Instrumentation and Control: ME- 441 Department of Mechanical Engineering UMT 1

Part I : Measurement and Instrumentation Temperature Measuring Instruments 2

Temperature is a measure of the thermal energy in a body, which is then relative hotness or coldness of a medium. Absolute zero is the temperature at which all molecular motion ceases or the energy of the molecule is zero. Heat is a form of energy; as energy is supplied to a system the vibration amplitude of its molecules and its temperature increases. The temperature increase is directly proportional to the heat energy in the system. Specific heat is the quantity of heat energy required to raise the temperature of a given weight of a material by 1°. 3 Basic terms related to Temperature and Heat

Thermal conductivity is the flow or transfer of heat from a high temperature region to a low temperature region. There are three basic methods of heat transfer; conduction, convection, and radiation. Linear thermal expansion is the change in dimensions of a material due to temperature changes. The change in dimensions of a material is due to its coefficient of thermal expansion that is expressed as the change in linear dimension α per degree temperature change. Volume thermal expansion is the change in the volume (b) per degree temperature change due to the linear coefficient of expansion. 4 Basic terms related to Temperature and Heat

Methods of temperature measuring can be categorized as follows: 1. Material Expansion 2. Electrical Resistance Change 3. Semiconductor characteristic change 4. Voltage generated by dissimilar metals 5. Radiated energy 5 Temperature Measuring Devices

Mercury in Glass Thermometer Mercury in glass is the most common direct visual reading thermometer. The device consisted of a small bore graduated glass tube with a small bulb containing a reservoir of mercury. The coefficient of mercury is several times greater than the coefficient of expansion of glass, so that as the temperature increases the mercury rises up the tube. The operating range of the mercury thermometer is from −30 to 800°F (−35 to 450°C). The toxicity of mercury, ease of breakage, and easily read digital thermometers has brought about the demise of the mercury thermometer. 6 Thermometers

Liquid in Glass Thermometer The liquid in glass thermometers are used to replace the mercury thermometer and to extend its operating range. The liquids used have similar properties to mercury, i.e., high linear coefficient of expansion, clearly visible, non wetting, but are nontoxic. These thermometers are accurate and with different liquids (each type of liquid has a limited operating range). have an operating range from −300 to 600°F (−170 to 330°C). Common liquid used in glass thermometer is Alcohol (Ethanol). 7 Thermometers

These thermometers are used where remote indication is required, as opposed to glass and bimetallic devices which give readings at the point of detection. Gas Thermometer They are known as Constant volume gas thermometer The gas is kept in a container at constant volume and the pressure and temperature vary, the ratio can be express as Gas thermometer is filled with a gas such as nitrogen at a pressure range of 1000 to 3350 kPa at room temperature. The device obeys the basic gas laws for a constant volume system [V1 = V2] giving a linear relationship between absolute temperature and pressure. It convert temperature information directly into pressure signals, so it is useful in pneumatic signals. A gas in a closed volume has a pressure of 120 psi at a temperature of 20oC. what will be the pressure at 100oC. 8 Pressure Spring Thermometers

9 Bimetallic Strip

If two strips of dissimilar metals such as brass and invar (copper-nickel alloy) are joined together along their length, they will flex to form an arc as the temperature changes. Their operating range is from −180 to 430°C. Bimetallic strip is a type of temperature measuring device that is relatively inaccurate, slow to respond. The bimetallic strip is extensively used in ON/OFF applications not requiring high accuracy, as it is rugged and cost effective. Extensively used as industrial control thermostat. 10 Bimetallic Strip

Resistance temperature devices (RTD) are either a metal film deposited on a former or are wire-wound resistors. The devices are then sealed in a glass or ceramic composite material. The electrical resistance of pure metals is positive, increasing linearly with temperature. These devices are accurate and can be used to measure temperatures from −300 to 1400°F (−170 to 780°C). The variation of resistance with temperature is given by where RT2 is the resistance at temperature T2 and RT1 is the resistance at temperature T1. 11 Resistance Temperature Devices/Detectors

