Understanding the Basics of Process Control Instrumentation and Their Classification

Process control instruments provide the information needed to ensure that a manufacturing process is carried out under the desired conditions. More than just monitoring what is happening in a circle, they can also regulate and control it. While most process control instrumentation falls into the category of process sensor, some subclasses may be more appropriate for the application at hand.

Most process instruments fall into two basic categories—control instruments and measurement instruments. As the names suggest, these two categories serve different purposes and offer advantages in specific manufacturing processes.

Control Instruments

Control instruments are used to regulate an operation or change its state. For example, a control instrument may be used to speed up, slow down, stop or reverse the direction of a process. Provide information, such as actual or target values, or calculate some weight, and actuate valves, transfer switches, and motors. Some significant examples of control instruments are:

• Control Valves: Used to actuate an operation, such as flow control. Control valves may be a quick-acting (single-use) type, typically made of plastic, or a slow-acting (reusable) type, typically cast iron. 
• Electric Motors: Used to drive mechanical equipment. Some typical types are induction motors and permanent-magnet synchronous motors. 
• Limit Switches: Used to sense position and trigger action to reach a specified position or condition.
• Pumps: Pumps are used to fill containers, such as bottles and tanks, and are often used in the fluid or process industries. Large pumps are high-speed, medium, low, and variable speed pumps.

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Measurement Instruments

Measurement instruments are used to collect information about a process. They don’t change the operation as much as control instruments do, but they may still be used to determine the sequence and rate of processing and monitor key process parameters. Also, they are used to indicate process conditions to help synchronize other control or measurement instruments. Some typical measurement instruments are:

• Temperature Sensors: Used to measure and indicate temperature. Temperature sensors are typically in the form of thermocouples, resistance temperature detectors, thermistors, and RTDs.
• Water Analyzer: Used to measure water composition (water content) in a liquid or gas. Also helpful in removing mineral contaminants in water from the internal and external atmosphere.
• Gas Analyzer: This analyzer measures various gases in an atmosphere of varying composition and concentration. One of the most commonly used types is the infrared analyzer.
• Pressure Gauges: This indicates pressure, typically atmospheric pressure or pressure at a specified location. Typical types include Bourdon tubes, mercury column, Bourdon tube with manometer, and also gauge barometer.

It should be clearly understood that process instrumentation comes in many different shapes and sizes. Not every process control equipment is just what it seems or may serve a slightly different function than its name would imply. An instrument’s purpose must be understood and followed by the right person at the right time.