1

Temperature Measurement

2

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

3

Source: Mechanical Engineering magazine, March 2010

4

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

5

Characteristics of thermocouples

Thermocouple tables Sensitivity

6

• 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)

7

• 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

8

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 thermometer

Pressure thermometer

9

Pressure measurement

10

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

11

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

12

Bourdon tube

• Pressure causes Bourdon tube to unwind• displacement transducer• Can also be used with an LVDT or strain

gauge

13

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

14

Force measurement

15

AF Strain Sensing:

APF Pressure Sensing:

amF Acceleration Sensing:

xkF Elastic Sensing: Beams, rings

Strain gauges

Piezoelectric elements

Force sensing

16

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

17

Flow measurement

18

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

MvQ

L

19

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

20

Principles of flow measurement

1 1 2 2v A v A

1 22 P PQ const

2 21 1 2 2

1 22 2

P v P vz z

g g g g

1v

2v

z1

z2

Bernoulli’s equation:

Conservation of mass:

For a horizontal pipe:2 2

1 2 1 2

2

P P v v

Pressure difference is a measure of flow rate

Q = volume flow rate

21

Orifice Plate

D d

22

Types of flowmeters

Orificesflow m eter

Venturi Tubeflow m eter

Flow Tubeflow m eter

Flow Nozzlesflow m eter

Pilot Tubesflow m eter

Others

PressureTypes

Turbineflow m eter

RecipocateingPiston flow m eter

Oval-Gearflow m eter

Others

M echanicalTypes

Hot-w ireflow m eter

Resistive-bridgeflow m eter

Others

Therm alTypes

Vortexflow m eter

Electrom agneticflow m eter

UltrasonicFlow m eter

Others

O therTypes

Flow m eters

23

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

24

Pitot static tube• Negligible obstruction of flow• Measures flow at a single point• Measures average flow velocity

02 sp pv

25

Turbine flowmeter• Speed of rotation of turbine is proportional to flow rate

26

Electromagnetic flowmeter• Used for electrically conductive fluids• Non-invasive device (no obstruction to fluid flow)• No pressure loss

27

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.