Measure 3rd lec

Mechanical Measurements
& Instrumentation
3rd Lecture
Nada Rikabi
nadarikabi@yahoo.com
6 May 2014

Today’s Lecture
 Temperature (definition, scales)
 Thermometer (definition, desired characteristics)
 Temperature measurement methods.
6 May 2014 Mechanical Measurement - 3rd year 2

What is Temperature ?
 It is the measure of how “hot” or “cold” an object.
 It is not energy nor heat.
 It is the measure of the motion (vibration or
translation) of the atom/molecules that make-up an
object.
 The greater the motion/vibration the greater T.
 The smaller the motion/vibration the lower T.

Diagrams show different objects with
different temperatures
Figure 2
Warm
Figure 1
Cold
Figure 3
Hot

Temperature Scales
 All temperature scales are based on some easily
reproducible states such as the freezing and boiling
points of water, which are also called the ice point and
the steam point, respectively.
 In thermodynamics, it is very desirable to have a
temperature scale that is independent of the properties
of any substance or substances. Such a temperature
scale is called a thermodynamic temperature scale

Temperature Scales
Temperature is measured in 3 common scales:
1. Celsius or Centigrade [ºC] – used in SI system.
[The ice and steam points were originally assigned
the values of 0 and 100°C, respectively].
2. Fahrenheit [ºF] – used in the English system.
[The corresponding values of the ice and steam
points are 32 and 212°F].
3. Thermodynamic temperature scale :
 Kelvin, K {note: units of Kelvin are not degrees K (ºK), just
K}, [SI system]
 Rankine scale, R. [English system]

Temperature Relationships
 The Kelvin scale is related to the Celsius scale by:
 The Rankine scale is related to the Fahrenheit scale by:
 The temperature scales in the two unit systems are related by:
 Therefore, when we are dealing with temperature differences
∆T, the temperature interval on both scales is the same.

What is a thermometer?
 A device to measure temperature or temperature
gradient, using a variety(‫)تنوع‬ of different principles.
 The word thermometer is derived from two smaller
word fragments: thermo from the Greek for heat and
meter from Greek, meaning to measure.
 A thermometer has two important elements:
1. The temperature sensor (e.g. the bulb on a
mercury thermometer) in which some physical
change occurs with temperature.
2. Some means of converting this physical change
into a value.

Desired Characteristics
Desirable Characteristics for a Thermometer:
 High sensitivity
 Large temperature range
 Accuracy
 Repeatability (precision).
 Easy calibration
 Fast response
 Easy and simple to use

Temperature measurement methods
Temperature measurement can be classified into a few general
categories:
 Based on thermal expansion
 Liquid-in-glass thermometer
 Bimetallic thermometer
 Electrical method
 Thermocouple
 Resistance – temperature detector
 Radiation method
 Infrared (IR) pyrometer

Thermometry Based on Thermal
Expansion
 Most materials exhibit a change in size with
changes in temperature.
 Since this physical phenomenon is well defined
and repeatable, it is useful for temperature
measurement.
 The liquid-in-glass thermometer and the
bimetallic thermometer are based on this
phenomenon.

The Liquid in Glass Thermometer
 The Liquid in Glass thermometer use
the variation in volume of a liquid in
temperature.
 They use the fact that most fluids
expand on heating. The fluid is
contained in a sealed glass bulb, and
its expansion is measured using a scale
etched in the stem of the thermometer.
Mechanical Measurement - 3rd year 13
 Liquid in Glass thermometers have been used in science,
medicine, metrology and industry for almost 300 years.
 Liquids commonly used include Mercury and Alcohol.
6 May 2014

The Liquid in Glass Thermometer
Structure
Two basic parts:
a. The bulb: Acting as a reservoir holding the
liquid whose volume changes with
temperature. The Bulb also acts as a sensor
or gauge which is inserted in the body
whose temperature is to be measured.
b. The Stem: containing the scale that is measuring
the temperature and a capillary through which the
liquid can accordingly expand and contract

Mercury in Glass Thermometer
 The operation of a mercury-in-glas thermometer is
based on the thermal expansion of mercury contained
in a glass capillary where the level of the mercury is
read as an indication of the temperature.
 Range: Covering a wide temperature range from -38
°C to 356 °C, and can be increased to 600 °C or
beyond.

