Temperature Measurement
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Transcript of Temperature Measurement
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Temperature Measurement
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How to measure temperature
Temperature can be measured by detecting changes in various temperature-dependent properties
Volume liquid-in-glass thermometerPressure gas thermometerDisplacement bimetallic stripVoltage ThermocoupleResistance RTD & thermistorRadiation spectrum Infra Red detectors
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Source: Mechanical Engineering magazine, March 2010
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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:
Thermocouple
21 2
nne a T a T a T
For certain pairs of materials, 1e a T
(a) Thermocouple; (b) equivalent circuit
Hot junction
Reference junction
http://www.thermoworks.com/products/logger/images/digisense_dualogr_lg.jpg
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Characteristics of thermocouples
Thermocouple tables Sensitivity
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• 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
Resistance Temperature Detectors (RTDs)
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• 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
Resistance thermometers orResistance Temperature Devices (RTDs)
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
Bimetallic thermometerPressure 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
Bellows
Diaphragm
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Application: sound measurement
1020log0.0002p
PS
• Sound is measured as sound pressure level:• Microphone: diaphragm-type pressure sensor
• Converts sound pressure into displacement• Displacement is commonly measured using
a piezoelectric-type transducer
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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: 1 2p p 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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AF Strain Sensing:
APF Pressure Sensing:
amF Acceleration Sensing:
xkF Elastic Sensing: Beams, rings
Strain gauges
Piezoelectric elements
Force sensing
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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
Use of strain gauges in a torque cellHydraulic load cell
Based on strain gauge measurement
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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
M = mass of materialL = length of conveyorv = velocityQ = mass flow rate
MvQL
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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
1 1 2 2v A v A
1 22 P PQ const
2 21 1 2 2
1 22 2P v P vz zg g g g
1v
2v
z1
z2
Bernoulli’s equation:
Conservation of mass:
For a horizontal pipe:2 2
1 2 1 2
2P P v v
Pressure difference is a measure of flow rate
Q = volume flow rate
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Orifice Plate
D d
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Types of flowmeters
O rificesflow m eter
Venturi Tubeflow m eter
Flow Tubeflow m eter
Flow Nozzlesflow m eter
Pilot Tubesflow m eter
O thers
PressureTypes
Turbineflow m eter
Recipocate ingPiston flow m eter
O val-G earflow m eter
O thers
M echanicalTypes
Hot-w ireflow m eter
Resistive-bridgeflow m eter
O thers
Therm alTypes
Vortexflow m eter
Electrom agneticflow m eter
UltrasonicFlow m eter
O thers
O therTypes
Flow m eters
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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
02 sp pv
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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.