The workings of an altimeter

[akafrank07]

An abstract from the internet site Flight Instruments (http://www.free-online-private-pilot-ground-school.com/flight-instruments.html)

"The other method of moving the pointers does not rely on changing air pressure, but the mechanical construction of the altimeter. Do not be confused by the fact that as the barometric pressure scale is moved, the indicator needles move in the same direction, which is opposite to the reaction the pointers have when air pressure changes. To illustrate this point, suppose the pilot lands at an airport with an elevation of 1,000 feet and the altimeter is correctly set to the current sea level pressure of 30.00 in. Hg. While the airplane is parked on the ramp, the pressure decreases to 29.50. The altimeter senses this as a climb and now indicates 1,500 feet.


When returning to the airplane, if the setting in the altimeter window is decreased to the current sea level pressure of 29.50, the indication will be reduced back down to 1,000 feet."

Do not be confused by the fact that as the barometric pressure scale is moved, the indicator needles move in the same direction, which is opposite to the reaction the pointers have when air pressure changes.

Relating to above the barometric pressure scale, is this referring to altimeter android wafers? what are the indicator needles? as i thought the pointers are the indicator needles? How does the above quoted sentence tie into the illustration/example?

Thanks

[cefey]

Ok, its late, Im tired, but Ill give it a try:)

Basically what they trying to say (in a complicated way), needle will move:
A. When pressure is changing. Either by climbing/descending. Or by changes in atmospheric pressure (airplane stands on the ramp, pressure changes, altimeter will show climb/descend.

B. You can adjust altimeter manually, by rotating the knob. Aka ""The other method of moving the pointers does not rely on changing air pressure, but the mechanical construction of the altimeter."

So, that how it fits into the example.
Pressure settings is 30.00
Atmospheric pressure is 30.00
Elevation of airfield is 1000 feet
Altimeter shows 1000 feeet

Then, atmospheric pressure drops down to 29.50.
Your pressure setting is still 30.00 and therfore altimeter shows 1500, aka climb of 500 feet.
Then you go into the airplane and change your pressure settings to 29.50, same as atmospheric pressure, and altimeter is now showing right altitude, 1000 feet.

[RTN11]

They couldn't make that sound anymore complicated!

It seems all they were getting at is that when you turn the knob to adjust QNH, you are moving the mechanical gears rather than the barometric capsule inside the altimeter.

[akafrank07]

Thanks for the reply guys and clarifying it for me :ok: