Connect and share knowledge within a single location that is structured and easy to search. In this answer reference was made to "sensitive" and "non-sensitive" altimeters. What is the definition of each, what are the differences, and why would one be used in preference to the other? All altimeters are "sensitive" to some extent an "insensitive" altimeter would be one where the internal pressure-sensing mechanism has failed: the pointer wouldn't ever move.
What the FAA calls a "sensitive altimeter" is simply more sensitive than a regular now often called non-sensitive one:. This altimeter is similar to what you'd find on an old Piper Cub: The scale is marked in thousands of feet, with ticks every feet. This altimeter is not adjustable for "barometric pressure", but it is adjustable: The little knob on the bottom rotates the face of the altimeter.
These altimeters can only be "set" on the ground, because you need to know your elevation to set the position of the face or needle. These show up from time to time: It's a non-sensitive altimeter same scale markings we saw above , but it's adjustable for barometric pressure.
The Kollsman window on this one is marked in inches of mercury. When you turn the knob on this altimeter the needle moves and the barometric pressure that corresponds to "zero feet" shows up in the Kollsman window. This makes it easier to read than the previous example, and easier to reset in flight based on radio information: You can set the local barometric pressure QNH in the Kollsman window and get your current altitude without having to be on the ground at a known elevation.
In this sensitive altimeter you can see that there is a foot scale large pointer , and a foot scale small pointer. This is a bit of an unusual example: It's a sensitive altimeter that is NOT adjustable for barometric pressure the knob simply adjusts the position of the pointers, and you set it to field elevation like you would with our first example.
It can't be easily reset in flight as you need to know your current altitude to reset the altimeter correctly. The non sensitive type is not adjustable for changes in barometric pressure no Kollsman window. But this is not a big problem since they also usually have a very small scale — meaning instead of the altimeter going around once for every thousand feet and having two or three hands, it will have one hand that only goes around once for about every 10 thousand feet.
You can know you altitude to within maybe feet if you look closely. Gary S. I imagine most of them they have been replaced by satellite systems by now. Its small linear motion is transmitted to the sector gear, via the rocking shaft and a connecting link which move the gears and pointers to show the altitude.
Kermode, , Pallett, Aneroid wafers are the pressure sensing element of the altimeter. They are formed by joining corrugated metal discs together, evacuating them close to vacuum and sealing them off. The characteristic of the capsule is dependent on the number and depth of its corrugations, thickness, area, and elasticity of the material. Under normal atmospheric conditions, capsules have the tendency to collapse.
The U-bracket which opens outward provides the spring tension to prevent the capsules from collapsing. A state of equilibrium is thus obtained. Pallett, When the atmospheric pressure decreases, the force that pushes on the aneroid wafers is lowered, but the spring tension of the does not change. This allows them to open out further. The reverse happens when atmospheric pressure rises.
The resulting linear motion from expansion and contraction of the capsule is extremely small. In addition to preventing the aneroid wafers from collapsing under normal atmospheric conditions, the bimetallic U-bracket also acts as a temperature compensating device. The temperature coefficient of the altimeter is largely determined by the elasticity of the aneroid material which varies with temperature.
Elasticity is the property of a material to return to its original dimension and shape when the applied force is removed. For example should the temperature at sea level decreases, the altimeter will over-read.
This increases the elasticity of the aneroid causing it to expand more. With increasing altitude, the same will occur but coupled with the decrease in air pressure, the capsule deflection will get progressively greater, hence the errors. The bimetallic U-shaped bracket bends inwards to oppose this extra expansion, thus cancelling out the error. The opposite occurs when ambient temperature is higher than the standard conditions.
An altimeter will only read correctly under conditions for which it is calibrated temperature; Standard Atmospheric Conditions which assumes a temperature lapse rate of 1. In this case an altimeter will show the pressure altitude. Under non-standard conditions, an altimeter reading will be in error and will instead show indicated altitude.
As altimeters are barometric instruments, its accuracy will be affected by changes in the ambient pressure. News: To better serve our users and the aviation safety community, SKYbrary is transitioning to a new, more flexible platform mid-November , providing users with a better service and easier access to the wealth of safety knowledge it offers.
If you wish to access the latest content from the SKYbrary team, please visit and bookmark www. A barometric altimeter consists of a barometric capsule linked to a pointer by a suitable mechanical or electronic system. The pointer moves across the dial in response to changes in barometric pressure.
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