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MEM322: CHAPTER 4 ( TEMPERATURE MEASUREMENT ), - Coggle Diagram
MEM322: CHAPTER 4
( TEMPERATURE MEASUREMENT )
INTRODUCTION
Law used in Temperature: ZEROTH LAW OF THERMODYNAMICS
If two bodies are in thermal equilibrium
with a third body, then they are all in
thermal equilibrium with each other
The
thermometer
is calibrated against a
standard
and is
subsequently used to read
unknown temperatures
TEMPERATURE SCALE
temperature scales are: Fahrenheit and Celsius
Kelvin an celsius scales are related by:
T: temperature in kelvin
t: temperature in celsius
Based on Carnot cycle:
T₁,T₂ : Reservoir of heat (T₁ > T₂ )
Q₁,Q₂: Heat energy (Q₁ <-> Q₂)
THERMOCOUPLE
temperature-measuring device consisting
of two dissimilar conductors that contact each other at one or more spots
choice of material ;
a. Ability to withstand the temperature at which they are used
b. Immunity from corrosion
c. Linearity characteristic
QUARTZ THERMOMETER
Piezo-electric crystal (quartz) provides a highly accurate and
sensitive method of temperature measurement based on change in its resonant frequency which is directly proportional to the temperature change
ADVANTAGES
Highly linear output.
Long-term stability and reliability
High resolution of the order of
0.001°C
Excellent repeatability in the
measuring range of -40 to 230°C
DISADVANTAGES
Limited measuring range -40 to 230°C
Piezo-electric crystals have strong
cross-sensitivity for pressure changes if
they occur simultaneously in the
temperature measuring system
Methods of Temperature measurement &
Temperature-Sensing Techniques
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
Bimetallic Thermometer.
Use the principle of solid expansion and consists of a ‘bimetal’ strip usually in the form of cantilever beam
usually employed as the low expansion metal
iron-nikel alloy containing 36% nickel
Brass is used as high expansion material for the measurement of low temperatures
Real world application: Thermostat in air conditioner
longer strip in helical form