Class: Senior Secondary School 1 (SS1 / SSS 1)
Term: Second Term
Week: 4
Age: 15 years
Duration: 45 minutes
Subject: Refrigeration And Air Conditioning
Curriculum Theme: Trade
Previous Lesson: Pressure And Its Measurement: Barometer and Manometer Operations.
Topic: Pressure and its measurement, Pressure measuring devices
Subject Matter: Bourdon gauge, pressure interpretation, gauge reading techniques

Specific Objectives

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

Cognitive Domain:

Define pressure and state its units.
Identify the main parts of a Bourdon gauge.
Explain the working principle of a Bourdon gauge.
Describe how to interpret pressure readings on a gauge.

Affective Domain:

Appreciate the importance of accurate pressure measurement in refrigeration and air conditioning systems.
Show interest in learning about various pressure measuring devices.

Psychomotor Domain:

Demonstrate how to read pressure values using a Bourdon gauge.
Apply proper techniques for accurate gauge reading.

Collaborate effectively with peers during practical demonstrations involving pressure gauges.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum (Senior Secondary Education)
State Unified Scheme of Work for Refrigeration and Air Conditioning
Refrigeration and Air Conditioning Technology by Billy C. Langley

Instructional Materials

The teacher will teach this lesson with the aid of:

A functional Bourdon gauge
A small compressor
Hose connectors
A dial chart showing different pressure units
Whiteboard and markers

Rationale for the Lesson

This lesson helps pupils understand pressure, which is a basic concept in refrigeration and air conditioning. Knowing how to measure and interpret pressure accurately is important for diagnosing system faults and ensuring proper operation of cooling equipment in everyday life.

Prerequisite/Previous Knowledge

Pupils should have a basic understanding of force and area, and simple measurement concepts.

Lesson Content/Board Summary

Pressure and its Measurement

What is Pressure?

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

Common units of pressure include:

Pounds per Square Inch (PSI)
Bar
Kilopascal (kPa)
Inches of Mercury (in Hg)

Pressure Measuring Devices

Various devices are used to measure pressure, depending on the application and range of pressure. One common device in refrigeration and air conditioning is the Bourdon gauge.

The Bourdon Gauge

A Bourdon gauge is a mechanical device used for measuring the pressure of liquids or gases. It is commonly used in refrigeration and air conditioning systems due to its simplicity and reliability.

The main parts of a Bourdon gauge include:

Bourdon Tube (C-shaped tube): This is a flattened, curved tube that forms the core of the gauge.
Linkage: A mechanism that connects the Bourdon tube to the pointer.
Pointer: An indicator that moves across the dial to show the pressure reading.
Dial: The face of the gauge with a calibrated scale indicating pressure values.
Socket/Inlet: The connection point for the fluid or gas whose pressure is to be measured.

Working Principle of a Bourdon Gauge:

When pressure is applied through the inlet, the fluid or gas enters the Bourdon tube. The internal pressure causes the C-shaped tube to try and straighten out. This straightening motion is transmitted through a linkage mechanism to the pointer, causing it to move across the calibrated dial and indicate the pressure value.

Pressure Interpretation

Pressure interpretation involves correctly understanding the readings displayed on the gauge. Gauges often have multiple scales for different units (e.g., PSI and Bar). It is important to read the correct scale relevant to the system being measured.

Most gauges in HVACR measure “gauge pressure,” which is the pressure relative to the surrounding atmospheric pressure.

Gauge Reading Techniques

To obtain accurate pressure readings from a Bourdon gauge, the following techniques should be observed:

Ensure Proper Connection: Connect the gauge securely to the pressure point of the system.
Eye Level Reading: View the pointer directly at eye level to avoid parallax error (distortion caused by viewing from an angle).
Tap Lightly: Sometimes, a light tap on the gauge casing can help the pointer settle accurately, especially if there are vibrations.
Identify Units: Clearly identify the unit of measurement being used (e.g., PSI, Bar) on the dial.
Read the Correct Scale: If the gauge has multiple concentric scales, ensure you are reading the scale appropriate for the system and unit.
Allow to Stabilize: Give the gauge a moment to stabilize before taking the final reading.

