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```AURT225667A
USE ANDIIIMAINTAIN
MEASURING
EQUIPMENT
– PART 2
AUR30405 CERTIFICATE
IN AUTOMOTIVE
MECHANICAL
TECHNOLOGY
1
AURT225667A
Use and maintain
measuring equipment
– Part 2
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Temperature
Heat = a form of energy.
All objects contain heat.
the same amount of heat,
the smaller object ends up
hotter.
20oC
+
20oC
hot
same
amount
of heat
hotter
Temperature = the intensity of heat.
These two objects are at the same
temperature, but the bigger one
must have more heat in it to be at
that temperature.
20oC
more heat
+
same
amount
of heat
less heat
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Safety
Take care when measuring temperature!
• Don’t touch hot objects or liquids!
• Wear a face mask or safety glasses
• Wear skin protection, like gloves and overalls.
Hot metal can burn your skin.
Hot liquids can splash and burn your face and eyes.
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Temperature scales
Celsius temperature scale = divided into 100 degrees.
• Freezing point of water = 0 degrees celsius (0o C)
• Boiling point of water = 100 degrees Celsius (100o C).
Temperatures above and below this range can also be measured.
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Thermometers
The most common thermometers used for automotive work are:
Digital
thermometer
Dial-type
thermometer
Liquid-in-glass
thermometer
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Liquid-in-glass thermometer
The liquid in this thermometer expands as it gets
hotter.
The height of the liquid shows the temperature.
What temperature is it here?
• Good for measuring liquid or gas temperatures
eg A/C systems or cooling systems
• Not so good for measuring the temperature of
solid objects.
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Dial-type thermometer
• A long thin metal probe senses the temperature
• Probe transfers heat to mechanism of
thermometer
• Pointer rotates around temperature scale and
shows temperature.
This thermometer shows a temperature of 90oC.
oC
• Good for measuring temperature of gases and
liquids
• If it has a wide, flat end on the probe, it can also
be used on solid objects.
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Digital thermometer
Shows the temperature on an electronic display.
There are two main types:
1. Contact digital thermometer
Place the measuring probe, or tip, in contact
with the item you want to measure.
2. Non-contact digital thermometer
Senses how much infra-red radiation (heat
!
Never look directly at the laser beam!
Never point it at another person’s eyes!
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Caring for Thermometers
Treat with care to maintain accuracy:
• Store glass and dial thermometers in
protective cases.
• Don’t drop or knock them.
• Clean plastic parts with a dry cloth.
• Check the calibration (accuracy) regularly.
• Replace the batteries in digital thermometers
regularly..
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Checking accuracy of thermometers
Test liquid-in-glass, dial-type, and contact digital
thermometers by:
• Measure the temperature of an
ice and water mixture - should be
0oC.
• Measure the temperature of
boiling water - should be 100oC
Point a non-contact digital thermometer at the surface of
the water for an approximate check of accuracy.
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Measuring pressure
Pressure = a force pushing on a surface or area.
This can be written as:
Pressure = Force
Area
OR
P= F
A
OR
P=F/A
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Units of pressure
• Area - measured in square metres = m2
Pressure = Force
• Force - measured in newtons = N
Area
• Pressure - measured in Pascals = N/m2 = Pa
1 N is a very small force acting on a
everyday use.
large area - not very practical for
So we use 1000 Pascals as the everyday unit of pressure.
It is called the kiloPascal or kPa.
Example: Average car tyre pressure is about
200,000 Pa
OR
200 kPa
Tyre Pressure
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SAFETY
!
TAKE CARE!
Measuring pressure can be dangerous. Always
relieve the pressure in a system before
connecting a pressure gauge.
• Leaks can spray liquids or gases into your eyes
• Leaks of flammable liquids can cause a fire.
!
Fuel Pressure
Hot cooling systems are especially
dangerous!
• Always wear protective equipment - face
• Keep a fire extinguisher close by.
