Temperature Measurement 1 How to measure temperature Temperature

Temperature Measurement 1

How to measure temperature Temperature can be measured by detecting changes in various temperature-dependent properties ØVolume ØPressure ØDisplacement ØVoltage ØResistance ØRadiation spectrum liquid-in-glass thermometer gas thermometer bimetallic strip Thermocouple RTD & thermistor Infra Red detectors 2

Source: Mechanical Engineering magazine, March 2010 3

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: For certain pairs of materials, Hot junction Reference junction http: //www. thermoworks. com/products/logger/images/digisense_dualogr_lg. jpg (a) Thermocouple; (b) equivalent circuit 4

Characteristics of thermocouples Thermocouple tables Sensitivity 5

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 6

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 7

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 8

Pressure measurement 9

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 10

Application: sound measurement • 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 11

Bourdon tube • Pressure causes Bourdon tube to unwind • displacement transducer • Can also be used with an LVDT or strain gauge 12

Manometer • Can be used to measure gauge pressure: • Can also measure differential pressure: • 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 13

Force measurement 14

Force sensing Elastic Sensing: Beams, rings Strain Sensing: Strain gauges Pressure Sensing: Piezoelectric elements Acceleration Sensing: 15

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 16

Flow measurement 17

Conveyor-based methods • To measure flow of solids or particles • Mass is measured with a load cell M = mass of material L = length of conveyor v = velocity Q = mass flow rate 18

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 19

Principles of flow measurement Bernoulli’s equation: Conservation of mass: z 2 z 1 For a horizontal pipe: Q = volume flow rate Pressure difference is a measure of flow rate 20

Orifice Plate D d 21

Types of flowmeters 22

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 23

Pitot static tube • Negligible obstruction of flow • Measures flow at a single point • Measures average flow velocity 24

Turbine flowmeter • Speed of rotation of turbine is proportional to flow rate 25

Electromagnetic flowmeter • Used for electrically conductive fluids • Non-invasive device (no obstruction to fluid flow) • No pressure loss 26

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. 27