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CHAPTER 4 : TEMPERATURE MEASUREMENT, INTRODUCTION, TEMPERATURE SCALES,…
CHAPTER 4 : TEMPERATURE MEASUREMENT
It also related and the important parameter in the thermodynamic and heat transfer operation
The law that is used in temperature measurement is Zeroth law of thermodynamic.
If two bodies are in thermal equilibrium with a third body, then they are all in thermal equilibrium with each other
In manufacturing, the product quality usually closely related with accurate measured and maintained of temperature.
Temperature is most widely measured quantity in process industry.
Common use scales : Fahrenheit & Celcius
T= t + 273.15
T: temperature in kelvin t: temperature in celsius
Scale are based on a specification of "the number of increment between freezing point and boiling point of water at a standard atmospheric temperature".
Carnot cycle
T1/T2 = Q1/Q2
T₁,T₂ : Reservoir of heat (T₁ > T₂) Q₁,Q₂: Heat energy (Q₁ <-> Q₂)
Temperature measurement method
non-electrical method
electrical method
radiation method
Temperature-sensing techniques
Changes in physical dimensions (non electrical method)
Liquid-in-glass thermometers
Bimetallic thermometers
Changes in gas or vapor pressure (non electrical method)
Constant-volume gas thermometers
Pressure thermometers
Changes in electrical properties (electrical method)
Resistance thermometers
.Thermistors
.Thermocouples
.Quartz Thermometer
Semiconductor-junction technology (electrical method)
Diodes
Integrated circuits
Changes in emitted thermal radiation (radiation method)
Total radiation pyrometers
Infrared pyrometer
Changes in chemical phase (non-electrical method)
Fusible indicators
Liquid crystal
Thermocouples remain the main temperature sensor, which half of industrial application
Bimetallic Thermometer
Use the principle of solid expansion and consists of a ‘bimetal’ strip usually in the form of cantilever beam
Two metals which having different coefficient of thermal expansion, joint together (welded/riveted)
Invar usually employed as the low expansion metal
Iron-nikel alloy containing 36% nickel
Coefficient of thermal expansion around 1/20 of ordinary metal
Use in high temperatures
Brass is used as high expansion material for the measurement of low temperatures
Brass is used as high expansion material for the measurement of low temperatures
Bimetallic thermometer usually function between -30 to 550°C.
Advantage: self generating type with low cost with no maintenance
Disadvantage: Inability to measure rapidly changing temperatures
Thermocouple
A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots
Produce voltage when the temperature of one of the spot differs from the reference temperature
Any junction of dissimilar metals will produce an electrical potential related to temperature
Material for thermocouples
Ability to withstand the temperature at which they are used
Immunity from corrosion
Linearity characteristic
Thermocouple can be broadly classified
Base metal thermocouples
Rare metal thermocouples
For increase its sensivity, thermocouples may be attached in series or parallel which known as thermopile
Advantages
Can made in small size and low capacity thermal
Less cost
They cover temperature range -200 to 3000 C
Disadvantages
The require insulation covering while using them
The output emf requires amplification in most applications
Thermocouple widely used as temperature measurement for gas turbine exhaust, diesel engines and as flame sensors in safety devices for gas-powered major appliances
Quartz Thermometer
Quartz highly accurate and sensitive method of temperature measurement based on the change in its resonant frequency which is directly proportional to the temperature change
To archive above properties the crystal is cut in the form of shear type LC (Linear coefficient) cut
Frequency of oscillation of 1 Hz gives a resolution of 0.001° C, thus temperature in the range of -40 to 230°C can be measured precisely and accurately
Disadvantages
strong cross-sensitivity for pressure changes if they occur simultaneously
Limited measuring range -40 to 230°C
Advantages
Highly linear output
Long-term stability and reliability
INTRODUCTION
TEMPERATURE SCALES
Methods of Temperature measurement & Temperature-Sensing Techniques