OAT gauge and aneroid capsule sensors
Two instruments sense via a simple mechanical element, no electronics.
OAT gauge (temperature) (PHAK ch.8):
- Sensing element: bimetallic strip, two dissimilar metals welded, twisted into a helix.
- One end anchored, other end drives pointer.
- Dial reads °C, °F, or both.
- Accurate OAT gives temperature lapse rate with altitude change.
Aneroid capsule (pressure) (AWH 8.2.1; PHAK ch.8):
- Sealed, partially evacuated flexible metal cell.
- One end fixed, other end moves coupling mechanism driving indicator hand.
- Contracts as pressure rises, expands as pressure falls.
- Altimeter uses same element: cell stack is main pressure-sensing part, calibrated in feet instead of inHg (PHAK ch.8).
- Altimeter graduated to standard atmosphere (AWH 8.4).
- Pitot-static lesson: how static pressure reaches the stack, blocked-line effects. This lesson: the element the pressure feeds.
Altimeter types:
| Type | Power / output |
|---|---|
| Basic | Static pressure is source of operation (PHAK ch.8) |
| Encoding | Electric. Computer measures pressure referenced to 29.92 inHg (standard sea level pressure, TC AIM AIR 1.5.1), sends it to transponder. Dialling local altimeter setting does not change output (IFH ch.3) |
- Encoding altimeter: pressure altimeter sending pressure altitude to ATC on the ground (PHAK glossary).
- Mode C readout limits: see transponder lessons.
Not these elements:
| Instrument | Senses via | Cite |
|---|---|---|
| VSI | Rate of pressure change: diaphragm + metered leak, not aneroid capsule | PHAK ch.8; vsi lesson |
| Heading indicator, turn coordinator | Spinning gyro, not bimetallic strip or capsule | gyroscopic-instruments lesson |
Trap: bimetallic strip senses temperature, aneroid capsule senses pressure.
References
- PHAK ch.8 (FAA-H-8083-25)
- AWH 8.2.1 (FAA-H-8083-28)
- IFH ch.3 (FAA-H-8083-15)