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.

Utilizing electricity to measure different temperatures is quite prevalent in the aviation industry. Different ranges of temperature tend to be measured in a more suitable way by one or the other sort of the system. Below mentioned are some measuring systems as well as indication systems that are largely seen in various sorts of aircraft.

Electrical-Resistance Thermometers

The main parts of electrical-resistance thermometers happen to be the indicator instrument, the element (or the bulb) that is temperature-sensitive & the connecting wires along with the plug connectors. Such type of a thermometer is largely used in various types of aircraft for measuring carburetor air, oil, free-air temperature etc. These are utilized for measuring low & medium temperatures between -70 degree Celsius and 150 degree Celsius.

The electrical resistance of most metals alters when the temperatures of those metals tend to change. A resistance thermometer works on this principle. Normally, the electrical resistance of metals tends to augment with increasing temperatures. Different metal alloys tend to have high temperature-tolerance coefficient, which means that their resistance differs significantly with temperatures. This property makes them highly suitable to be used in devices used for sensing temperatures. The metal resistors are in contact with the liquid or the area where the temperatures are to be measured. This is wired to a device in the cockpit detector. Optionally, the instrument dial tends to be calibrated in degrees Celsius or degrees Fahrenheit instead of ohms. As the measured temperature tends to change, the metal resistance also changes, and the indicator shows the exact extent to which the change is happening.

A common electrical-resistance thermometer appears to be like any other such thermometer. For use in multi-engine aircraft, these detectors are available in 2 different forms. Most such indicators themselves compensate for cabin temperatures. A thermally-sensitive resistor is designed to have its own resistance for every temperature value within its operating range. The temp-sensitive resistive element happens to be the length or coil made of nickel or manganese wire or any other such suitable insulating material alloy. The resistor tends to be protected by a sealed metal tube that is connected to a hexagonal head. The 2 ends of the coil are soldered or welded to a socket designed to accept the plugs of the plug connector.

Such indicators include the resistance measuring devices. They sometimes use a modified form of the Wheatstone Bridge circuit that works on the principle of balancing 1 unknown resistance with other such known resistances. 3 resistance values of the same are connected in a bridge circuit that is diamond-shaped. One resistor, whose value is unknown, also happens to be one part of the circuit. The unknown resistance shows the temperature bulb’s resistance of the electric-resistance thermometer system. A galvanometer is fixed at 2 points in the circuit.

If the temperature makes the resistance of the bulb the same as the other resistances, there’s no potential difference between the 2 points in the circuit. So, no current will flow in the galvanometer’s leg of the circuit. As the temperature of the bulb changes, so does its resistance, and the bridge tends to become imbalanced, leading to the current to flow through the galvanometer in 1 direction or the other. The pointer of a galvanometer is basically the pointer of a gauge. The time of it moving against that dial-face that is calibrated in degrees, the temperature is shown. Many such pointers are equipped with a zero adjustment screw on the front of the instrument. This adjusts the tension of the pointer reset spring when the bridge is at the balancing point (this happens to be that position where the bridge circuit tends to balance & no current flows through that meter).

One more method to determine the temperatures using electrical-resistance thermometers is to do it by utilizing a ratiometer. In the Wheatstone bridge detectors, there are failures due to fluctuations in the mains voltage. The ratiometers tend to be more stable, & they can provide much larger accuracy. As suggested by their name, ratiometer electrical-resistance thermometers help in measuring the ratio of current.

The sensor part of the resistance bulb of such types of thermometers is primarily similar to the one explained above. The circuit consists of a variable resistance as well as a fixed resistance to give the reading. This consists of 2 power supplies; each having a coil that is mounted on one of the sides of that pointer mounted in the field of a large-sized permanent magnet. The changing current passing through the coils leads to varied fields reacting with the much larger permanent magnet’s field. Such an interaction turns the cursor against a dial that has been calibrated in degrees Fahrenheit/Celsius, providing the indication of the temperature being measured.

The ends of the magnetic poles of such types of permanent magnets tend to be closer at the topmost point than at the bottom-point. It leads to the flux’s magnetic field lines among the poles to concentrate at the tip. When the 2 coils release their own magnetic field, the one that is stronger tends to interact & deflect down into the weaker one, which happens to be the less concentrated part of the permanent field of the magnet; however, the field of the weaker coils moves up into the more concentrated large magnet’s flux field. This leads to a sort of balancing effect, which alters but tends to remain balanced as the coil field becomes strengthened and varies with the temperatures & currents that are flowing in the coils.

