Air Temperature Indicator in Aircraft – PART 1

Air Temperature Indicator in Aircraft

The temperatures of many objects should be known prior to the flight for the correct operation of an aircraft. Such equipment acts as the air temperature indicator in aircraft! These aircraft temperature measuring equipment include the intake air, engine oil, free air, carburetor mixture, engine’s cylinder heads, heater duct as well as turbine engine exhaust  temperature. These happen to be all the things that require proper monitoring of temperatures. Several other temperature measurements should be known as well. Various thermometers are being utilized for collecting & displaying temperature data. 

Non-Electrical Temperature Sensors 

The physical properties of many of the materials alter when they are exposed to any alterations in the temperatures. These alterations tend to be continuous, for example, the expansion/contraction of solids, liquids, & gasses. The coefficient of such an expansion happening with various materials tends to be different, and is individual for every such material. 

  • Many people are quite familiar with the liquefied mercury thermometer most of us use at home. As mercury increases in temperature, it expands and goes up along a narrow channel with a scale that can be utilized for reading the temperature associated with that expansion. Such mercury thermometers are of no use at all in the aviation industry.
  • Bimetallic thermometers, on the other hand, are quite useful in the aviation field. The element used for sensing the temperature measurement of such bimetallic thermometers is made of 2 different metal strips that are glued together. Every metal tends to expand as well as contract at different rates as the changes in temperature happen. 1 of the ends of this bimetallic strip tends to be fixed, while the other end is twisted. A pointer is attached to the threaded end, which is inserted into the body of the instrument. When the bimetallic strip gets heated, the 2 metals tend to expand. Because they have different expansion rates, and are connected to each other; the coiled end will try to loosen if one metal expands faster than the other. This leads to a movement in the pointer over the instrument dial. As the temperature tends to drop, the metals tend to contract at different rates; this tightens the coil & helps in the movement of the pointer in the opposite direction.
  • Direct-range bimetallic temperature gauges have been in use much frequently in light aircraft for measuring free air temperatures or the outside air temperatures (OAT). In such an application, the collection probe is inserted through the aircraft’s windshield for exposing it to atmospheric air. The end that is coiled on the gauge head’s bimetallic strip is just inside the windshield where the pilot can read it.
  • The bourdon tube too is utilized for direct reading non-electrical temperature gauges for simple & light aircraft. Bourdon tube gauge’s dial face with temperature scale tends to be the indicator of the temperature. Such an operation is based on the uniform expansion of the steam that has been generated by the volatile liquid present in a closed space. Such pressure from the vapor varies with temperature directly. By filling a sensing bulb with this type of volatile liquid and then connecting it to a Bourdon tube, it helps in acting as an indicator of the rise & fall of the vapor pressure owing to the changes happening in the temperature. Calibrating the handle in degrees (Fahrenheit/Celsius) instead of psi gives a reading of the temperature. In this type of meter, a sensor bulb tends to be placed in the region where the temperature is being measured. A lengthy capillary tube tends to connect the bulb to a Bourdon tube in the instrument’s housing. The capillary tube’s much narrower diameter ensures that the evaporating liquid is light and it also remains mainly in the sensor bulb. Sometimes oil temperature is also measured like this.

Heatcon Sensors

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