12

13

14

What is the resistance of a platinum resistor at 250°C, if its resistance at 20°C is 1050 Ω? Resistance devices are normally measured using a Wheatstone bridge type of system, but are supplied from a constant current source. Care should also be taken to prevent electrical current from heating the device and causing erroneous 15 Resistance Temperature Devices/Detectors

Thermistors are a class of metal oxide (semiconductor material) which typically have a high negative temperature coefficient of resistance. Thermistors have high sensitivity which can be up to 10 percent change per degree Celsius, making them the most sensitive temperature elements available, but with very nonlinear characteristics. The nonlinear characteristics are as shown in Fig. 8.5 and make the device difficult to use as an accurate measuring device without compensation, but its sensitivity and low cost makes it useful in many applications 16 Thermistor

17 Thermistor

Thermocouple Thermocouples are formed when two dissimilar metals are joined together to form a junction. An electrical circuit is completed by joining the other ends of the dissimilar metals together to form a second junction. An electromotive force exists at the junction formed between two dissimilar metals. A current will flow in the circuit if the two junctions are at different temperatures. This is called as Seebeck effect. 18

Thermocouple The current flowing is the result of the difference in electromotive force developed at the two junctions due to their temperature difference. Voltage difference between the two junctions is measured; the difference in the voltage is proportional to the temperature difference between the two junctions. Thermocouple can only be used to measure temperature differences. If one junction is held at a reference temperature the voltage between the thermocouples gives a measurement of the temperature of the second junction. 19

Thermocouple Three effects are associated with thermocouples. Seebeck effect. It states that the voltage produced in a thermocouple is proportional to the temperature between the two junctions. Peltier effect. It states that if a current flows through a thermocouple one junction is heated (puts out energy) and the other junction is cooled (absorbs energy). Thompson effect. It states that when a current flows in a conductor along which there is a temperature difference, heat is produced or absorbed, depending upon the direction of the current and the variation of temperature. 20

Thermocouple 21

Thermocouple 22

Thermopiles Thermopile is a number of thermocouples connected in series. Increase the sensitivity and accuracy by increasing the output voltage when measuring low temperature differences. Each of the reference junctions in the thermopile is returned to a common reference temperature. 23

Pyrometers (Non-Contact Device) Used for temperature measurements in furnaces Pyrometers are devices that measure temperature by sensing the heat radiated from a hot body. Fixed lens focuses heat energy onto a thermopile. Furnace temperatures, for instance, are normally measured through a small hole in the furnace wall. The distance from the source to the pyrometer can be fixed and the radiation should fill the field of view of the sensor. 24

Application Consideration 25

Application Consideration 26

Application Consideration 27 Thermal time constant A temperature detector does not react immediately to a change in temperature. The reaction time of the sensor or thermal time constant is a measure of the time it takes for the sensor to stabilize internally to the external temperature change. When the temperature changes rapidly, the temperature output reading of a thermal sensor is given by

Application Consideration 28 Thermal time constant The time constant of a system tc is considered as the time it takes for the system to reach 63.2 percent of its final temperature value after a temperature change. A copper block is held in an ice–water bath until its temperature has stabilized at 0°C, it is then removed and placed in a 100°C steam bath, the temperature of the copper block will not immediately go to 100°C, but its temperature will rise on an exponential curve as it absorbs energy from the steam, until after some time period (its time constant) it will reach 63.2°C, aiming to eventually reach 100°C. During the second time constant the copper will rise another 63.2 percent of the remaining temperature to get to equilibrium, i.e., (100 − 63.2) 63.2 percent = 23.3°C, or at the end of 2 time constant periods, the temperature of the copper will be 86.5°C. At the end of 3 periods the temperature will be 95° and so on.

Application Consideration 29 Thermal time constant