Temperature Sensing and Measuring Devices PDF

Unit A-9 • Energy Plant Instrumentation and Controls Objective 4 Describe the types of temperature sensing and measuring devices. Temperature Measurement Glass Stem Thermometers Glass stem thermometers (also called liquid-in-glass thermometers) are used extensively in laboratories and industrial s...

Unit A-9 • Energy Plant Instrumentation and Controls Objective 4 Describe the types of temperature sensing and measuring devices. Temperature Measurement Glass Stem Thermometers Glass stem thermometers (also called liquid-in-glass thermometers) are used extensively in laboratories and industrial settings. These thermometers employ the thermodynamic principle of volumetric expansion of liquids. The thermometer has a liquid-filled bulb, and a glass stem with a uniform bore. When the bulb is heated, the liquid expands through the glass stem until the liquid reaches the temperature of the substance being measured. The temperature is read at the point where the liquid reaches the highest point in the glass stem. The temperature scale is either etched onto the glass stem (Figure 42 (a)), or attached beside the stem (Figure 42 (b)). Liquid-in-glass thermometers are available in a variety of temperature ranges. However, the type of thermometer used must be selected according to the range of temperatures being measured. When in use, the fluid contained within the thermometer must not freeze or boil. Some thermometers have ranges from -40 to 40°C. Others measure from 10 to 400°C. Very accurate thermometers are generally quite long and have a small range. Liquid-in-glass thermometers are fragile. For this reason, they are not suited for use in locations where they may be subject to vibration or mechanical injury. Figure 42 – Glass Stem Thermometers -100 Uniform bore -90 -80 -70 -60 -50 -40 -30 -20 -10 0 Bulb Laboratory Thermometer (a) Industrial Thermometer (b) Alcohol-filled liquid-in-glass thermometers are used extensively for home, laboratory, medical, and industrial applications. Toluene, butane, and other organic liquids are used in liquid-in-glass thermometers for higher temperatures. 2-34 4th Class Edition 3 • Part A Introduction to Process Measurement • Chapter 2 Mercury-filled thermometers were quite common in the past. However, mercury has been banned in many countries due to its hazardous and toxic effects on health and the environment. Check site-specific policies and jurisdictional regulations regarding the use of mercury-filled thermometers, to find out if they are permissible. Filled System Thermometers The filled system thermometer, illustrated in Figure 43, can be used to provide an indication of temperature, or to produce a control signal proportional to the measured temperature so it can be used for recording and controlling at a distance. A basic system consists of a temperature sensitive bulb, a capillary tube, a pressure-sensing device (such as a bourdon tube, bellows, or diaphragm), and an indicating or transmitting device. The system is completely filled with fluid (liquid, gas, or vapour). With an increase in measured temperature, the fluid in the bulb expands, increasing the pressure in the bulb, capillary, and the pressure-sensing element. Figure 43 shows a temperature controller operated by a filled-system thermometer. The controller has a pressure-sensing element that responds to the increase in fluid pressure in the bulb. This activates a switch, according to the temperature on the set point dial. Some filled system thermometers are only indicators. Others may be used to drive temperature transmitters or recording devices. Figure 43 – Temperature Control with Filled System Thermometer Controller Capillary Bulb Bimetal Thermometers The bimetal thermometer consists of two thin strips of metal, with different coefficients of expansion, laminated together. When one end is fixed, as illustrated in Figure 44(a), the free end deflects in nearly direct proportion to the change in temperature. Brass and invar are the metals often used, because the coefficient of linear expansion of brass is over twenty times higher than that of invar. 4th Class Edition 3 • Part A 2-35 Unit A-9 • Energy Plant Instrumentation and Controls Figure 44 – Bimetal Thermometers, Bimetal Strip Flat bimetal strips deflect only slightly, unless made very long. To amplify the deflection and still maintain compactness, bimetal strips may be wound into a helix or spiral. Figure 44(b) shows an industrial bimetal thermometer that uses a helical bimetal element whose motion is transmitted to a pointer by a shaft. Bimetal thermometers are far sturdier and break-resistant than liquid-in-glass thermometers. They are also easier to read. Bimetal thermometers are available for very low temperatures (-70°C), and very high temperatures (550°C), with spans ranging from 50 to 450°C. Thermocouples The thermocouple is one of the most widely used temperature sensing devices. It consists of two wires, each made of a different metal or alloy (dissimilar metals). These wires are connected at one end to form the measuring junction, as shown in Figure 45. The free ends of the two wires are connected to a measuring instrument, either directly or by means of extension wires. The connection to the instrument is called the reference junction. When the measuring and reference junctions are at different temperatures, the thermocouple produces a voltage, which causes a current to flow. The magnitude of this generated DC voltage is a function of the temperature difference between the two junctions. Figure 45 – Basic Thermocouple Circuit Scale Calculated in °C (°F) Pointer Millivoltmeter Magnet Moving Coil Thermocouple Reference Junction 2-36 4th Class Edition 3 • Part A Introduction to Process Measurement • Chapter 2 A basic thermocouple and measuring circuit is illustrated in Figure 45. The thermocouple circuit is connected to a millivolt meter whose scale is calibrated in degrees Celsius. An increase in temperature will cause an increase in current flow through the moving coil, and a corresponding movement of the pointer on the temperature scale. In actual practice, the measuring junction of a thermocouple is placed at the point of temperature measurement, while the meter with the reference junction may be some distance away. Extension wires from the reference junction to the millivolt meter have no effect on the output, as long as both ends of the wire are at the same temperature. Various combinations of metals may be used depending on the temperature range required. Some types of thermocouples and their approximate temperature ranges are shown in Table 1. Table 1 – Thermocouple Ranges Type of Thermocouple Wire Materials