Thermocouple indicators are fundamental sensors used to indicate and regulate temperature of an aircraft engine. Despite being frequently used, thermocouples are not always considered when discussing aircraft equipment calibration. Similar to other aircraft equipment, thermocouple indicators have the ability to drift; therefore, it is extremely important they undergo regular and consistent calibration.
HOW THERMOCOUPLE INDICATORS OPERATE
Thermocouple indicators consist of two heterogeneous metal wires. These wires are joined at one end and produce a voltage corresponding to temperature. One junction of the wire is considered the measuring junction, where the wire is exposed to the heat to measure the temperature. The other junction is consider the reference junction. The reference junction maintains a known temperature.
Due to a wide range of temperature preferences, a variety of thermocouple indicators exist. Each type consists of different temperature ranges and sheath material.
WHY CALIBRATION IS NECESSARY
Just like every other aircraft equipment that is used to process critical measurements, thermocouple indicators needs to be calibrated. An area of extreme importance in the aviation industry is the production of carbon fiber components. This means that temperature must be accurately controlled and repeatability needs to be present to ensure engine consistency.
In order to confidently know temperature readings are coming back accurate and repeatable you keep the following in mind when deciding to calibrate:
- The bimetals of a thermocouple indicator, along with the wire, will change with time and frequent use. Notably, change of both the wire and bimetals will be more prompt if the thermocouple indicator is used at elevated temperatures.
- Thermocouples can damage over time – usually during service. This can happen mechanically or by corrosion, which leads to deterioration of both junctions.
SETTING A CALIBRATION SCHEDULE
Thermocouple indicators should have a set calibration schedule just like every other aircraft equipment. By setting a schedule, the chances of the indicator drifting are slim and risk factors from improper temperatures are minimized. When establishing a schedule the following should be considered:
- Type of Use/Frequency of Use
- Damage Risk
- Rate of Drift — this can be determined through calibration records
Risks that could occur from not calibrating thermocouple indicators could cause severe damage to the aircraft engine and could also possess hazardous safety issues.