Air Temperature Indicator in Aircraft – PART 4

Thermocouple Temperature Indicators

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.