

Once the instrument is put into service, regularly check the zero point, span, wiring, power supply, grounding, communication, alarm outputs, and interlock status.

When troubleshooting on-site, follow this sequence: check the power supply, then the signal, then the wiring, then the parameter settings, and finally the sensing element and process conditions.

The classification of automation instruments is not difficult to remember, as each type performs a specific function.

For instance, measuring instruments collect data; control instruments handle signal processing; actuators perform physical actions in the field; display and recording instruments show and save data; regulating and control instruments manage control and interlocks; and specialized measuring instruments detect unique parameters.

When actually on-site, do not view an instrument as an isolated device. Consider it within the context of the entire control loop, and many problems will become easier to diagnose.