Unit 55: Instrumentation and Control principles


Unit 55:

Instrumentation and Control




Unit code:

QCF level:

Credit value:


The aim of this unit is to introduce learners to the principles and practice of instrumentation and control in process industries

Unit abstract

This unit is intended to give learners an appreciation of the principles of industrial instrumentation. The unit will also give learners an understanding of the techniques used in industrial process control and enable them to predict controller settings and make adjustments to achieve stability in such a control system.

Learners will investigate instrumentation systems terminology and the components that make up a system. Learners will then look at where instrumentation systems and controllers are applied in process control schemes. Finally, learners will examine the components of regulating units and their applications.

Learning outcomes

On successful completion of this unit a learner will:

1     Understand instrumentation systems used in process control

2     Understand process control systems and controllers

3       Understand the use of regulating units.

Unit content

1      Understand instrumentation systems used in process control

System terminology: use of correct terminology eg accuracy, error, repeatability, precision, linearity, reliability, reproducibility, sensitivity, resolution, range, span, zero drift, hysteresis

Sensors/transducers: those used to measure pressure eg resistive, strain gauge, inductive, capacitive, semiconductor, ceramic, piezoelectric, linear variable differential transformer (LVDT); level eg conductivity, capacitive, ultrasonic, radar, nucleonic, load cells, radiometric, microwave, hydrostatic, sonar; flow eg ultrasonic, Coriolis, vortex, magnetic, differential pressure; temperature eg resistance, thermocouple, semiconductor, radiation pyrometers; displacement eg diffraction grating, lasers, variable resistance, ceramic, piezoelectric, LVDT

Transmitters/signal converters: current to pressure; pressure to current; microprocessor-based (smart); digital; analogue; optical; wireless

Transmission medium: pneumatic; hydraulic; electrical; fibre-optic; wireless

Signal conditioners: industry-standard devices; industry-standard signal ranges and conversion between them

2      Understand process control systems and controllers

Need for process control: quality; safety; consistency of product; optimum plant performance; human limitations; efficiency; cost; environmental

Process controller terminology: deviation; range; span; absolute deviation; control effect; set point; process variable; manipulated variable; measured variable; bumpless transfer; process variable tracking; direct and reverse acting; offset; on-off control; two step control; cycling; three-term control (proportional band, gain, proportional, proportional with integral, proportional with integral and derivative, proportional with derivative)

System terminology: distance velocity lags; transfer lags; multiple transfer lags; capacity; resistance; dead time; reaction rate; inherent regulation; open loop; closed loop; load; supply; static gain; dynamic gain; stability; loop gain

Tuning techniques: Zeigler-Nichols; continuous cycling; reaction curve; ¼ decay methods; tuning for no overshoot on start-up; tuning for some overshoot on start-up

Represent systems using: P and I diagrams; loop diagrams; wiring diagrams; constructing and using diagrams to appropriate standards

3      Understand the use of regulating units

Regulating unit terminology: body; trim; plug guide and seat; valve; stem; bonnet; packing gland; yoke; actuator; motor; stroke; direct and reverse action; air fail action; repeatability; CV; turndown; flow characteristics; linear, equal percentage, quick-opening, modified parabolic, split range

Characteristics of regulating units: dampers; power cylinders; louvres; valve positioners; valves (globe, ball, diaphragm, gate, double seated, 3-way, solenoid, split bodied, butterfly)

Learning outcomes and assessment criteria

Learning outcomes
Assessment criteria for pass

On successful completion of
The learner can:

this unit a learner will:

LO1 Understand instrumentation
describe the terminology used in process

systems used in process


evaluate a range of sensors and transducers with

reference to manufacturers’ terminology

explain the construction and operation of modern

sensors used to measure pressure, level, temperature

and flow

describe typical applications for the sensors examined

explain signal conditioning and transmission

LO2 Understand process control
explain the need for process control

systems and controllers
describe process control terminology

determine the medium required for successful


name sensors, conditioners and display units for a range

of specific purposes

evaluate tuning techniques

describe the control actions required for different


represent systems using standard diagrams

LO3 Understand the use of
identify the main parts of a regulating unit

regulating units
evaluate a regulating unit with reference to standard

terminology, including manufacturers’ specifications

select the plug characteristics required for a specified


describe the characteristics of a range of regulating


describe the use of valve positioners.


This unit is designed to be stand alone but has links to Unit 48: Analytical and Chemical Composition Measurement and Unit 46: Plant Process and Control.

Essential requirements

There are no essential requirements for this unit.

Employer engagement and vocational contexts

Visits to industrial installations will be of value to supplement learning activities and provide learners with a perspective on scale and application of process instrumentation and process control hardware.