07 Temperature

Question 1

How do you convert from Celsius to Kelvin?

Answer 1

Degrees in Celsius + 273 = Kelvin

Question 2

Name the different methods of measuring temperature

Answer 2

• Direct Reading / Thermal Expansion
• Electrical Resistance
• Electrical Thermocouple
• Radiation / Pyrometry

Question 3

Describe the ‘Direct Reading’ / ‘Thermal Expansion’ method of measuring temperature,

• What type of reading
• How it works

Answer 3

• 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

Question 4

Describe the ‘Electrical Resistance’ method of measuring temperature,

• What type of reading
• How it works

Answer 4

• 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

Question 5

Describe the ‘Electrical Thermocouple’ method of measuring temperature,

• What type of reading
• How it works
• Where it is used in Jet and Prop AC

Answer 5

• 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

Question 6

Where on a piston engine would you find a Electrical Thermocouple sensor?

Answer 6

In the rear cylinder, which will be the hottest

Question 7

Describe the ‘Radiation’ / ‘Pyrometry’ method of measuring temperature,

• What type of reading
• How it works

Answer 7

• 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

Question 8

Describe a Total Air Temperature Probe (TAT Probe)

• Where it is located
• What it is made from
• How it functions

Answer 8

• 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

Question 9

What is the formula for the Total Air Temperature?

Answer 9

TAT (Total Air Temp) = SAT + Ram Rise

Question 10

What is Ram Rise and what is it’s formula?

Answer 10

• Ram Rise is an increase in temperature due to movement
• Caused by Adiabatic Compression
• Ram Rise = (TAS / 100)^2

Question 11

What is Measured Ram Rise?

Answer 11

• Ram Rise corrected by a ‘Recovery Factor’
• Less than the theoretical Ram Rise
• Measured Ram Rise = Ram Rise x Recovery Factor

Question 12

What is the formula for the Ram Air Temperature?

Answer 12

Ram Air Temperature = SAT + Measured Ram Rise

Question 13

What are the errors associated with the Total Air Temperature Probe?

Answer 13

• Icing
• Radiation (sunlight)
• Friction Heating (boundary layer)
• Heating (adiabatic compression)
• Instrument (manufacturing imperfections)

Question 14

Describe an Aspirated probe

Answer 14

• 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

Question 15

How is the electromotive force in thermocouple created?

Answer 15

Created due to the temperature gradient between the different metals

Question 16

When a thermocouple is heated what happens?

Answer 16

• Voltage increases

Question 17

What is the formula relating the electromotive force and the temperature in a thermocouple?

Answer 17

Emf = K x T

Where K is a constant

Question 18

What is the formula for TAT using Mach Number?

Answer 18

TAT = SAT x (1 + (0.2 x Kr x Mac^2))

• All temps in kelvin
• Kr is recovery factor