Pressure and Temperature Sensing - ATPL 022 — Instrumentation

Two quantities dominate the gauges of any aircraft: pressure and temperature. Between them they report the health of engines, fuel, oil, hydraulics and air systems, and their sensors reappear inside half the instruments of the modules ahead. This lesson builds both measurement chains from the definition to the failure case: what the quantity is, the units it comes in, the sensors that feel it, where the aircraft measures it, and what a wrong indication is trying to tell you.

Pressure, absolute and differential

Pressure is force spread over area: how hard a gas or liquid pushes on every square metre of whatever contains it.

A single word covers three different measurements, and instruments are honest about which one they make:

Absolute pressure — measured from a perfect vacuum: zero means no pressure at all. Ambient air pressure is an absolute pressure.
Differential pressure — the difference between two pressures, each fed to one side of the sensor. Cabin differential (cabin air against outside air) is the classic example, and the airspeed indicator of the next module is nothing but a differential-pressure gauge.
Gauge pressure — the everyday middle case: a differential measurement with ambient air as the reference. An oil-pressure reading of zero on the ground means the oil is at ambient pressure, not in a vacuum.

The pressure units

Four units share the cockpit:

Pascal (Pa) — the SI unit, one newton per square metre: so small that aviation works in hectopascals (hPa).
Bar — 100,000 Pa, close to sea-level atmospheric pressure, so 1 hPa = 1 millibar.
Inches of mercury (in Hg) — pressure measured by the height of a mercury column it supports, the unit of American altimeter settings.
Pounds per square inch (psi) — the imperial force-over-area unit, standard for oil, hydraulic and tyre pressures.

The exam expects conversions among them:

1 bar = 100,000 Pa = 1,000 hPa — and 1 hPa = 1 mb.
1 bar ≈ 14.5 psi — the standard atmosphere of 1,013.25 hPa is about 14.7 psi.
1,013.25 hPa = 29.92 in Hg — the same standard atmosphere in both altimeter-setting units.

The four pressure sensors

Mechanical pressure sensing uses four families, chosen by how large the pressure is and which kind of measurement is wanted.

Aneroid capsule — two corrugated metal discs sealed together around a near-vacuum, with no inlet at all. Only the outside pressure can squeeze it, so its thickness measures absolute pressure: the altimeter's sensing element. (Aneroid means "without liquid": it replaced the mercury column.)
Bellows — an accordion-folded chamber, in effect a deep stack of capsules. The folds give a large movement for a small pressure change. Seal and evacuate one side and it reads absolute pressure like the capsule; feed a pressure to each side and it reads differential.
Diaphragm — a thin flexible wall dividing a case into two chambers, one pressure on each side. It bulges toward the lower pressure, so its deflection is a differential measurement directly.
Bourdon tube — a flattened metal tube bent into a C. Internal pressure tries to round out the flat cross-section, which makes the C uncurl, and the moving tip drives a pointer through a linkage. Robust and happiest at high pressures: the classic oil- and hydraulic-pressure gauge.

What the aircraft measures, and how it reads

The airframe applies those four sensors everywhere a pressure carries information:

Liquid pressures — fuel feed pressure, oil pressure, hydraulic system pressure: the classic bourdon-tube duties.
Air pressures — bleed-air ducts and the air-conditioning system, where a duct pressure confirms the supply the packs depend on.

And engine pressures:

manifold absolute pressure (MAP) — on a piston engine, the absolute pressure in the induction manifold that sets how much charge reaches the cylinders.
engine pressure ratio (EPR) — on many turbofans, turbine outlet pressure divided by compressor inlet pressure, used as the thrust-setting parameter.

Readings reach the pilot as a needle on a dial, a vertical tape, or a digital value on the engine display, usually colour-banded:

green for the normal range.
amber for caution.
red for a limit.

When the pressure reading is wrong

A pressure indication fails in two ways, and they call for different reactions:

Loss of pressure sensing — a blocked or leaking sense line, a failed transmitter: it typically drives the indication to zero or freezes it, while the system behind it may be perfectly healthy.
An incorrect indication — worse: a reading that looks alive but is wrong invites either a needless shutdown or misplaced trust.

The defence in both cases is cross-checking: a real oil-pressure loss starves the bearings, so the oil temperature rises with the falling pressure; a zero oil pressure with a steady, normal oil temperature points at the sensing, not the oil. The same logic pairs a suspect fuel pressure with the flowmeter and engine behaviour, and a suspect duct pressure with the pack's actual output. One gauge is a claim; two agreeing gauges are a diagnosis.

Temperature and its units

Temperature measures how energetically the molecules of a substance are moving: the hotter the gas in a turbine, the faster its molecules strike whatever they meet. Three scales share the work:

Kelvin (K) — the SI scale, counted from absolute zero, the temperature at which molecular motion is at its minimum; there are no negative kelvins.
Celsius (°C) — the same size of degree, but starting at the ice point, 273 K, so ordinary weather sits in small familiar numbers.
Fahrenheit (°F) — puts the ice point at 32 and the boiling point at 212, dividing the same span into 180 smaller degrees.

What the aircraft measures in temperature

Temperature probes are scattered through the aircraft in three families:

Gas temperatures — the ambient air outside (the OAT the next module returns to), bleed-air and air-conditioning ducts, the engine's air inlet, and above all the turbine's own gas path: exhaust gas temperature (EGT) and its gas-turbine-outlet siblings, the engine's most life-limiting readings.
Liquid temperatures — fuel (cold-soaked fuel approaches its freezing limit on long high flights), oil and hydraulic fluid.
Component temperatures — electrical machines and their power electronics, generators and the transformer rectifier units (TRUs) that turn AC into DC, and pumps for fuel and hydraulics, including the power transfer unit (PTU) that lets one hydraulic system drive another's pressure.

Indications read like the pressure family's: dials, tapes or digital values, colour-banded, with EGT's red line the most famous limit mark in the cockpit. And the cross-checking habit carries over unchanged: pressure and temperature of the same system are read as a pair, because a genuine failure moves both.

What the exam asks

Unit conversions and the sensor-to-application match are the bank's favourites here. The overview, following the learning objectives:

Define pressure, absolute pressure and differential pressure

List the pressure units and state the relationships between them

List and describe the sensor types used according to the pressure to be measured

Identify the aircraft's pressure measurements and read their indications

Explain the implications of loss of pressure sensing and of incorrect indications

Explain temperature, list its units, and convert between them

Identify the aircraft's temperature measurements and read their indications