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Temperature Measurement 1 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 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: 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 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: 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 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: 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 13 Force measurement 14 Force sensing Elastic Sensing: F kx Beams, rings Strain Sensing: F A Strain gauges Pressure Sensing: F PA Acceleration Sensing: F ma Piezoelectric elements 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 Q Mv L 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 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 20 Orifice Plate D d 21 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 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 v 2 p0 ps 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 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