Pressure altimeter

Hikers and mountain climbers use wrist-mounted or hand-held altimeters, in addition to other navigational tools such as a map, magnetic compass, or GPS receiver.

[2] The most common unit of measurement used for altimeter calibration worldwide is hectopascals (hPa), except for North America (other than Canada[3] ) and Japan where inches of mercury (inHg) are used.

Because barometric pressure changes with the weather, hikers must periodically re-calibrate their altimeters when they reach a known altitude, such as a trail junction or peak marked on a topographical map.

[7] On one end of the spectrum, a low-altitude demonstration jump with water landing and no free fall might waive the mandated use of altimeters and use none at all.

In contrast, a jumper doing freeflying jumps and flying a high performance canopy might use a mechanical analogue altimeter for easy reference in free fall, an in-helmet audible for breakaway altitude warning, additionally programmed with swoop guide tones for canopy flying, as well as a digital altimeter on an armband for quickly glancing the precise altitude on approach.

Another skydiver doing similar types of jumps might wear a digital altimeter for their primary visual one, preferring the direct altitude readout of a numeric display.

The design evolved further to drum-type altimeters, the final step in analogue instrumentation, where each revolution of a single needle accounted for 1,000 feet (300 metres), with thousand foot increments recorded on a numerical odometer-type drum.

The reference pressure, in inches of mercury in Canada and the United States, and hectopascals (previously millibars) elsewhere, is displayed in the small Kollsman window,[9] on the face of the aircraft altimeter.

In aerospace, the mechanical stand-alone altimeters which are based on diaphragm bellows were replaced by integrated measurement systems which are called air data computers (ADC).

Digital barometric pressure sensor for altitude measurement in consumer electronic applications
Digital wrist-mounted skydiving altimeter in logbook mode, displaying the last recorded jump profile.
Skydiver in free fall, making use of a hand-mounted altimeter. The analogue face is visible, showing colour-coded decision altitudes. The depicted altimeter is electronic, despite using an analogue display.
Speaking altimeter with helmet for skydiving
An old altimeter intended for use in aircraft
A drum-type aircraft altimeter, showing the small Kollsman windows at the bottom left (hectopascals) and bottom right (inches of mercury) of the face.
A chart showing how much the true altitude of an aircraft is below the altimeter reading ("indicated altitude") without correcting for temperature. The colder the ambient temperature, the lower the plane is — thus the saying "From hot to cold, look out below". [ 8 ]
Diagram showing the internal components of the sensitive aircraft altimeter.