For instance, if the temp-bulb’s resistance is the same as the value of the fixed resistance (R), then the same values of current will flow through the coils. The torque(s) thus produced by the magnetic fields by each coil tends to be the same & cancels out any motion in the larger field. The pointer is in the vertical position. As the temperature of the bulb augments, so does its resistance. This leads to the current through one of the coils in the circuit-branch to augment. This leads to a stronger magnetic field being produced in one coil than in the other one. As a result, the torque on one of the coils increases, and it is pulled down to the weak point of the higher magnetic field. Simultaneously, lesser current flows through the resistance of the sensor bulb and the second coil, causing that coil to form a weaker magnetic field, which is pulled up into the stronger flux region of the permanent magnet’s magnetic field. The pointer halts turning when the fields tend to reach a new equilibrium point, which is directly related to the resistance of the sensing bulb. This would be the opposite if the temperature of the thermosensitive bulb were to drop.

Ratiometer’s temperature measurement systems are being utilized for measuring outside air, engine oil, carburetor air as well as other temperatures in many of such types of aircrafts. These happen to be particularly useful for measuring temperature conditions wherein accuracy is more significant or where there are large fluctuations in the supply voltages.

Thermocouples happen to be the circuits or connections between 2 metals that are unlike. The metals meet at 2 different junctions. If 1 of these junctions gets heated to a higher degree temperature than the other junction, an electromotive force is generated inside the circuit. Such voltage tends to be directly linked to the temperature. Thus, the temperature can be determined by measuring the magnitude of this force. A voltmeter tends to be placed on the cooler of the 2 thermocouple junctions. This would be calibrated in Celsius/Fahrenheit (as required). As the high-temp junction (or also called the hot junction) becomes hotter, the electromotive force thus generated becomes greater & the temperature reading on the meter becomes greater!

A thermocouple is utilized for measuring high temperatures. 2 most common applications of this are measuring the cylinder-head-temperatures (CHT) in the reciprocating engines, and second is the measuring of the exhaust-gas-temperatures (EGT) in the turbine engine. The wires of the thermocouples tend to be made from several metals as per the max temperature they are being exposed. Iron & constantan/copper & constantan happen to be the most commonly used ones for measuring CHT. Chromel & alumel tend to be utilized in turbine EGT thermocouples.

The amount of voltage generated while heating different metals tends to be measured in the units of millivolts. So, the wires of thermocouples are formed in such a way that they give a certain amount of resistance (that is usually very low) in the thermocouple circuit. Their materials, lengths or cross-section sizes could not be changed without compensating for the resulting change in the total resistance. Any wire connecting back to the voltmeter should be designed from the similar metal as that of the thermocouple’s part wherein it has been connected. For instance, a copper wire tends to be connected to the copper part of the hot junction, & a constantan wire has been connected to the constantan segment.

The thermocouple’s hot junction has different shapes as per the application(s). The 2 most common ones happen to be the pack one and the bayonet one. In a gasket, 2 dissimilar metal rings tend to be pressed together for forming a gasket, which could be installed either under the spark plug or the cylinder that is retaining the nut. In the bayonet one, the metals are connected within a perforated protective casing. The bayonet type of the thermocouple fits into a hole or well inside the cylinder head. In turbine engines, these are installed in the turbine inlet/outlet, & then extend through the casing into the gas-stream. Note that the cylinder that runs hottest under most operating conditions is selected to mount the thermocouple to read the CHT. This cylinder’s location happens to vary for different engines.

The thermocouple circuit’s cold junction happens to be inside of the instrument housing. As the electromotive force generated inside the circuit tends to vary according to the temperature difference between the hot & cold junctions, it is much needed to compensate for temperature changes in the cockpit of the detector mechanism that affect the cold junction. This is achieved by a bimetallic spring that has been connected to the detector mechanism. It basically works the same way as the bimetallic thermometer. With the wires disconnected from the detector, the cockpit temperature around the instrument panel could be read from the indicator dial. CHT’s number LED indicators also tend to be quite common in the modern type of the aircraft.

EGT happens to be a critical variable in the operations of turbine engines. Such EGT indicator systems tend to show the visual temperatures inside the cockpits of turbine’s exhaust gasses once they tend to leave that particular turbine unit. In some types of these turbine engines, the exhaust gas temperatures are measured at the inlet of the turbine unit. Such values are called the Turbine-Inlet-Temperature (i.e., TIT) detection systems.

