Aircraft Instruments - Air Temperature Measurement PDF

Chapter 2 Air Temperature Measurement 2.1 Introduction In this chapter, the aircraft instruments used to measure the air temperature will be introduced. Firstly, some definitions will presented as follows: ∗ Temperature: It is the measure of the ”hotness” or ”coldness” of the substance or...

Chapter 2 Air Temperature Measurement 2.1 Introduction In this chapter, the aircraft instruments used to measure the air temperature will be introduced. Firstly, some definitions will presented as follows: ∗ Temperature: It is the measure of the ”hotness” or ”coldness” of the substance or the quality of heat. ∗ Ice Point: It is the temperature at which equilibrium exists between ice and vapour saturated air at a pressure of 760 mmHg. ∗ Steam Point: It is the temperature at which equilibrium exists between liquid water and its vapour at a pressure of 760 mmHg. ∗ Fundamentals Interval: It is the temperature interval or change between two fixed points. ∗ Temperature Scale: The fundamentals interval is divide into a number of equal parts or degrees the division being in accordance with two scale notations, Celsius and Fahrenheit. Celsius Scale: The fundamental interval is divided into 100 degrees, the ice point being taken as 0◦ C and the steam point as 100◦ C. One Celsius degree is thus 1/100 of the fundamental interval. Fahrenheit Scale: The fundamental interval is divided into 180 degrees, the ice point being taken as 32◦ F and the steam point as 212◦ F. One Fahrenheit degree is thus 1/180 of the fundamental interval. Kelvin Scale: This temperature is −273.15◦ C or 273.15◦ C below freezing point, and is known as absolute zero or 0 on the Kelvin scale. 2.2 Methods of Temperature Measurements In most forms of temperature measurement, the variation of some property of substance with temperature is utilized. These variations may be summarized as: 9  1. Most substance expand as their temperature rises; thus, a measure of temperature is obtainable by taking equal amounts of expansion to indicate equal increment of temper- ature. 2. Many liquid, when subjected to a temperature rise, experience such motion of their molecules that there is a change of state from liquid to vapour. Equal increment of temperature may therefore be indicated by measuring equal increment of the pressure of the vapour. 3. Substance change their electrical resistance when subjected to varying temperature, so that a measure of temperature is obtainable by taking equal increment of resistance to indicate equal increments of temperature. 4. Dissimilar metals when joined at their ends produce an electromotive force (Thermo e.m.f) dependent on the difference in temperature between the junctions. Thermometers are classified according to: Substance whose temperature is measured: 1. Oil thermometer 2. Coolant thermometer 3. Gas thermometer (for jet engine) 4. Cylindrical head thermometer (for air cooled piston engine). 5. Cabin air thermometer. 6. Atmosphere air thermometer Physical principle of measurement: 1. Dilation thermometer. (Expansion type) 2. Bimetallic thermometer. (Expansion type) 3. Pressure type thermometer (volume expansion of liquids). 4. Pressure type thermometer (vapor or gas pressure changed). 5. Electrical resistance thermometer. (Electric type) 6. Thermocouples thermometers. (Electric type) 7. Optical Pyrometer. (Light) 8. Thermograph. (Infrared) In the coming sections, the most popular air thermometers will be presented including their construction and principle of operation. 2.3 Bimetallic Thermometer The physical characteristics of most materials change when exposed to changes in temperature. The changes are consistent, such as the expansion or contraction of solids, liquids, and gases. 10 The coefficient of expansion of different materials varies and it is unique to each material. The direct reading thermometer is the simplest type of thermometer, and it only indicates SAT. It consists of a bi-metallic element in the shape of a helix, which expands and contracts when subjected to temperature changes as shown in Figure 2.1.As the temperature changes, the helix winds or unwinds, and causes a pointer to rotate against a scale calibrated in degrees Celsius. This type of thermometer is normally only fitted on low-speed light aircraft, and passes through the fuselage or canopy with the sensitive element projecting into the airflow. The element is also shielded so that it is not directly affected by solar radiation. Figure 2.1: Bimetallic Thermometer. 2.4 Electrical Resistance Thermometer The use of electricity in measuring temperature is very common in aviation. The following measuring and indication systems can be found on many types of aircraft. Certain tempera- ture ranges are more suitably measured by one or another type of system. The principle parts of the electrical resistance thermometer are the indicating instrument, the temperature-sensitive element (or bulb), and the connecting wires and plug connectors. Electrical resistance thermometers are used widely in many types of aircraft to measure car- buretor air, oil, free air temperatures, and more. They are used to measure low and medium temperatures in the −70◦ to 150◦ range. The indicator contains a resistance-measuring instrument. Sometimes it uses a modified form of the Wheatstone bridge circuit shown in Figure 2.2. The Wheatstone-bridge meter operates on the principle of balancing one unknown resistor against other known resistances. Three equal values of resistance [A, B, and C] are connected into a diamond shaped bridge circuit. A resistor with an unknown value [D] is also part of the circuit. The unknown resis- tance represents the resistance of the temperature bulb of the electrical resistance thermometer system. A galvanometer is attached across the circuit at points X and Y. 2.5 Thermocouple Temperature Indicators A thermocouple is a circuit or connection of two unlike metals. The metals are touching at two separate junctions. If one of the junctions is heated to a higher temperature than the 11 Figure 2.2: The internal structure of an electric resistance thermometer indicator features a bridge circuit, galvanometer, and variable resistor, which is outside the indicator in the form of the temperature sensor. other, an electromotive force is produced in the circuit. This voltage is directly proportional to the temperature. So, by measuring the amount of electromotive force, temperature can be determined. A voltmeter is placed across the colder of the two junctions of the thermocouple. It is calibrated in degrees Fahrenheit or Celsius, as needed. The hotter the high temperature junction (hot junction) becomes, the greater the electromotive force produced, and the higher the temperature indication on the meter. Figure 2.3 represents a thermocouples combine two unlike metals that cause current flow when heated. Figure 2.3: A thermocouples combine two unlike metals that cause current flow when heated. 12 2.6 Air Temperature Gauge Errors Similar to an instruments, air temperature gauges are affected by internal and external factors causing errors in the reading. The errors are as follows: 1. Instrument error: It results from manufacturing imperfections. 2. Environmental error: It results from solar heating of the temperature sensor. 3. Heating error: It results from friction and air compression. It accounts for the greatest error in high speed aircraft. 13