Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: Second Term
Week: 5
Age: 15 years
Duration: 45 minutes
Subject: Refrigeration And Air Conditioning
Curriculum Theme: Trade
Previous Lesson: Pressure And Its Measurement: Bourdon Gauge and Pressure Interpretation.
Topic: Pressure and its measurement
Subject Matter: Practical pressure measurement, reading interpretation, safety in measurement

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Instructional Materials

The teacher will teach this lesson with the aid of:

A functional pressure tester.
Various pressure gauges (e.g., compound gauge, high-pressure gauge).
Hoses for connecting gauges.
Safety gloves and safety goggles.
A chart showing different units of pressure.

Rationale for the Lesson

This lesson helps pupils understand how to measure pressure, which is a fundamental skill in refrigeration and air conditioning. Knowing how to accurately measure and interpret pressure readings enables pupils to diagnose system faults and ensure efficient operation, thereby preparing them for practical applications in the trade.

Prerequisite/Previous Knowledge

Pupils should have a basic understanding of force, area, and simple tools used in workshops.

Lesson Content/Board Summary

Pressure and its Measurement

Definition of Pressure

Pressure is defined as the force applied perpendicular to the surface of an object per unit area over which that force is distributed.

The standard formula for pressure is: Pressure = Force / Area.

Units of Pressure

The following are common units of pressure:

Pascals (Pa) – the SI unit.
Pounds per square inch (psi).
Bars (bar).
Kilopascals (kPa).
Millimeters of mercury (mmHg).
Inches of mercury (inHg).

Practical Pressure Measurement

Pressure is measured using instruments called pressure gauges. These gauges are connected to a system to display the pressure inside.

Steps for practical pressure measurement:

Ensure the system is safe and depressurized if necessary before connecting.
Select the appropriate pressure gauge for the expected pressure range.
Connect the gauge hose securely to the service port of the system.
Open the valve to allow system pressure to reach the gauge.
Observe the needle on the gauge to read the pressure value.
Close the valve and disconnect the gauge carefully after measurement.

Reading and Interpretation of Pressure Gauges

Pressure gauges have a dial with markings indicating pressure units and values. It is important to read the gauge at eye level to avoid parallax error.

How to interpret readings:

Identify the unit of measurement indicated on the gauge (e.g., psi, kPa, bar).
Note the major and minor divisions on the dial.
Read the value indicated by the needle, estimating between minor divisions if necessary.
For compound gauges, differentiate between positive pressure (above atmospheric) and vacuum (below atmospheric).

Safety in Pressure Measurement

Working with pressurized systems requires strict adherence to safety rules to prevent accidents.

The following are important safety precautions:

Always wear personal protective equipment (PPE), such as safety gloves and safety goggles.
Ensure hoses and fittings are in good condition and correctly connected.
Never exceed the maximum pressure rating of the gauge or the system components.
Avoid quick disconnections of hoses from pressurized systems.
Handle refrigerants and pressurized gases with care, as they can cause injury.
Work in a well-ventilated area.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and asks them what they understand by “pressure” in everyday life (e.g., pressure in a car tyre, water pressure). The teacher then introduces the topic of pressure measurement in refrigeration and air conditioning.
Pupils’ Activity: Pupils respond to the teacher’s questions and listen attentively to the introduction.
Learning Point: Pupils are introduced to the concept of pressure and its relevance.

Step 2: Definition and Units of Pressure

Time: 8 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines pressure, states its formula, and explains various units of pressure (Pa, psi, bar, etc.) using a chart or whiteboard. The teacher emphasizes the importance of understanding these units.
Pupils’ Activity: Pupils listen, take notes, and ask questions for clarification regarding the definitions and units.
Learning Point: Pupils understand the definition of pressure and can identify its common units.

Step 3: Types of Pressure Gauges

Time: 7 minutes
Teaching Skill: Illustration/Classification
Teacher’s Activity: The teacher briefly introduces different types of pressure gauges relevant to refrigeration (e.g., high-pressure gauge, low-pressure/compound gauge) and explains their basic function, showing physical examples.
Pupils’ Activity: Pupils observe the gauges, ask questions about their uses, and note down the types.
Learning Point: Pupils can identify different types of pressure gauges and their basic functions.

Step 4: Practical Demonstration of Pressure Measurement

Time: 10 minutes
Teaching Skill: Demonstration/Practical Application
Teacher’s Activity: The teacher demonstrates how to connect a pressure gauge to a simulated system (or a safe, low-pressure system if available) following the correct steps, while emphasizing safety precautions. The teacher shows how to open and close valves and observe the needle.
Pupils’ Activity: Pupils observe the demonstration carefully, noting the connection procedures and safety measures. They may ask questions about the process.
Learning Point: Pupils observe the practical steps involved in measuring pressure.

Step 5: Reading and Interpretation of Pressure Gauges

Time: 7 minutes
Teaching Skill: Interpretation/Analysis
Teacher’s Activity: The teacher guides pupils on how to correctly read the values from the pressure gauge demonstrated, explaining how to avoid parallax error and interpret the different scales (e.g., psi and kPa on the same gauge). The teacher might use a large diagram of a gauge.
Pupils’ Activity: Pupils practice reading values from the gauge or diagram, asking for clarification on interpretation.
Learning Point: Pupils learn to accurately read and interpret pressure gauge values.

Step 6: Safety in Pressure Measurement

Time: 3 minutes
Teaching Skill: Emphasizing/Cautioning
Teacher’s Activity: The teacher lists and explains the critical safety precautions to be observed when performing pressure measurements, reiterating the use of PPE and proper handling of equipment.
Pupils’ Activity: Pupils listen and note the safety precautions, understanding their importance.
Learning Point: Pupils understand the essential safety measures for pressure measurement.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define pressure and state two common units.
List three important steps when connecting a pressure gauge to a system.
Mention two safety precautions to observe during pressure measurement.
How do you avoid parallax error when reading a pressure gauge?

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 2 minutes
Teaching Skill: Summarization/Assignment
Teacher’s Activity: The teacher summarizes the key points of the lesson, reiterating the importance of accurate and safe pressure measurement. The teacher assigns homework: “Draw a typical pressure gauge and label its parts, indicating at least two units of measurement.”
Pupils’ Activity: Pupils listen to the summary and copy down the homework.
Learning Point: Pupils recall the main aspects of the lesson and have an assignment to reinforce learning.

Lesson Keywords

Pressure – Force applied perpendicular to a surface per unit area.
Pressure Gauge – An instrument used to measure the pressure of a fluid or gas.
Psi – Pounds per square inch, a common unit of pressure.
Pascal (Pa) – The SI unit of pressure.
Parallax Error – An error in reading an instrument due to viewing it from an angle instead of directly.
Compound Gauge – A type of pressure gauge that can measure both positive pressure and vacuum.

Differentiation

For pupils who grasp concepts quickly, they can be tasked with identifying different types of pressure gauges and their specific applications. For pupils needing more support, the teacher will provide closer supervision during the practical demonstration, allowing them to practice connecting and reading gauges multiple times with assistance.

Note for teachers using this lesson plan

Ensure all demonstration equipment is in good working order and that safety procedures are strictly followed. If a live system is unavailable, use diagrams, videos, or a non-pressurized setup to simulate the practical aspects. Emphasize the real-world consequences of incorrect measurements or unsafe practices.