Certain types of thermocouples happen to be utilized for measuring this EGT/TIT. Such thermocouples tend to be placed at intervals circling round the circumference of that engine’s turbine case or the exhaust pipe. The small thermocouple voltages are usually increased & are utilized to drive the servo motor that moves the indicator pointer. Moving the digital drum pointer out of the cursor movement is quite a common phenomenon. The EGT indicator happens to be in a hermetically sealed device. The instrument scale tends to range from 0 degrees Celsius to 1200 degree Celsius, with a vernier scale in the upper right corner of the dial and a power failure warning flag in the lower part of the dial.

The TIT display system visually displays the temperature of the gasses entering the turbine on the instrument panel. Multiple thermocouples could possibly be utilized with an intermediate TIT voltage. Inside the sensor are 2 thermocouples that contain 2 junctions, which happen to be electrically independent. One of these 2 thermocouples happens to be in parallel to send signals to the cockpit detector. Another set of parallel thermocouples provides signals for temperature to the engine monitoring system as well as the control system. Every such circuit happens to be electrically independent, thus ensuring the reliability of the two systems.

The circuits for the other 3 engines happen to be quite identical to that system. The detector includes a bridge circuit, a switch, a 2-phased motor for the purpose of driving the indicator, and a feedback potentiometer. Also included is a voltage reference circuit, amplifier, shutdown indicator, power supply, & overheat warning light. The output of the amplifier activates the adjustable field of the 2-phase motor, which positions the detector master pointer & the digital detector. The motor even utilizes a feedback potentiometer for providing a buzzer signal in order to stop the drive motor at the time when the correct pointer position (relative to that of the temperature signal) is reached. The voltage-reference-circuit gives a precisely regulated reference voltage inside the bridge circuit for avoiding errors generated from fluctuations in the input voltage of the indicator’s supply of power.

The warning light telling about over temperature tends to illuminate when TIT happens to reach the predetermined limit. An test switch (external) is ideally installed for the engines overheating warning lights to be tested simultaneously. While making use of such a test-switch, an over-temperature signal tends to get simulated inside every bridge circuit for detector temperature control.

Digital cockpit instruments do not require the use of resistance-type indicators or the regulated servo-driven thermocouples for communicating temperature information to the pilot. The sensor’s resistance & the voltage values tend to be entered inside a suitable computer wherein they are adjusted, processed, monitored & printed on the cockpit panels. They are even sent to other computers that need temperature data to control & monitor various integrated systems.

Air temperature happens to be quite a significant parameter on which various types of performance monitoring as well as different control variables tend to depend. During the flight, the air temperature (static) is constantly changing, & accurate measurements are quite hard to get. Below Mach 0.2, a simple resistance-type/bimetallic temperature gauge could possibly give relatively correct readings for air temperature measurements. At higher speeds however, friction, air compressibility & boundary layer behaviors tend to create difficulty with correct temperature measurements. Total-air-temperature (i.e. TAT) happens to be the static air temperature added to any temperature increase due to the rapid movements of the aircraft in the air. The rise in the temperatures is called ram rise. TAT sensors are specifically designed for capturing such values accurately & sending signals for pilot detection and usage in different engine systems as well as aircraft systems.

Simpler TAT systems tend to consist of the sensors as well as the detectors with in-built resistance-balancing circuits. The airflow through the sensor has been designed in such a way that air (at the exact temperature) tends to hit the platinum alloy resistive element. Sensors are designed in such a way that they help in detecting any changes in the temperatures versus the variations in the element resistance. When the pointer is placed in the bridge circuit, it moves in response to the imbalance caused by the variable resistance.

More advanced systems tend to utilize the technology of signal correction as well as amplified signals that are sent to the servo motor for adjusting the cockpit indicators. Such systems consist of precisely regulated power supply as well as fault monitoring. These usually utilize digital drum-type readouts; however, they can also be sent to the LCD controllers for illuminating the LCD screens. Various LCD displays tend to be multi-functional, and they are also able to display static air temperatures & actual air speed. In a totally digital system, the correction signals tend to feed to the ADC. They can be properly processed there for cockpit displays or systems that need temperature data.

The designs of the TAT sensors/probes are indicated by the possibility of ice formation in icy cool conditions. Sensors that are left unheated might stop working properly. Adding a heating element comprises accurate data collection. The heating of the sensors should not affect the sensor element’s resistance.

During the design phase, special attention tends to be paid to the air flow as well as material conductivity. Some of the TAT sensors direct the airflow through the devices for influencing the ambient airflow so that it directly flows into the platinum sensors without additional energy from the sensor heaters.

Heatcon Sensors

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Air Temperature Indicator in Aircraft