Temperature Range (°C) Type J Iron – Constantan 0 - 815 Type K Chromel – Alumel -185 to 1260 Type B Platinum/Rhodium – Platinum 0 to 1860 Type T Copper – Constantan -300 to 400 Thermocouples have non-linear response to temperature change. However, their response is almost linear over their designed temperature range, making them useful temperature sensors. Thermocouples are frequently used as pilot flame detection devices for smaller burners and appliances. Because they are suitable for high temperature, thermocouples are commonly used to transmit high temperature process conditions, like superheated steam temperature. Thermistors A thermistor is a solid state, resistance temperature sensor that may have either a negative or a positive temperature coefficient. Figure 47 shows the response of a thermistor with a negative temperature coefficient. As its temperature increases, this type of thermistor’s resistance decreases. Thermistors look similar to glass diodes or small transistors (Figure 46), and provide quick temperature response. Because the response is non-linear, thermistors are only suitable for measuring temperature over very small spans. However, they are highly accurate for these very small spans. Thermistors are commonly used in HVAC thermostats and temperature transmitters. Thermistors are not capable of measuring temperatures above 315°C. Figure 46 – Thermistor (Solid State) Temperature Sensor (Courtesy of Honeywell Inc.) 4th Class Edition 3 • Part A 2-37 Unit A-9 • Energy Plant Instrumentation and Controls Thermistor Resistance (Ω) Figure 47 – Resistance vs Temperature Relationship for Thermistors 4000 Temperature Resistance Curve 1000 200 0 50 100 150 200 Temperature °C (Courtesy of Honeywell Inc.) Resistance Temperature Devices (RTDs) The resistance temperature device (RTD) is based on the principle that conductors change in resistance, proportional to their temperature. As a conductor increases in temperature, it increases in resistance. RTDs are made of fine gauge wires wrapped around a non-conducting core, or deposited on a flat ceramic plate. Platinum, nickel, copper, or nickel-iron alloy (Balco) wires are commonly used. The fine wire is exposed to the process condition, and its resistance is measured and converted to temperature. A Balco RTD provides a relatively linear change in resistance from -70 to 200°C. These RTDs are small and respond quickly to changes in temperature. A platinum RTD has a very linear and stable response from -240 to 650°C. Platinum RTDs are the most expensive, but are the most accurate. Figure 48 shows a platinum element type RTD on a ceramic base. The two wires connected to the platinum wires would be connected to an ohmmeter, which is reading in temperature. Figure 48 – Platinum Element RTD Sensor 2-38 4th Class Edition 3 • Part A Introduction to Process Measurement • Chapter 2 Thermowells A thermowell is a metal sheath used to house temperature sensors, such as liquid-in-glass thermometers, bimetal thermometers, filled system thermometer sensing bulbs, thermocouples, thyristors, and RTDs. The thermowell: a) Protects the sensing element from the action of harmful atmospheres, corrosive fluids, or mechanical damage. b) Permits installation of a sensing element into a pressurized system. c) Permits easy replacement of defective sensing elements without isolating process lines. The thermowell is a metal tube that is closed at one end. The closed end is inserted into a pipe or vessel. Three types of thermowells are shown in Figure 49. The thermowell may be socket welded, flanged, or threaded into a pipe or vessel. The temperature-sensing device is screwed into the open end of the thermowell. Figure 49 – Mounting Styles of Thermowells Socket Weld Flanged Threaded Pyrometer To measure high temperature, or temperature from a source that is some distance away from the operator, a pyrometer may be used. Both optical and radiation pyrometers are available. These devices operate on the principle that heat is a form of radiant energy that can be measured. As temperature increases, the radiated heat increases. The optical pyrometer uses a very narrow band of radiation wavelengths (that of visible light) to measure the temperature of a heated body. Bodies vary in colour predictably, from dull red to bright yellow, with increasing temperature. The optical pyrometer in Figure 50 uses this principle. In a typical optical pyrometer, the radiation source is viewed through a scope with a lens and an eyepiece. Inside the scope is a small lamp heated by the current from a battery. The current is adjustable with a rheostat. A milliammeter is connected in the lamp circuit. A red optical filter is positioned between the eyepiece and lamp. When looking through the eyepiece, the temperature source is seen as a bright circle. The image of the lamp’s filament is seen in the center this circle. The rheostat is adjusted until the brightness of the filament is equal to that of the temperature source. When the lamp filament appears as bright as the temperature source, the filament image disappears into the heat source image. If the filament is brighter than the source image, it appears bright against a dark background. If the filament is not as bright as the source image, it appears dark against a lighter background. 4th Class Edition 3 • Part A 2-39 Unit A-9 • Energy Plant Instrumentation and Controls When the images merge, the temperature is read from the attached milliammeter scale (Figure 50), or (as in some other varieties) read from the dial of the rheostat. The rheostat scale or the milliammeter scale can be calibrated directly in degrees of temperature. Figure 50 – The Radiation Pyrometer Principle Heat Source Lamp Filament Filter Eye Piece Lens Rheostat Rheostat Milliammeter Battery Infrared Thermometer (Pyrometer) An infrared thermometer (Figure 51) is a digital device that measures the temperature of an object at a distance by detecting its infrared energy emissions. The device has a laser to assist with aiming, so the correct surface temperature reading can be made. The thermometer senses emitted, reflected, and transmitted infrared energy, which is collected and focused onto a detector. Electronic circuitry translates the signal into a temperature reading which the unit then displays. Infrared temperature detectors: • Are inexpensive, compact, and portable • Are easy to use • Display temperatures in several units of measurement • Can measure temperatures of less accessible objects from a distance • Work in ranges from -40°C to 800°C However, infrared temperature detectors only measure surface temperatures. Figure 51 – Infrared Thermometer (Pyrometer) 2-40 4th Class Edition 3 • Part A