VIEWS: 3 PAGES: 48 POSTED ON: 9/10/2012
Thermal Sensor Thermal Sensors • The control of process needs to be able to sense controlled or regulated variables • One physical variable that is important in industrial process control is temperature Definition of Thermal Energy • Substances around us are composed of atoms • When substance received energy, it will vibrates – Vibrates at equilibrium – solid – Vibrates and rotating – liquid – Vibrates and rotating and moving freely - gas Definition of Temperature • Temperature is the measurement of thermal energy containing in each molecule of substance (Joules/Molecule) • Example 10,000 J of energy – In a glass of water – In a tank of water Temperature Scale and Calibration Points Absolute Temperature Scale • Assigns zero temperature to a material that has no thermal energy – Kelvin (K) Scale – Rankine (°R) Scale • The transformation between the two scales is 5 T ( K ) T ( R) 9 Relative Temperature Scale • Shift the zero index up; zero degree does not mean zero thermal energy • Celsius scale relates to Kelvin T ( C ) T ( K ) 273.15 • Fahrenheit relates to Rankine T ( F ) T ( R) 459.6 Relationship between F and C • To transform from Celsius to Fahrenheit, note that the two scales differ by size of degree and scale shift of 32 9 T ( F ) T ( C ) 32 5 Relationship to Thermal Energy • Average thermal energy WTH of a molecule can be found from the absolute temperature in K from 3 WTH kT 2 • Where k = 1.38x10-23 J/K is Boltzmann’s constant Example A material has a temperature of 335 K. Find the temperature in °R Example Given temperature of 144.5 °C, express this temperature in (a) K and (b) °F Example A sample of oxygen gas has a temperature of 90 °F. If its molecular mass is 5.3x10-26 kg, find the average thermal speed of a molecule. Metal Resistance and Temperature • Metal is composed of atoms. • Add up all atoms vibrations constitutes thermal energy stored in the metal • The energy bands of metal is shown below • Valance band and conduction band are overlapped • Valance electron can stay in the conduction band and moves freely, conducting current • As metal received energy, stationary atom vibrates more and more • Conduction electrons collide more and more with vibrating atoms. Thus, resists the flow of current (increase in resistance) Graph of Temperature vs. Resistance Resistance vs. Temp Approximation Resistance vs. Temperature Approximation • A straight line approximation of resistance vs. temperature R(T ) R(T0 ) 1 0T Where R(T ) approximation of resistance at temperature T R(T0 ) resistance at temperature T0 T T T0 0 fractional change in resistance per degree of temperature at T0 where 1 R2 R1 0 T T R(T0 ) 2 1 Example A sample of metal resistance vs. temperature has the following measured values: Find the linear approximation of resistance vs temperature between 60° and 90° F Resistance Temperature Detectors (RTD) • A temperature sensor based on the changed of resistance of metal due to the change in temperature • Metals used are – Platinum (Repeatable, expensive) – Nickel (No quite repeatable, inexpensive) Characteristics of RTD • Sensitivity – Value of α0 is sensitivity – on the order of 0.004/°C for platinum and 0.005/°C for nickel • Response time – Due to the slowness of thermal conductivity, the response time is 0.5 to 5 s or more • Construction – Metal wire is wound into coil and is protected in sheath or productive tube • Signal conditioning – The RTD is generally used in a bridge circuit with compensate line • Dissipation constant – Because RTD is resistance, there is an I2R power dissipation in the device (Self-heating) – Can cause erroneous reading – Dissipation constant (PD) is provided in RTD spec. • A power required to raise the RTD temp by 1 C • The increase in temperature is found as P T PD T temperature rise becuase of self-heating in C P power dissipated in the RTD from the circuit in W PD dissipation constant of the RTD in W/ C Example An RTD has α0 = 0.005/°C , R = 500 , and a dissipation constant of PD = 30 mW/°C at 20 °C. The RTD is used in a bridge circuit with R1 = R2 = 500 and R3 a variable resistor used to null the bridge. If the supply is 10 V and the RTD is placed in a bath at 0 °C, find the value of R3 to null the bridge. Thermistors • Thermistor is made of semiconductor instead of metal • The gap energy ΔWg exists between the band • When the temperature of material increases, valance electrons gain additional energy until exceeds the band gap • Valence electrons are free to move and conduct current as temperature increase • As current conducts more, resistance decreases Resistance vs. Temperature Thermistor Characteristics • Sensitivity – Typically, 10% resistance change per 1 °C • Construction – Can be fabricated in discs, bead, and rods • Response time – Typically, 0.5 s – For poor thermal contact, response time can be 10 s • Signal conditioning – Bridge circuit • Dissipation constants – In milliwatts/°C Example A thermistor is to monitor room temperature. It has a resistance of 3.5 k at 20 °C with a slope of -10%/C. The dissipation constant is PD = 5 mV/C. It is proposed to use the thermistor in the divider circuit as shown to provide a voltage of 5.0 V at 20 ° C. Evaluate the effect of self- heating Thermocouples • When any conductor (such as a metal) is subjected to a thermal gradient, it will generate a voltage • Two different metals A and B are used to close the loop with connecting junction at T1 and T2 Seebeck Effect • Seebeck effect T2 QA QB dT T1 • where emf produced in voltage T1 , T2 junction temperatures in K QA , QB thermal transport constants of the two metals • In practice, an approximation linear relationship exists as T2 T1 • where constant in V/K T1 , T2 junction temperatures in K Example Find the Seebeck emf for a material with α = 50 V/°C if the junction temperatures are 20 °C and 100 °C. Thermocouple Characteristics • The setup of thermocouple is arranged as shown • Junction TM is exposed to temperature to be measured • The reference junction TR is a known temperature • The voltage produce is a function of (TM- TR) Thermocouple Types • Certain standard configuration of thermocouples have been adopted Each type has its particular features such as range, linearity, inertness to hostile environments sensitivity Thermocouple Polarity • If the reference temperature is less than the measured temperature, the output would be positive Thermocouple Tables • Table gives the voltage output related to reference and measured temperature with TR = 0 °C increment TM VM • To find a corresponding temperature to a certain output voltage, need interpolation TH TL TM TL VM VL VH VL • To find a measured voltage for a particular temperature VH VL VM VL TM TL TH TL Example A voltage of 23.72 mV is measured with a type K thermocouple at a 0 °C reference. Find the temperature of the measurement junction. Example Find the voltage of a type J thermocouple with a 0 °C reference if the junction temperature is -172 °C. Thermocouple Sensors • Sensitivity – Type J: 0.05 mV/°C – Type K: 0.006 mV/°C • Construction – Welded or twisted junction between 2 metals – Protective cover • Time response – Related to size of the wire and any protective material – Small TC, 10 to 20 ms – Large TC, 10 to 20 s • Signal conditioning – Output is very small, need high gain differential amplifier