07 Temperature Flashcards
How do you convert from Celsius to Kelvin?
Degrees in Celsius + 273 = Kelvin
Name the different methods of measuring temperature
- Direct Reading / Thermal Expansion
- Electrical Resistance
- Electrical Thermocouple
- Radiation / Pyrometry
Describe the ‘Direct Reading’ / ‘Thermal Expansion’ method of measuring temperature,
- What type of reading
- How it works
- Form of Direct reading
- Uses a bi-metallic strip of Invar and Brass
- Each has a different coefficient of expansion so the bar bends
- Can be Coiled to give a rotary action and to amplify the reading
Describe the ‘Electrical Resistance’ method of measuring temperature,
- What type of reading
- How it works
- Form of Remote indicating
- A sensor sends a signal to a Wheatstone bridge
- A change of resistance with temperature changes the signal which gives the reading
Describe the ‘Electrical Thermocouple’ method of measuring temperature,
- What type of reading
- How it works
- Where it is used in Jet and Prop AC
- Form of Remote Indicating
- Probes are located in the area where temperature is to be read
- These are linked in parallel in the event of one failing
- There are two ‘Junctions’ a Hot and Cold one
- The hot is located in the area to be measured and the cold is in the cockpit
- The difference in the resistance of these two areas are then compared to give the reading
- Used in a Jet engine to measure the exhaust gas temperature
- Used in a Prop Piston engine to measure the temp of the cylinders
Where on a piston engine would you find a Electrical Thermocouple sensor?
In the rear cylinder, which will be the hottest
Describe the ‘Radiation’ / ‘Pyrometry’ method of measuring temperature,
- What type of reading
- How it works
- Form of Remote Indicating
- As a material is heated, it releases electromagnetic radiation
- A sensor detects the frequency of the radiation emitted
- A higher frequency indicates a higher temperature
Describe a Total Air Temperature Probe (TAT Probe)
- Where it is located
- What it is made from
- How it functions
- On the outside of the AC, outside the boundary layer, out of direct sunlight
- Body is made from Nickel plated Beryllium, Sensor is made from Platinum
- Air enters and is forced to change direction by 90 degrees
- This forces the air to reduce in velocity
- The sensor then reads the temperature
What is the formula for the Total Air Temperature?
TAT (Total Air Temp) = SAT + Ram Rise
What is Ram Rise and what is it’s formula?
- Ram Rise is an increase in temperature due to movement
- Caused by Adiabatic Compression
- Ram Rise = (TAS / 100)^2
What is Measured Ram Rise?
- Ram Rise corrected by a ‘Recovery Factor’
- Less than the theoretical Ram Rise
- Measured Ram Rise = Ram Rise x Recovery Factor
What is the formula for the Ram Air Temperature?
Ram Air Temperature = SAT + Measured Ram Rise
What are the errors associated with the Total Air Temperature Probe?
- Icing
- Radiation (sunlight)
- Friction Heating (boundary layer)
- Heating (adiabatic compression)
- Instrument (manufacturing imperfections)
Describe an Aspirated probe
- Similar to a TAT probe but with the addition of Aspirated air
- Air is pumped in to the probe from behind the sensor
- Causes a flow of air through the probe even when the AC is not moving
- Because air is pulled through behind the sensor, this air is never in contact with the sensor
How is the electromotive force in thermocouple created?
Created due to the temperature gradient between the different metals
