Ch. 5 temperature measurement


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Ch. 5 temperature measurement

  2. 2. INTRODUCTION  What is Temperature? Temperature is defined as the degree of hotness or coldness p g measured on a definite scale. Hotness and coldness are the result of molecular activity. As the molecules of a substance move faster, the temperature of that substance increases. p  What is Heat? gy Heat is a form of energy and is measured in calories or BTU's (British Thermal Units).  Why temperature is measured as a process variable? There are changes in the physical or chemical state of most substances when they are heated or cooled. The measurement of temperature is also important for protection of the equipment, as uncontrolled high or low temperatures can cause structural deterioration of pipelines and vessels.
  3. 3. Heat Transfer The flow of heat is transferred in three ways: convection, conduction, and radiation. a) C d ti ) Conduction  When heat is applied to one part of a substance, it is transferred to all p parts of the substance. The movement is from molecule to molecule. Gases and liquids are poor conductors. The flow of heat by conduction takes place most effectively in solids. 
  4. 4. b) Convection  Heat transferred by the actual movement of portions of a gas or liquid from one place to another is called convection. This movement is caused by changes in density due to rising temperat re For example, in a forced air heating s stem the warm air temperature. e ample system, arm entering the room through the supply duct is less dense, and therefore, lighter than the cooler air already in the room. As the warm air-cools, it drops and moves through the cool air return and back through the heating system system.
  5. 5. c) Radiation  Heat energy is transferred in the form of rays sent out by the heated substance as its molecules undergo internal change. Only energy is transferred. The direction of the flow of heat is from the radiating source. The radiant energy is then absorbed by a colder substance or object. Radiation takes place in any medium (gas, liquid, or solid), or in a vacuum vacuum.
  6. 6. Temperature Scales      Absolute zero is the temperature at which the movement of molecules completely stops Temperature value on given scale can be converted to express on other Scales: C = ( F-32) x 5/9 F = ( C x 9/5) + 32 K = ( C + 273.2)
  7. 7. Temperature Measuring Sensors  The common temperature measuring sensors are: 1.Thermometers, 2.Thermocouples, 3.Resistance Temperature Detectors (RTDs)
  8. 8. 1. Thermometers a)Fill d Filled Th Thermometers t  These are thermometers filled with either a liquid such as mercury or an evaporating fluid such as used in refrigerators. In both cases the inside refrigerators of the sensor head and the connecting tube are completely full.  Any rise in temperature produces expansion or evaporation of the liquid so the sensor becomes pressurised. The pressure is related to the temperature and it may be indicated on a simple pressure gauge.
  9. 9.  The problems with glass thermometers are that they are: 1.Mercury solidifies at -40oC. 2.Alcohol boils at around 120oC. 3.Accurate 3 Accurate manufacture is needed and this makes accurate ones expensive. 4.It is easy for people to make mistakes reading them. them
  10. 10.  b) Bimetallic Strips  A bimetallic strip is constructed by bonding t b di two metals with different t l ith diff t coefficients of thermal expansion.  If heat is applied to one end of the strip, the metal with the higher p, g coefficient of expansion will expand more readily than the lower one.  As a result, the whole metallic strip will bend in the direction of the metal with the lower coefficient.
  11. 11.  Bimetallic strip applications:  it can be used as a fast acting thermostat to control air temperature.  They are usually used to measure process parameters for local readout.
  12. 12. 2. Thermocouples p  A thermocouple consists of two pieces of dissimilar metals with their ends joined together (by twisting soldering or welding). twisting, welding)  When heat is applied to the junction, a voltage, in the range of millivolts (mV), is generated. A thermocouple is therefore said to be self-powered. Shown is a completed thermocouple circuit.
  13. 13.  In order to use a thermocouple to measure process temperature, one end of the thermocouple has to be kept in contact with the process d f h h l h b k h h while the other end has to be kept at a constant temperature. The end that is in contact with the process is called the hot or measurement p junction. Circuit emf = Measurement emf - Reference emf  If circuit emf and reference emf are known, measurement emf can be calculated and the relative temperature determined.  To convert the emf generated by a thermocouple to the standard 4-20 mA signal, a transmitter is needed.
  14. 14. Types of thermocouples Thermocouples exist in many different types, each with its own color codes for the dissimilar-metal wires. Here is a table showing the more common thermocouple types and their standardized colors, along with some distinguishing yp , g g g characteristics of the metal types to aid in polarity identification when the wire colors are not clearly visible:
  15. 15.  An output of 40 millivolts at 1000ºF can be compared to 30 mv for type J and 22 mV for type K. Type E has more tendencies to change characteristics with time than type K J, K and T.
  16. 16. Three Basic Types Of Thermocouple Assembly yp p y Some thermocouple assemblies are manufactured as follw:  Ground junction  Ungrounded junction  Exposed junction
  17. 17. o The exposed junction is often used for the measurement of static or flowing non-corrosive gas temperatures where the response ti time must b minimal. t be i i l o The ungrounded junction often is used for the measurement of static or flowing corrosive gas and liquid temperatures in critical electrical applications. pp o The grounded junction often is used for the measurement of static or flowing corrosive gas and liquid temperatures and f i fl i i d li id d for high-pressure applications.
