Pressure sensing Flashcards
what instruments rely only on sensing pressure?
Altimeter
VSI
ASI
machmeter
Pressure sensing threats?
blockages
leaks
Units of pressure
1 Pa= 1 N/m2
1 mb= 1hPa
ISA pressure= 1013.25 hPa, 29.92inHg, 14.7psi
When is psi usually used?
for oil pressure, hydraulic pressure and differential cabin pressure
Gauge pressure
Dif between pressure to be measured and atmospheric
zero- referenced against ambient air pressure
absolute pressure
Dif between pressure to be measured and a vacuum
differential pressure
Dif between two sampled pressures
Aneroid capsule
single or multilayered
sealed partially evacuated
leaf spring(to stop capsule collapsing)
aneroid means no flow
thin sheets of brass(naturally springy)
other measuring devices
pressure capsules- not sealed, lets pressure in and out
diaphragms- flexible membrane move back and forth
burden tube- partially flatten tube wound into a curve, relatively stiff so measures high pressures
MEM tube- convert pressures to electrical signals
Static pressure
result of weight of air above object
fluid so acts in all directions
reduces as alt increases
only static instruments= VSI & altimeter
Pitot pressure
Also known as stagnation or total pressure
dynamic + static = pitot pressure
dynamic pressure
known as impact pressure
can’t directly measure
pressure relative to motion
dynamic= pitot - static
increases as airspeed increases
= 1/2pV2
pitot tube
parallel to longitudinal axis
needs heat element
drain from bottom
Static port
pairs at right angles to airflow
manoeuvre errors reduced by pairs
alternate static on light aircraft usually read low due to suction
combined probe
both pitot and static
common on light but not on large transport
