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1. Pressure Probes
- Aneroid Capsules (Low Pressure)
Closed capsules with a (almost total) vacuum inside them. The pressure in the case around the capsule will either compress or expand them and the magnitude of this movement will be shown on the instrument.
Example; Altimeter or manifold air pressure (in vicinity of the air inlet valve)
- Bellow Capsule (Medium Pressure)
Same principle as the Aneroid Capsules but different materials are used and the capsule has a lower travel (expansion/contraction) per unit of pressure difference so that it can be used to measure higher pressures.
Example; Fuel pressure in small aircraft
- Bourdon Tube (Medium to High Pressure)
A oval shaped tube, curved to form a three-quarter circle and sealed at one end. The pressure to be measured is fed into the other end. The stretching of the tube can be converted into rotary movement of a pointer on an instrument.
Example; Oil pressure or pressure in a hydraulic system
2. Temperature Sensing
- Resistance Thermometer
Consists of a "Wheatstone Bridge", a circuit containing a resistor with a positive temperature coefficient (PTC) which means that resistance increases as temperature increases.
- Thermocouples
Consists of 2 different metal alloys which are directly connected on one side, called the "hot junction". On the other side, the "cold junction", their ends are embedded into an insulator used for cooling.
If the hot junction is heated, a direct current is introduced. The ability of the thermocouple to produce this current (is electromotive force EMF) corresponds to the temperature difference multiplied by a constant K:
EMF = K x Temperature difference
- Aneroid Capsules (Low Pressure)
Closed capsules with a (almost total) vacuum inside them. The pressure in the case around the capsule will either compress or expand them and the magnitude of this movement will be shown on the instrument.
Example; Altimeter or manifold air pressure (in vicinity of the air inlet valve)
- Bellow Capsule (Medium Pressure)
Same principle as the Aneroid Capsules but different materials are used and the capsule has a lower travel (expansion/contraction) per unit of pressure difference so that it can be used to measure higher pressures.
Example; Fuel pressure in small aircraft
- Bourdon Tube (Medium to High Pressure)
A oval shaped tube, curved to form a three-quarter circle and sealed at one end. The pressure to be measured is fed into the other end. The stretching of the tube can be converted into rotary movement of a pointer on an instrument.
Example; Oil pressure or pressure in a hydraulic system
2. Temperature Sensing
- Resistance Thermometer
Consists of a "Wheatstone Bridge", a circuit containing a resistor with a positive temperature coefficient (PTC) which means that resistance increases as temperature increases.
- Thermocouples
Consists of 2 different metal alloys which are directly connected on one side, called the "hot junction". On the other side, the "cold junction", their ends are embedded into an insulator used for cooling.
If the hot junction is heated, a direct current is introduced. The ability of the thermocouple to produce this current (is electromotive force EMF) corresponds to the temperature difference multiplied by a constant K:
EMF = K x Temperature difference
