measure 3rd lec
TRANSCRIPT
Mechanical Measurement - 3rd year 2
Today’s Lecture
Temperature (definition, scales)
Thermometer (definition, desired characteristics)
Temperature measurement methods.
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Mechanical Measurement - 3rd year 3
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.6 May 2014
Mechanical Measurement - 3rd year 4
Diagrams show different objects with different temperatures
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Figure 2 Warm
Figure 1Cold
Figure 3 Hot
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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
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Mechanical Measurement - 3rd year 6
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]
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Temperature Relationships
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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.
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Mechanical Measurement - 3rd year 9
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.
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Desired Characteristics
Desirable Characteristics for a Thermometer:
High sensitivity Large temperature range Accuracy Repeatability (precision). Easy calibration Fast response Easy and simple to use
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Temperature measurement methodsTemperature 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
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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.
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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.
Liquid in Glass thermometers have been used in science, medicine, metrology and industry for almost 300 years.
Liquids commonly used include Mercury and Alcohol.
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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.
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b. The Stem: containing the scale that is measuring the temperature and a capillary through which the liquid can accordingly expand and contract
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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.
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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 هش6. Uniform expansion 6. Mercury vapour is poisonous سام
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Alcohol Thermometer Uses ethyl alcohol. Range -118°C to 80°C
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Advantages : Disadvantages
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)
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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 ?
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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
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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. 6 May 2014
Mechanical Measurement - 3rd year 21
Bimetallic Thermometer Bimetal strips can be fabricated
into coils, spirals, and disks. Frequently used in on-off
temperature control (thermostats).
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Bimetallic thermometer
Range of use: -65 to 430 °CAccuracy: varies with range ± 0.5 to 12°CMajor application areas include:
− Circuit breakers for electrical heating devices (iron, oven, heater, grills etc)
− air-conditioning thermostats− Wall thermometers
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Bimetallic Thermometer
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Thermocouple A thermocouple is a junction between two different
metals that produces a voltage related to a temperature difference.
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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.
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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.
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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.
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Then V becomes:
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Thermocouple MaterialsThe three most common thermocouple materials for moderate temperatures are Iron-Constantan (Type J), Copper-Constantan (Type T), and Chromel-Alumel (Type K).
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Capabilities and Limitations Advantages Self powered (does not require a current) Wide Range Rugged Simple InexpensiveDisadvantages extremely low voltage output (mV) Need reference point Non-Linear6 May 2014
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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.
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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.
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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.
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Resistance Temperature Detector (RTD)
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Advantages : Disadvantages
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
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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.
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Infrared (IR) pyrometer
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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
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Infrared (IR) pyrometer
Advantages:1. High temperature measurement (greater than
1300°C)2. Wide range3. Fast response (in the ms range).4. Measuring moving object
5. Measuring objects which are difficult to access (high-voltage parts, great measurement distance).
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