14.13. A th le is a temperature sensor that generates a voltage proportional to
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14.13. A th le is a temperature sensor that generates a voltage proportional to the measured temperature difference from a reference temperature. The thermal capacitance of the probe tip limits its ability to track rapidly changing temperatures. The response is described by a first-order transfer function relating output voltage vT(t) to tip temperature T(t) H(s) ts 1 where K is a constant and t i s. An electronic amplifier is used to amplify and process the output voltage as shown in Fig. 14.30. Amplifier Thermocouple Figure 14.30: Thermocouple temperature sensor and amplifier. (a) Determine the frequency response of the thermocouple sensor. At what frequency has the amplitude response rolled off to 0.707 (-3 dB) of the low-frequency response? useful bandwidth of the thermocouple system. A prototype amplifier has a transfer function 1 Ha(s) K where Ka is a constant. If ri is adjusted to be l s, what value of t2 will double the cut-off frequency of the system? Sketch the combined thermocouple-amplifier frequency response. What range of frequencies will have a response less than 10% (-20 dB) of the low-frequency response?Explanation / Answer
TEMPERATURE MEASUREMENT SYSTEMS:
Introduction Temperature measurement can be accomplished using several types of sensing mechanisms. Temperature measurement systems generally consist of a sensor, a transmitter, an external power supply (for some types of systems), and the wiring that connects these components. The temperature measurement sensors most commonly used in engineering applications are thermocouples, resistance temperature detectors (RTD’s), and infrared (IR) thermometers; these devices are described in detail in the following paragraphs. Integrated circuit (IC) temperature transducers and thermistors also are commonly used but have more limitations than thermocouples, RTD’s, and IR thermometers. Table 4.2-1 lists some of the advantages and disadvantages of these types of temperature measuring devices.
Other types of temperature sensors include bimetallic devices, fluid expansion devices, and change-of-state devices. Bimetallic temperature sensors relate temperature to the difference
in thermal expansion between two bonded strips of different metals. Fluid expansion devices, such as the common thermometer, measure temperature as a function of the thermal expansion of mercury or organic liquid, such as alcohol. Change-of-state temperature sensors change appearance when a specific temperature is reached. One major drawback of these types of sensors is that they do not readily lend themselves to automatically recording temperatures on a continuous or periodic basis. The following paragraphs describe temperature measurement systems that are based on three types of temperature sensors: Section 4.2.2 describes thermocouples, Section 4.2.3 describes RTD’s, and IR thermometers are described in Section 4.2.4. For each type of system, the system components, operation, accuracy
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