What Is a Height Gauge?

The altimeter is a high-precision instrument of various ranges designed for many industrial applications and detection agencies.

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The altimeter is a high-precision instrument of various ranges designed for many industrial applications and detection agencies.

Chinese name

Altimeter

Foreign name

Altimeter

Pinyin

o dù jì

Category

measuring instrument

The instrument includes a high-precision grating device, a measuring rod device of a dense bead guide, and a high-performance high-speed sampling.

The barometric altimeter is an instrument that is placed in an aircraft when it detects airborne objects and uses the relationship between air pressure and altitude to measure the altitude (also known as absolute altitude) of the aircraft by observing the pressure.

Everyone knows that the pressure in water is only determined by the water depth, P = gh. Atmospheric pressure is similar and is caused by the gravity of the surface air. As the altitude rises, the thickness of the air on the ground decreases and the pressure drops. Therefore, the altitude value can be obtained by measuring the atmospheric pressure at the location and comparing it with the standard value. This is the basic working principle of the barometric altimeter. Let the atmospheric pressure at sea level be P0 and the atmospheric pressure at the location be P, then the altitude h = (P0-P) / ( * g). With the increase of altitude, the temperature and pressure gradually decrease, resulting in a decrease in density. Formulas that do not take this into account have no practical value.

It is assumed that the density decreases uniformly with height, h = 0 at sea level, = 0, h = r (thickness of the atmosphere) at the outer boundary of the atmosphere, and = 0, so there is = 0 (h0-h) / h0, then the altitude h The atmospheric pressure at the place is the sum of the atmospheric masses at h0 to h, because it is a linear relationship. Using the knowledge of the difference series, the atmospheric pressure at the altitude h should be P (h) = 0 (h0-h) ^ 2 / (2h0), and the standard atmospheric pressure P0 and air density 0 at sea level are known. Taking P0 = 101kPa and air density 0 = 1.2kg / m ^ 3, we can calculate h0 = 8400 meters from this, so the altitude The expression of should be modified to h = h0-sqrt (P / P0).

In fact, the thickness of the gas that contributes to the surface atmospheric pressure is indeed only in the order of tens of kilometers. More precisely, 99% of the atmospheric mass is concentrated within 30km of the surface, of which 50% is within 5.6km, and the upper layer is above 100km. Although the atmosphere has an important impact on the global environment, its density is already quite low.

Of course, the assumption of this linear relationship is only a rough approximation. From the hydrostatic equilibrium conditions, it can be concluded that the atmospheric density decreases exponentially with increasing altitude. However, this sentence only applies when the atmosphere is statically stable. Only approximated, NASA gives an empirical formula for near-earth atmospheric temperature and pressure. All barometric altimeters use machinery or circuits to reproduce the corresponding relationship between these pressures and altitudes, but due to climate change The difference in air density is completely impossible to estimate, which is a common problem with such altimeters. Therefore, it is better to use GPS based on three-dimensional geometry when highly accurate values are required.