Temperature Measurement

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 2

Source: Mechanical Engineering magazine, March 2010

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

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

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

Pressure measurement

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

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

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

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

Force measurement

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

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

Flow measurement

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

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

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

Orifice Plate D d

Types of flowmeters

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

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

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

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

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.