Transmission Oil Pressure
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Pressure gauges
The most common types are:
Conventional gauges - have a circular scale with a
pointer showing pressure.
Choose a gauge which will display the measured
pressure somewhere near mid-scale on the gauge.
Digital gauges - have an electronic
display. Must be handled carefully.
The maximum pressure range is written
on the gauge face.
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Gauge pressure
Atmospheric pressure = pressure of the air all around us.
Almost all gauges read zero (0 kPa) when they are not
connected to anything, even though they are open to
atmospheric pressure.
This is gauge pressure = the pressure above atmospheric
pressure.
Most vehicle specifications use gauge pressure.
These gauges show gauge pressure:
• Tyre pressure
• Oil pressure
• Cooling system pressure
• Fuel pressure.
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Absolute pressure
Absolute pressure gauges start at
absolutely zero = there is no pressure at
all.
An absolute pressure gauge not
connected to anything shows
atmospheric pressure – the pointer is
above the zero mark.
Atmospheric pressure = 100 kPa absolute.
Absolute pressure = 0 kPa absolute
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Measuring a vacuum
A vacuum has less than atmospheric pressure.
It can also be called sub-atmospheric pressure.
1. Ordinary gauge
Needs a scale extending below zero to show
“negative pressure”, up to -100kPa.
Vacuum
2. Absolute pressure gauge
Shows absolute pressure.
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Using pressure gauges
1. Choose a suitable gauge.
2. Put on PPE.
3. Clean the pressure test point.
4. De-pressurise the system.
5. Connect pressure gauge to test point.
6. Return pressure to the system.
7. Check for leaks.
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Wait for the needle to stabilise (settle down).
Note where the tip of the needle points to the scale.
What do these
1
3
2
Sometimes the needle vibrates
because of pressure fluctuations.
A damping valve can be used to
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Caring for pressure gauges
• Treat gently to keep them accurate - don’t drop or knock them.
• Choose a gauge to suit the expected pressure range of the system
being tested.
• If you apply more pressure than its highest scale reading, the gauge
will be damaged.
• Check gauge reads correctly when not connected to anything - 0
kpa or 100 kpa absolute.
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Why measure electricity?
We need to measure electrical quantities in electric
circuits to check whether systems are working correctly.
You can’t see electricity
BUT
you can
see
and
feel
its effects.
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What is an electrical circuit?
Components and wires arranged so
that an electric current can flow.
Here is a very simple
automotive electrical
circuit.
battery
Light
light globe
plastic-covered
electrical wires
5W
12V
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Automotive electrical circuits
The frame or body of a vehicle
is often used in place of one of
the connecting wires.
vehicle frame
• Usually connected to the negative battery terminal
• Called an earth connection.
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Parts of a circuit
All electric circuits have:
Power supply - source of
electrical energy
eg. a battery
Conductors – things to
carry electric current
between components
eg. copper
wires with
plastic covering.
component which uses
the electrical energy to do
useful work
eg. light globe,
electric motor,
audio system
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Control of circuits
Circuits must be complete (no breaks)
for electric current to flow.
Switch closed
Closed circuit, current flowing
Switch open
Open circuit, no current flowing
Put a switch in the circuit = we can
control the flow of electricity.
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Electrical features of the circuit
Electricity has three aspects:
1. Voltage
2. Current
3. Resistance
Voltage
This is electrical pressure.
• Comes from the battery
• Pushes electrical energy around the circuit
• Measured in volts (V, sometimes E)
5W
12V
• Does useful work eg makes the globe light up.
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Electrical features of the circuit
contd…
Current
This is the amount of electrical energy
that flows around the circuit.
• measured in amperes (A, sometimes I)
• normally flows from positive (+) to
negative (-).
5W
12V
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Electrical features of the circuit
28
contd…
Resistance
All components have some resistance eg connecting
wires, light globe, battery.