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 questions about everyday experiences related to pressure, such as inflating a tyre or opening a carbonated drink. The teacher then introduces the topic: Pressure and its measurement.
Pupils’ Activity: Pupils respond to questions and listen attentively.
Learning Point: Pupils recall prior knowledge and are introduced to the lesson topic.

Step 2: Explanation of Pressure

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines pressure as force per unit area and lists common units of pressure (PSI, Bar, kPa) using the whiteboard. The teacher explains why understanding pressure is important in refrigeration and air conditioning.
Pupils’ Activity: Pupils listen, take notes, and ask questions for clarification.
Learning Point: Pupils understand the definition and units of pressure.

Step 3: Introduction to Pressure Measuring Devices

Time: 5 minutes
Teaching Skill: Introduction/Categorisation
Teacher’s Activity: The teacher explains that various devices are used to measure pressure and introduces the Bourdon gauge as a primary tool in their field of study.
Pupils’ Activity: Pupils listen and prepare for the detailed explanation.
Learning Point: Pupils are aware of the existence and relevance of pressure measuring devices.

Step 4: Explanation of Bourdon Gauge

Time: 10 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher presents a Bourdon gauge, identifies its main parts (Bourdon tube, linkage, pointer, dial, inlet), and explains the working principle: how pressure causes the tube to straighten and move the pointer. The teacher uses the dial chart to show different scales.
Pupils’ Activity: Pupils observe the Bourdon gauge, identify its parts, and listen to the explanation of its working principle.
Learning Point: Pupils identify parts of a Bourdon gauge and understand how it measures pressure.

Step 5: Pressure Interpretation and Reading Techniques

Time: 8 minutes
Teaching Skill: Demonstration/Instruction
Teacher’s Activity: The teacher explains how to interpret pressure readings, emphasizing the importance of reading the correct scale and unit. The teacher demonstrates proper gauge reading techniques, such as viewing at eye level, tapping lightly, and ensuring a stable reading.
Pupils’ Activity: Pupils pay close attention to the demonstration and practice mentally.
Learning Point: Pupils learn how to correctly interpret and read pressure values from a gauge.

Step 6: Practical Application/Demonstration

Time: 5 minutes
Teaching Skill: Practical Application/Demonstration
Teacher’s Activity: The teacher connects the Bourdon gauge to a small compressor (or a simulated pressure source) using hose connectors and demonstrates how the pointer moves as pressure builds up. Pupils are encouraged to observe the readings and practice reading them.
Pupils’ Activity: Pupils observe the practical demonstration and attempt to read the gauge.
Learning Point: Pupils see the Bourdon gauge in action and apply reading techniques.

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 mention two units of pressure.
List three main parts of a Bourdon gauge.
Explain briefly how a Bourdon gauge works.
State two important techniques for accurate gauge reading.

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
Teacher’s Activity: The teacher summarizes the key points of the lesson, reiterating the importance of pressure measurement in refrigeration and air conditioning. The teacher assigns homework: “Draw a Bourdon gauge and label its parts.”
Pupils’ Activity: Pupils listen to the summary and copy the homework assignment.
Learning Point: Pupils consolidate their understanding of the lesson.

Lesson Keywords

Pressure – Force applied perpendicular to a surface per unit area.
Bourdon Gauge – A mechanical device used to measure the pressure of fluids or gases.
PSI – Pounds per Square Inch, a unit of pressure.
Bar – A metric unit of pressure.
Dial – The face of a gauge with a calibrated scale.
Parallax Error – An error in reading an instrument due to viewing it from an angle.

Differentiation

For pupils who grasp concepts quickly, the teacher can challenge them to research other types of pressure measuring devices (e.g., manometers, digital gauges). For pupils who require more support, the teacher can provide additional one-on-one guidance during the practical demonstration and simplify explanations of the Bourdon gauge’s working principle.

Note for teachers using this lesson plan

Ensure the Bourdon gauge used for demonstration is in good working condition. Emphasize safety precautions when working with pressurized systems, even with a small compressor. Encourage active participation and hands-on interaction where possible to enhance learning.