Mercury in Glass Thermometer
Advantages : Disadvantages
1. Cheap 1. Display is harder to read
2. Durable ‫متين‬ 2. Does not work below -39ºc (hg freezing point)
3. Accurate
4. Easily calibrate 4. Slow response
5. Does not stick to glass 5. Fragile ‫هش‬
6. Uniform expansion 6. Mercury vapour is poisonous ‫سام‬

Alcohol Thermometer
 Uses ethyl alcohol.
 Range -118°C to 80°C
1. Low freezing point -1180C 1. Non-uniform expansions
2. Safe liquid 2. Sticks to glass
3. Cheap 3. Slow reaction to temperature changes
4. Low boiling point 800C
5. Less durable (alcohol evaporates)

Example
The length of the mercury thread is found to be 20 mm and
100 mm at 00C and 1000C respectively on an ungraduated
thermometer. What is the temperature corresponding to the
length of mercury thread of 40mm ?
1000C
00C
x
100 mm
40 mm
20 mm
Solution:
Let x is the required temperature.
Ɵ = (lƟ – l0) / (l100 – l0) x 100°C
= (40 – 20) / (100 – 20) x 100°C
= 25 °C

Bimetallic Thermometer
 Bimetallic thermometer
consists of two pieces of
different metals welded
together.
 Each piece of metal
strip(‫)شريط‬ is having a
different coefficient of
expansion.
 The strip bends when it is
subjected to heat.
 If one end is fixed, other end
displaces in response to
temperature changes.

 Bimetal strips can be fabricated
into coils, spirals, and disks.
 Frequently used in on-off
temperature control (thermostats).
Bimetallic thermometer
Range of use: -65 to 430 °C
Accuracy: varies with range ± 0.5 to 12°C
Major application areas include:
− Circuit breakers for electrical heating devices (iron,
oven, heater, grills etc)
− air-conditioning thermostats
− Wall thermometers

Thermocouple
 A thermocouple is a junction between two different
metals that produces a voltage related to a temperature
difference.
 Thermocouples are among the
easiest temperature sensors to
use and obtain and are widely
used in science and industry.
 Range: Over different types
they cover from -250C to
+2500C.

Thermocouple Principle
 Thomas Seebeck discovered that when
two wires composed of dissimilar metals
are joined at both ends and one of the ends
is heated, a continuous current will flow
across this thermoelectric circuit.
 By measuring the so-called “Seebeck
Voltage” across the open circuit, the
temperature can be determined by the
equation:
 where the proportionality constant, αAB, is
the Seebeck coefficient for metals A and B
(V/K), and T is the absolute temperature.

 The voltmeter reading is:
 One way to determine the temperature of J2 is to physically put
the junction into an ice bath, forcing its temperature to be 0 °C
and establishing J2 as the Reference Junction.
Then V becomes:

Thermocouple Materials
The three most common thermocouple materials for moderate
temperatures are Iron-Constantan (Type J), Copper-Constantan
(Type T), and Chromel-Alumel (Type K).

Capabilities and Limitations
Advantages
 Self powered (does not require a current)
 Wide Range
 Rugged
 Simple
 Inexpensive
Disadvantages
 extremely low voltage output (mV)
 Need reference point
 Non-Linear

Electric Thermometers
 Electric thermometer includes electrical
resistance thermometer, semiconductor
thermometer (thermistor) and thermocouple
thermometer.
 Platinum resistance thermometer and
thermocouple thermometer are explained in
this course.

Resistance Temperature Detector
(RTD)
 An RTD is a temperature sensor
that operates on the measurement
principle that a material’s
electrical resistance changes with
temperature.
 By supplying an RTD with a constant current and
measuring the resulting voltage drop across the resistor, the
RTD’s resistance can be calculated, and the temperature
can be determined.
 The RTD element is made from a pure material, typically
platinum (the most commonly used), nickel or copper.

Resistance Temperature Detector
(RTD)
1. Most accurate 1. Expansive
2. Most repeatable 2. Current source required
3. Wide range (-196°C to 660°C) 3. Sensitive to vibration
4. Change in resistance is linear 4. Fragile
5. Good stability at high temperature 5. Slowest response time
6. Easy to calibrate

Pyrometer
 Pyrometers are also known as Radiation
thermometers.
 A pyrometer, is a non-contact instrument that detects
an object’s surface temperature by measuring the
temperature of the electromagnetic radiation (infrared
or visible) emitted from the object.
 Idea: Every object whose temperature above the
absolute zero emits radiation.

Infrared (IR) pyrometer
6 May 2014 32
The target emits infrared radiation (IR) according to its
temperature.
The detector converts (IR) received into electrical signals, which
are then emitted as temperature values by the electronic system

Infrared (IR) pyrometer
Advantages:
1. High temperature measurement (greater than
1300°C)
2. Wide range
3. Fast response (in the ms range).
4. Measuring moving object
5. Measuring objects which are difficult to access
(high-voltage parts, great measurement distance).

Measure 3rd lec

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