A measurement starts at the sensor

Which pressure does the gauge actually measure?

A number without a reference can be correct and still answer the wrong question.

Absolute, gauge and differential pressure

Pressure is force per unit area. Absolute pressure uses a vacuum reference; gauge pressure is relative to the surrounding atmosphere; differential pressure compares two selected pressures. A hydraulic gauge reading zero therefore need not imply a vacuum. It normally means that the pressure difference measured by that gauge is zero. Establish the reference before comparing two indications or applying a conversion.

A sealed evacuated capsule responds to absolute pressure. A diaphragm or bellows can compare the pressures on opposite sides; a Bourdon tube tends to straighten as its internal pressure exceeds the reference outside it. Each converts pressure into displacement. An electrical transducer then converts deformation into a signal, allowing a remote display without routing a high-pressure fluid line into the flight deck.

Calibration is a relationship, not a promise

One bar equals 100,000 Pa, or 100 kPa; one hPa equals 100 Pa. Approximately 1 psi equals 6.895 kPa and 1 inHg equals 33.864 hPa. Convert the number and the reference together. If ambient pressure is 95 kPa and gauge pressure is 205 kPa, absolute pressure is 300 kPa. This addition applies only when the stated gauge reference is that ambient pressure.

A blocked sensing line, electrical supply loss, offset or stuck pointer can produce a believable indication. Compare related evidence: a pump command is not proof of pressure, and pressure alone is not proof of useful flow. The model adds a known offset so that a smooth responsive display can still be wrong. Follow the aircraft procedure for disagreement rather than assuming the most convenient indication is correct.

Why can different sensors show different temperatures?

Trace heat transfer and signal conversion before trusting the displayed digits.

Temperature and the measuring element

Temperature describes thermal state; heat is energy transferred because of a temperature difference. Kelvin has the same interval size as Celsius, with K = °C + 273.15. Fahrenheit uses °F = 1.8 × °C + 32. Ratios in gas laws require absolute temperature: twice the Celsius number does not imply twice the thermodynamic temperature. A conversion changes the scale, not the physical condition.

A resistance temperature detector uses a predictable resistance change; a thermistor has a stronger often nonlinear resistance response; a thermocouple produces a small voltage related to the temperature difference between junctions. A thermocouple installation needs reference-junction compensation to infer the sensed temperature. The appropriate sensor depends on range, response, accuracy, environment and the measurement required, rather than one technology being universally best.

Location, lag and interpretation

Oil in a sump, oil at an engine inlet and a bearing housing can have different temperatures. A probe measures its local thermal condition, not an instantaneous average of an entire system. Installation and airflow influence response. Heating a probe to prevent icing can introduce an error unless its design compensates for the heating. A correct electrical signal can therefore represent a local condition different from the one the pilot assumes.

During a rapid change, thermal inertia makes the indicated value lag the fluid or gas temperature. When comparing two displays, check sensor location, units, response and validity before diagnosing a real system difference. A warning threshold and the sensor response are separate parts of the protection chain. In the laboratory, switch Celsius and Fahrenheit while holding the physical temperature fixed, then add bias and distinguish a unit conversion from a measurement fault.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.