Instrumentation terms

Instrumentation terms and definition

instrumentation

Instrumentation is a branch of science which deals with the measurement and control of process variables in a process.

Process

It is defined as the adoption of series of steps or methods to acquire a predefined result.

In other words, it can be defined as a physical system we want to measure and control by implementing instrumentation devices

Examples of process are:

boilers

oil refinery

chemical plants

power plants etc

Measurement

It is a way to associate physical quantities with numbers, so by measurement we can find the number which shows us the amount or quantity of an entity.

Process Variable

It is a variable in the process that we are attempting to maintain, measure or control in a process.

process variable maybe

pressure

temperature

flow

level

quality

vibration etc.

Setpoint

In-process control system, the setpoint is a point we set for a process value to be achieved this is a value we desire to maintain a process variable.

suppose we want to maintain the temperature to 100 C°, then 100 is our set point.

Controller

It is an instrument or device which receives a process variable value measured in the process by a transmitter or sensor, compares this process variable with a set point, and then at output give a control signal to final control equipment to achieve the desired value of process variable.

Primary sensor elements

any sensor which senses or measures a process variable (PV) directly from the process and shows in analog value like an electrical signal or mechanical movement or resistance etc.

For different process variables, different PSE is used.

pressure: Bourdon tube, diaphragm, strain gauge, etc

Temperature: thermometer, thermocouple, RTD, etc.

Final control element

A device that affects directly the process variable, FCE receives an output signal of the controller and directly changes the PV, e.g a VFD (Variable Frequency Drive), control valve, heater, etc.

Transducer

A device that converts one form of energy into another form of energy, same like PSE (primary sensing elements)

A transducer is also defined as a device that converts a standard signal into another standard signal, like an I/ P converter.

Transmitter

A device that converts signal from a PSE (primary sensing elements) to a standard signal of 4 to 20 mA or 0 to 5 volt and transmits this standard signal to a display or DCS or plc.

examples are pressure transmitter

temperature transmitter

vibration transmitter etc.

Manipulated variable

This is the output signal of a controller to the FCE (final control equipment).

because it manipulates the process variable to setpoint by changing the position or function of FCE (final control equipment).

Automatic mode

In this mode, a controller-generated output signal controls the final control equipment to maintain the process variable to the desired set point.

Manual mode

In this mode, automatic mode is bypassed and human control the process variable in his way. In manual mode controller's decision-making ability is off.