Aircraft Sensors Questions and Answers

Answer: A

Correct answer: The change in electrical resistance of a metal with temperature

A resistance temperature detector exploits the fact that a metal's resistivity rises with temperature in a close to linear way. Platinum is the usual choice, being stable, repeatable and linear over a wide range.

The device is passive, so it needs an excitation current to be read - which distinguishes it from a thermocouple, whose junction of dissimilar metals generates its own voltage from a temperature difference. Bimetallic expansion and capacitance change are the mechanisms of quite different instruments.

Answer: B

Correct answer: Has a large non-linear change

A thermistor is a semiconductor, and its resistance changes far more steeply with temperature than a metal's - typically several per cent per degree against 0.4 for platinum - but the relation is strongly non-linear, roughly exponential.

The trade is sensitivity against linearity and range. A thermistor gives a large signal over a narrow band, which suits monitoring around a set point; an RTD gives a smaller but linear signal over hundreds of degrees. Both are passive and both need excitation.

Answer: C

The junction survives the gas path where a platinum element would not, and being self-generating it keeps working with the excitation supply lost. Its output depends on a difference, so cold junction compensation is required.

Answer: D

Correct answer: The change in its electrical resistance as it is stretched

A strain gauge is a fine conductor bonded to the surface. Stretching it makes it longer and thinner, and both changes raise its resistance, since \( R = \rho L/A \).

The change is very small - a few parts in a thousand for useful strain - which is why the gauge is read in a bridge rather than with an ohmmeter. It is a passive device requiring excitation; a piezoelectric element, by contrast, generates charge under stress and needs none.

Answer: A

Correct answer: Fractional resistance change to strain

The gauge factor is defined as

\[ GF = \frac{\Delta R/R}{\varepsilon} \]

so it relates a fractional resistance change to the strain producing it, and is itself dimensionless. For a metal foil gauge it is about 2, meaning a strain of 0.001 gives a resistance change of 0.002 - two parts in a thousand.

Semiconductor gauges reach 100 or more but are far less stable with temperature. The ratio of strain to stress is the reciprocal of Young's modulus, a property of the material rather than of the gauge.

Answer: B

Correct answer: Turns a tiny change into a voltage

The gauge's resistance changes by a few parts in a thousand, which no practical instrument reads directly with the required precision. A Wheatstone bridge converts that change into a differential voltage measured against a balanced reference.

Because the bridge output is zero when balanced, the small change appears as a small voltage on a zero baseline rather than as a tiny variation on a large one - which is what makes it amplifiable. The gauge factor is a property of the gauge and excitation is still required.

Answer: C

Correct answer: Four active gauges, two in tension and two in compression

A full bridge puts an active gauge in every arm, arranged so that two see tension and two compression - on opposite faces of a beam, for instance.

Each contributes to the output in the same sense, so the signal is four times that of a single gauge. And since all four sit in the same thermal environment, a temperature change shifts all of them together and cancels in the bridge. Sensitivity and thermal compensation therefore come from the same arrangement.

Answer: D

Correct answer: Cancel the effect of temperature on the active gauge

A dummy gauge is bonded to an unstrained piece of the same material and placed in the adjacent bridge arm. It sees the same temperature as the active gauge but no strain.

Both resistances therefore shift together with temperature, and the bridge output is unaffected, while the strain-induced change in the active gauge alone remains. This matters because thermal effects on a gauge are comparable with the strain signal itself, so an uncompensated measurement drifts with the ambient conditions.

Answer: A

An absolute sensor therefore reads atmospheric pressure when open to the air, which is why a manifold pressure gauge does not read zero at rest. It is normal behaviour. A gauge sensor references ambient and a differential sensor references another supplied pressure.

Answer: B

Correct answer: Resistors diffused into a diaphragm

Piezoresistive sensors are made by diffusing resistors into a silicon diaphragm. Pressure deflects the diaphragm, straining the resistors, and their resistance changes - the strain gauge principle in an integrated form.

The last option describes a piezoelectric device, which generates charge under stress and cannot hold a static reading. That difference matters: a piezoresistive sensor measures steady pressure, while a piezoelectric one responds only to change and is used for dynamic measurements.