Temperature Measurement

1. Temperature Measurement

2. How to measure temperature Temperature can be measured by detecting changes in various temperature-dependent properties • Volume  liquid-in-glass thermometer • Pressure  gas thermometer • Displacement  bimetallic strip • Voltage  Thermocouple • Resistance  RTD & thermistor • Radiation spectrum  Infra Red detectors

3. Source: Mechanical Engineering magazine, March 2010

4. 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

5. Characteristics of thermocouples Sensitivity Thermocouple tables

6. 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 thermistorsdepending 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

7. 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

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 Pressure thermometer Bimetallic 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 Diaphragm Bellows

11. Application: sound measurement • Sound is measured as sound pressure level: • Microphone: diaphragm-type pressure sensor • Converts sound pressure into displacement • Displacement is commonlymeasured 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: • 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

14. Force measurement

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

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 Based on strain gauge measurement Hydraulic load cell Use of strain gauges in a torque cell

17. Flow measurement

18. 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

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 Bernoulli’s equation: Conservation of mass: z2 For a horizontal pipe: z1 Q = volume flow rate Pressure difference is a measure of flow rate

21. Orifice Plate D d

22. Types of flowmeters

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

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.