lý thuyết Temperature

Level 1 - TemperatureRMT Training - 05 /98 1 Fundamental Training Fundamental Training Level 1 Level 1 - TemperatureRMT Training - 05 /98 2 Topics: Slide No: • Why measure temperature? 3 - 5 • Temperature terminology 6 - 8 • Temperature measurement technology 9 - 13 • Temperature sensors 14 - 40 • Sensor accessories 41 - 52 • Temperature transmitter 53 - 64 • Exercise 65 - 67 Contents Contents Level 1 - TemperatureRMT Training - 05 /98 3 • Because temperature affects: • Temperature is critical to the following process: – rate of reaction – viscosity – state of a matter – strength of materials – quality & taste of food – safety of a process – Pulp & Paper – Food Industry Pasteurisation – Vacuum Packaging – Chemical Industry Why measure temperature? Why measure temperature? Level 1 - TemperatureRMT Training - 05 /98 4 • Safety – to prevent explosion as a result of excessive temperature • Efficiency – example:- Air -Conditioning » accurate temperature measurement prevent the supplier from overcooling the air, which saves energy and increases efficiency • Product Quality & Yield – variation from optimum temperature result in » very little production of the desired product » creation of waste product – precise temperature measurement ensures efective separation of products in » distillation column » catalytic cracking processes Why measure temperature? Why measure temperature? 4 Common Reasons 4 Common Reasons Level 1 - TemperatureRMT Training - 05 /98 5 • Custody Transfer – amount of material that is bought & sold – extremely important to know exact temperature when determining volumetric flow rate of gas – amount of material contained in a specific volume of gas » decreases with rising temperatures » increases with falling temperatures – inaccurate temperature measurement result in » over or under-charging customers during custody transfer Why measure temperature? Why measure temperature? 4 Common Reasons 4 Common Reasons Level 1 - TemperatureRMT Training - 05 /98 6 Steam Cold Water Hot Water Load Disturbance TT TIC I/P Temperature terminology Temperature terminology Temperature Control Loop Temperature Control Loop • Temperature Loop Issues: – Fluid response slowly to change in input heat – Requires advanced control strategies • Feedforward Control Level 1 - TemperatureRMT Training - 05 /98 7 373 100° 672 212° 273 0° 492 32° 0 -273° 0 -460° °C = 5/9 (°F - 32 ) °F = 9/5 (°C) + 32 K = 273 +°C R = 460 + °F Kelvin & Rankine are Kelvin & Rankine are absolute scales absolute scales BOILING POINT OF WATER ICE POINT ABSOLUTE ZERO kELVIN CELSIUS RANKINE FAHRENHEIT Temperature terminology Temperature terminology Temperature Measurement Scales Temperature Measurement Scales Level 1 - TemperatureRMT Training - 05 /98 8 Differential Temperature Differential Temperature °C = 5/9 °F °F = 9/5 °C K = °C °R = °F Example: 2 points in a process differ in temperature by 100 °C. These 2 points differ by 180 °F i.e. 180 = 9/5(100) Whereas, they also differ by 100K Example #1 Example #1 20°C = 20 + 273 = 293K 20°C = 9/5*(20) +32 = 68°F Temperature terminology Temperature terminology Temperature Measurement Scales Temperature Measurement Scales Level 1 - TemperatureRMT Training - 05 /98 9 Temperature Measurement Temperature Measurement Technology Technology METALS change in VOLUME in response to change in TEMPERATURE & DISSIMILAR METAL STRIPS having different COEFFICIENT of VOLUME CHANGE. Example: Bimetallic Thermometer Thermocouple (discussed later) Bimetallic Thermometer The degree of deflection of 2 dissimilar metals is proportional to the change in temperature. One end of the spiral (wounded from a long strip of material) is immersed in the process fluid and the other end attached to a pointer. Level 1 - TemperatureRMT Training - 05 /98 1 0 Example: Vapour Pressure Thermometer A bulb connected to a small bore capillary which is connected to an indicating device. Indicating device consist of a spiral bourdon gauge attached to a pointer. The bulb is filled with a volatile liquid and the entire mechanism is gas tight and filled with gas or liquid under pressure. Basically the system converts pressure at constant volume to a mechanical movement. Temperature Measurement Temperature Measurement Technology Technology Expansion & Contraction of FILLED THERMAL FLUIDS [...]... 