Temperature Measurement
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How to measure temperature
Temperature can be measured by detecting changes
in various temperature-dependent properties
Volume
Pressure
Displacement
Voltage
Resistance
Radiation spectrum
 liquid-in-glass thermometer
 gas thermometer
 bimetallic strip
 Thermocouple
 RTD & thermistor
 Infra Red detectors
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Source: Mechanical
Engineering magazine,
March 2010
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Thermocouple
Thermoelectric effect: when any two different metals are connected together, an
emf that is a function of the temperature is generated at the junction between the
metals:
2
n
e  a1T  a2T 
For certain pairs of materials,
 anT
e
a1T
Hot junction
Reference junction
http://www.thermoworks.com/products/logger/images/digisense_dualogr_lg.jpg
(a) Thermocouple; (b) equivalent circuit
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Characteristics of thermocouples
Thermocouple tables
Sensitivity
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Resistance Temperature Detectors (RTDs)
•
Varying resistance devices
• Rely on the fact that the
resistance of a metal varies with
temperature
•
Also known as resistance
thermometers or thermistors
depending on material used (metal
or semiconductor)
• Variation can be non-linear,
resulting in inconvenient
measurement
• Platinum exhibits most linear
behavior
• Platinum is also chemically inert
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Resistance thermometers or
Resistance Temperature Devices (RTDs)
• Two common designs:
• Coil wound on mandrel
• Film deposited on substrate
• Wheatstone bridge: used to measure
resistance change for an RTD
• Excitation voltage has to be chosen
carefully: while a high voltage is
desirable for high sensitivity, this causes
self-heating
http://www.extech.com/instrument/products/400_450/407907.html
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Mechanical temperature sensing devices
• Liquid-in-glass thermometer
• Bimetallic thermometer
• Commonly used as a thermostat (on-off
switch in control applications)
• When displacement is measured, it acts as a
thermometer
• Tip displacement: against a calibrated scale,
or electrical output such as LVDT
• Pressure thermometer
Liquid-in-glass thermometer
Pressure thermometer
Bimetallic thermometer
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Pressure measurement
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Diaphragm & bellows
• Pressure causes displacement of
diaphragm (thin sheet), which can be
measured by a displacement transducer
• Can be used with an LVDT or strain
gauge
Diaphragm
Bellows
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Application: sound measurement

• Sound is measured as sound pressure level: S p  20 log10 

0.0002


• Microphone: diaphragm-type pressure sensor
• Converts sound pressure into displacement
• Displacement is commonly measured using
a piezoelectric-type transducer
P
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Bourdon tube
• Pressure causes Bourdon tube to unwind
• displacement transducer
• Can also be used with an LVDT or strain
gauge
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Manometer
• Can be used to measure gauge pressure: p   gh
• Can also measure differential pressure: p1  p2   gh
• Type of liquid
• Water is cheap & convenient
• Water evaporates & is difficult to see
through
• Not to be used if reacts with fluid
• Well-type: need only to measure liquid
level in one tube
• Inclined-type: better sensitivity
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Force measurement
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Force sensing
Elastic Sensing:
F  k x
Beams, rings
Strain Sensing:
F   A
Strain gauges
Pressure Sensing:
F  PA
Acceleration Sensing:
F  ma
Piezoelectric
elements
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Load cell
• Force produces measurable displacement
• Design objectives:
• Achieve linear input/output relation
• Make the instrument less sensitive to
forces not applied along sensing axis
Based on strain gauge measurement
Hydraulic load cell
Use of strain gauges in a torque cell
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Flow measurement
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Conveyor-based methods
• To measure flow of solids or particles
• Mass is measured with a load cell
Q
Mv
L
M = mass of material
L = length of conveyor
v = velocity
Q = mass flow rate
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Pipe flow
• Flow measurement :
• Local properties (velocity, pressure, temperature, density, viscosity)
• Integrated properties (mass flow rate, volumetric flow rate)
• Global properties (visualization of entire flow)
• If the velocity profile is known, it is enough to measure one velocity (centerline)
to determine the total flow rate
• Otherwise, cross-section must be mapped by a grid of velocity data
Laminar Flow
Turbulent Flow
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Principles of flow measurement
v2
Bernoulli’s equation:
P1 v12
P2 v22

 z1 

 z2
 g 2g
 g 2g
Conservation of mass:
z2
v1
z1
For a horizontal pipe:
Q  const
2  P1  P2 

v1 A1  v2 A2 
P1  P2

v12  v22

2
Q = volume flow rate
Pressure difference is a measure of flow rate
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Orifice Plate
D
d
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Types of flowmeters
Flowmeters
Pressure
Types
Mechanical
Types
Thermal
Types
Other
Types
Orifices
flowmeter
Turbine
flowmeter
Hot-wire
flowmeter
Vortex
flowmeter
Venturi Tube
flowmeter
Recipocateing
Piston flowmeter
Resistive-bridge
flowmeter
Electromagnetic
flowmeter
Flow Tube
flowmeter
Oval-Gear
flowmeter
Others
Ultrasonic
Flowmeter
Flow Nozzles
flowmeter
Others
Others
Pilot Tubes
flowmeter
Others
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Differential pressure meters
• Rely on the insertion of some device info a fluid-carrying pipe to obstruct the
flow, thus creating a pressure difference
• Obstruction-type meters or flow-restriction meters
• Common devices: orifice plate, Venturi tube, flow nozzle
• Pressure difference usually measured with a differential pressure transducer
• Advantage: no moving parts; robust, reliable & easy to maintain
• Disadvantage: permanent loss of pressure
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Pitot static tube
• Negligible obstruction of flow
• Measures flow at a single point
• Measures average flow
velocity
v
2  p0  ps 

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Turbine flowmeter
• Speed of rotation of turbine is proportional to flow rate
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Electromagnetic flowmeter
• Used for electrically conductive fluids
• Non-invasive device (no obstruction to fluid flow)
• No pressure loss
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Hot wire anemometer
• Consists of an electrically heated fine wire which is immersed in the
flow.
• As the fluid velocity increases, the rate of heat flow from the heated
wire to the flow stream increases.
• Thus a cooling effect on the wire occurs, causing its electrical
resistance to change.
• In a constant current anemometer, the fluid velocity is determined from
measurement of the change in resistance.
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