• restricts (reduces) the amount of current flowing
• measured in ohms (Ω)
For a given voltage:
5W
12V
Higher resistance
lower current
Lower resistance
higher current
Most resistance is in the light
globe, generates heat and light
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Electrical quantities
Quantity
Measurement
Unit
Symbol
Measured
with:
Voltage
volts
V (or E)
voltmeter
Current
amperes
(‘amps’)
A (or I)
amp meter
(ammeter)
Resistance
Ohms
Ω (or R)
ohm-meter
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Types of electrical circuits
Series Circuits
Current has only one path to
follow.
Series
Parallel circuits
Current has more than one
path.
Parallel
Series-parallel circuit
A combination of a series circuit and
parallel circuit.
Series-parallel
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Safety precautions
1. Wear safety glasses.
2. Wear protective clothing and
footwear.
3. Don’t wear metal jewellery.
4. Keep flames and sparks away
from batteries.
5. Work in well ventilated areas when an
engine is running.
6. Be careful with high voltage systems eg
7. Keep clear of engine fans.
8. Report any unsafe conditions.
9. Ask for help with unfamiliar systems.
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Electrical test equipment
Three basic test instruments:
1. Voltmeter
2. Ammeter
3. Ohmmeter.
Analogue meter
- uses a conventional
needle or pointer to show
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The digital multimeter
• Can measure all three aspects of
electricity - voltage, current and
resistance
• Comes with a pair of red and black
Two types are available:
1. Switched range type
2. Auto ranging type.
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Digital multimeter
1. Switched range
• Many positions on the central
selector knob
• Choose the range or scale which is
best for the measurement you want
to make.
2. Auto ranging
• fewer positions on the central selector
knob.
• switch to the quantity to be measured
(i.e. volts, amps, ohms), and the meter
automatically adjusts to the range being
measured.
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Switched range multimeter
Turn the rotary switch to select the aspect of electricity you want to measure.
Digital display
DC / AC switch
On-Off switch
Rotary function
& range switch
Ranges for Volts,
Amps, Ohms etc.
20A socket (+)
Volts (+) socket
uA, mA socket (+)
Common (-) socket
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Connecting multimeters
To measure Volts :
The voltmeter is
connected in parallel with
the circuit.
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Connecting multimeters
To measure Amps:
To measure Resistance:
Amp meter (ammeter) is wired in
series with the circuit.
Ohm-meter must not be
connected to circuit power
battery is disconnected
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Using a multimeter
1. May need to select “DC”
on some multimeters
2. Select quantity to be
measured (volts,
amps or ohms)
3. Select appropriate
range
DO NOT try to
measure AC
quantities!
4. Select correct test
sockets for the test
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Multimeter testing ranges
Volts 0-700V, 0-200V, 0-20V, 0-2V 0200mV (millivolts)
Amps 0-20A
0-200mA (milliamps)
Ohms 0-200MΩ, 0-20MΩ, 0-2M Ω,
(Megohms, or millions of
ohms)
0-200k, 0-20k, 0-2k (kilohms,
thousands of ohms)
Can measure very large amounts of
current, voltage or resistance
(many millions of volts or ohms) ...
... or very small amounts (millionths
of a volt or amp).
When you don’t know the range:
• then adjust downwards until you
0-200 Ω.
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Connecting the multimeter
1. To measure voltage, use COM and VΩ Hz
sockets.
11.75 V
63.22 mA
2. To measure current up to 200mA (200
milliamps), use COM and µA mA sockets.
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Connecting the multimeter
3. To measure current up to 20A, use
COM and 20A sockets.
contd…
8.34 A
1.84 MΩ
4. To measure resistance (ohms), use
COM and VΩ Hz.
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Important points to remember
Rotary switch
Select correct unit to be
measured eg volts, amps, ohms.
Meter test
Measuring
current
for test being carried out.
Make sure meter is connected in
series with the circuit.
immediately after use, to avoid
short circuits.
Make sure power is OFF.
Measuring
resistance
ON/OFF switch Make sure meter is switched OFF
after use to save the batteries.
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ISBN: 978-1-876838-65-2
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