05 /98 Level 1 - Temperature Temperature Sensors RTDs 1 4 What is an RTD ? – Resistance Temperature Detector » flow » Operation depends on inherent characteristic of metal (Platinum usually): electrical resistance to current changes when a metal undergoes a change in temperature If we can measure the resistance in the metal, we know the temperature! Platinum resistance changes with temperature Wire-wound... 1 - Temperature Temperature Sensors RTDs What is RTD Calibration? – The real RTD curve is found by “characterizing” an RTD over a specific temperature range or point 2 3 » Temperature Range Characterization ⇒ Calibration certificate provided with sensor » Temperature Point Characterization ⇒ Calibration certificate provided with sensor Customer Receives RTD-specific Resistance Data generated vs Temperature. .. RTD-specific Resistance Data generated vs Temperature Chart: (RTD “characterized”) o C Ohms 0.0 99.997 1.0 100.38 Temperature Bath 2.0 100.77 - One temperature 3.0 101.16 - Multiple temperatures RMT Training - 05 /98 Level 1 - Temperature Temperature Sensors RTDs With a Real RTD, the Resistance vs Temperature relationship of the sensor is NOT the same curve that is programmed into the transmitter R vs T... capability • Dual element adds only $5 over single element RTD » Reduce the risk of a temperature point failure • Supports Differential Temperature Measurement RMT Training - 05 /98 Level 1 - Temperature Temperature Sensors RTDs The IEC 751 standard curve (programmed into all our transmitters) describes an IDEAL Resistance vs Temperature relationship for Pt100 α = 0.00385 RTDs RESISTANCE (OHMS) Every RTD... 1 - Temperature Temperature Measurement Technology Collection of THERMAL RADIATION from an object subjected to HEAT 1 2 Example: Radiation Pyrometry Infers temperature by collecting thermal radiation from process and focusing it on a photon detector sensor The sensor produces and output signal as radiant energy striking it releases electrical charges RMT Training - 05 /98 Level 1 - Temperature Temperature... C(t-100)t3] = 103.90 Level 1 - Temperature Temperature Sensors RTDs 1 6 Relative Resistance (RT / R0) Platinum vs other RTD materials 6 4 Most linear Nickel 5 Thermistor Balco Most Repeatable 3 Platinum 2 Positive Slope 1 0 °C -100 °F -148 Most Stable 0 100 200 300 400 32 212 392 572 762 500 600 700 932 1112 1292 Temperature RMT Training - 05 /98 Level 1 - Temperature Temperature Sensors RTDs 1 7 Why... (voltmeter) that measures the potential created by the temperature difference between the two ends Process Temperature Hot junction DT + Cold junction “40 millivolts!,” Tommy Seebeck yelled in a heated debate MV - The junction of two dissimilar metals creates a small voltage output proportional to temperature! RMT Training - 05 /98 Level 1 - Temperature Temperature Sensors Thermocouples 2 9 How does a Thermocouple... to the temperature difference between the hot and cold junction! (T2 - T1) =∆T Hot junction Measurement Junction Heat ∆T T2 Millivolts 0.000 0.591 1.192 1.801 TYPE E THERMOCOUPLE RMT Training - 05 /98 Reference Junction T1 80 Voltage (mV) C 0 10 20 30 MV - Thermoelectric Voltage vs Temperature Chart: o + Cold junction 60 IEC 584 40 20 0 -500 -20 0 500 1000 Temperature (oC) Level 1 - Temperature Temperature... Typically use copper wires for Red extension from the sensor White White Black Green Green Level 1 - Temperature Temperature Sensors RTDs 2-wire or 4-wire RTD ? • If the sensing element is at 20°C, – What would be the temperature measured at the end of the extension wire using a 2-wire assembly – What would be the temperature measured at the end of the extension wire using a 4-wire assembly 2-wire RTD Red... C This means that the temperature measured at the end of the cable would be 21.8 Deg C RMT Training - 05 /98 1 8 6 metres of copper extension wire, lead resistance = 0.06 ohms/metre (1 ohm = 2.5 deg C approx) White Error for a 4 wire assembly As the lead resistances can be accounted for the temperature measured at the end of the cable would be 20.0 Deg C Level 1 - Temperature Temperature Sensors RTDs . 68°F Temperature terminology Temperature terminology Temperature Measurement Scales Temperature Measurement Scales Level 1 - TemperatureRMT Training - 05 /98 9 Temperature Measurement Temperature. FAHRENHEIT Temperature terminology Temperature terminology Temperature Measurement Scales Temperature Measurement Scales Level 1 - TemperatureRMT Training - 05 /98 8 Differential Temperature Differential. undergoes a change in temperature. » If we can measure the resistance in the metal, we know the temperature! Level 1 - TemperatureRMT Training - 05 /98 1 5 Temperature Sensors Temperature Sensors RTDs RTDs How