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.

Lower Range Value

A transmitter is used to measure in range it has two points LRV and URV. The lower or zero side is called the lower range value.
The minimum value a transmitter can measure is called LRV.

If the range is 0 to 100 percent then 0 is LRV (lower range value)
If the range is 30 to 90 then 30 is LRV (lower range value).

Upper Range Value

The upper or higher value a transmitter can measure is called the upper range value (Upper Range Value).
If the range is 0 to 100 percent then 100 is URV (upper range value)
If the range is 0 to 900 then 900 is URV (upper range value)

Span

 The difference between LRV and URV is called the span of a transmitter.
If LRV is 0 and URV is  100 then 100 is span.
If LRV is 30 and URV is 90 then span is = 90-30 =60
If LRV is -30 and URV is 30 then span is 60.

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