  18. 18. T/C Operation and Installation Aspects Extension cables  In every thermocouple circuit there must be both a measurement y p junction and a reference junction: this is an inevitable consequence of forming a complete circuit (loop) using dissimilar-metal wires.  As we already know, the voltage received by the measuring instrument from a thermocouple will be the difference between the voltages produced by the measurement and reference junctions.  Since the purpose of most temperature instruments is to accurately measure temperature at a specific location, the effects of the reference junction’s voltage must be “compensated” for by some means, either a special circuit designed to add an additional canceling voltage or by a i l i it d i d t dd dditi l li lt b software algorithm to digitally cancel the reference junction’s effect.
  19. 19. Thermocouple reference tables  When using thermocouples, temperature reference tables t bl can be used to convert the mV signal from the b dt t th V i lf th thermocouple into a temperature reading.  All table values are referenced to a cold junction temperature of 0° C. if the reference junction of a thermocouple is not at 0°C, the tables can still be p , used by applying an appropriate Correction to compensate f th diff t for the difference between the reference b t th f junction and 0 ºC.
  20. 20. 3. Resistance Temperature Detectors  The resistance of a conductor usually increase as the temperature increase .if the properties of that conductor are known, the temperature can be calculated from the measured resistance.  Any conductor can be used to construct an RTD, but a few have been identified as having more described characterstics than others. The characteristics which are desired include include. 1.Stability: in the temperature range to be measured. The material must not melt, correde, embattle or change electrical characteristics when subjected to the environment in which it will operate. 2.Linearity: The resistance change with temperature should be as liner as possible over the rang of interst to simplify readout. 3.High resistively: Less material is needed to manufactor an RTD with a specified resistance when the matrial has a high characteristic resistively. i ti l 4.Workability: The material must be suitable for configuring for insertion into the media.
  21. 21.  Platinum has been accepted as the material, which best fit all the criteria and has been generally accepted for industrial measurement between –300 and 1200° F (-150 and 650 °C ).  RTDS are commercially available with resistances from 50 to 1000 ohms.  Platinum RTD known as Pt100, because it has 100 Ohm resistance at 32°F ( 0°C) and increase resistance 0.385 ohms for f every °C of temperature rise. ft t i  When the resistance of the RTD is found by measurement, the temperature can b calculated: be l l d °C = ( Ohms reading – 100 ) / 0.385  the accuracy of this calculation is determined primarily by the accuracy of the reading.
  22. 22. RTD Connections 2-wire 2 wire connection:  To detect the small variations of resistance of the RTD, a temperature transmitter in the form of a Wheatstone bridge is generally used. The circuit compares the RTD value with three known and highly accurate resistors.  A problem arises when the RTD is installed some distance away from the transmitter. Since the connecting wires are long, resistance of the wires changes as ambient temperature fluctuates. The variations in wire resistance would introduce an error in the transmitter To eliminate this transmitter. problem, a three-wire RTD is used.
  23. 23. 3-wire RTD connection:  The connecting wires (w1, w2, w3) are made the same length and therefore the same resistance. The power supply is connected to one end of the RTD and the top of the Wheatstone bridge.  It can be seen that the resistance of the right leg of the Wheatstone bridge is R1 + R2 + RW2.  The resistance of the left leg of the bridge is R3 + RW3 + RTD.  Since RW1 = RW2, the result is that the resistances of the wires cancel and therefore the effect of the connecting wires is eliminated eliminated.
  24. 24. Thermistors   Thermistor is resistance temperature element made from a semiconductor material and basically do the same job as an RTD. These elements g generally have a negative Temperature coefficient (NTC) but positive y g p ( ) p temperature coefficients are also available over a limited range. They are only used for a typical range of -20 to 120oC and are commonly used in small hand held thermometers for every day use. The d Th advantage of a Th t f Thermistor i it i hi hl sensitive t t i t is is highly iti to temperature t changes making them useful in temperature trip alarms. Unfortunately they posses highly non-linear resistive properties which restrict their useful range
  25. 25. Thermowells  Thermowells e The mo ell are used to p ote t the dete to and so that the detector can ed protect detector nd o th t dete to n be changed without interrupting the process. One downside of using a thermowell is the time delay it introduces into the measurement system due to thermal lag.  Thermowlls should be installed where a good representative sample of the process fluid temperature can be measured measured.  The optimum immersion length of a thermowell depends on the application pp  If the well is installed perpendicular to the line, the tip of the well should be between one half and one third of the pipe diameter.  If the well is installed in an elbow, the tip should point towards the flow.  The speed of response of a sensor in a thermowell will be slower than that of an unprotected buib. Keeping the clearance between bulb and pocket down to an absolute minimum and filling the space with oil or glycol ( (antifreeze) can reduce this effect. )