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Freescale Semiconductor, Inc.

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Barometric Pressure Measurement Using Semiconductor Pressure Sensors
Prepared by: Chris Winkler and Jeff Baum Discrete Applications Engineering


The most recent advances in silicon micromachining technology have given rise to a variety of low–cost pressuresensor applications and solutions. Certain applications had previously been hindered by the high–cost, large size, and overall reliability limitations of electromechanical pressure sensing devices. Furthermore, the integration of on–chip temperature compensation and calibration has allowed a significant improvement in the accuracy and temperature stability of the sensor output signal. This technologyallows for the development of both analog and microcomputer–based systems that can accurately resolve the small pressure changes encountered in many applications. One particular application of interest is the combination of a silicon pressure sensor and a microcontroller interface in the design of a digital barometer. The focus of the following documentation is to present a low–cost, simpleapproach to designing a digital barometer system.

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Figure 1. Barometer System

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Table 1.Altitude versus Pressure Data
Altitude (Ft.) 0 500 1,000 6,000 10,000 15,000 Pressure (in–Hg) 29.92 29.38 28.85 23.97 20.57 16.86

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Figure 1 shows the overall system architecture chosen for this application. This system serves as a building block, from which more advanced systems can be developed. Enhanced accuracy, resolution, and additional features can beintegrated in a more complex design. There are some preliminary concerns regarding the measurement of barometric pressure which directly affect the design considerations for this system. Barometric pressure refers to the air pressure existing at any point within the earth’s atmosphere. This pressure can be measured as an absolute pressure, (with reference to absolute vacuum) or can be referenced to someother value or scale. The meteorology and avionics industries traditionally measure the absolute pressure, and then reference it to a sea level pressure value. This complicated process is used in generating maps of weather systems. The atmospheric pressure at any altitude varies due to changing weather conditions over time. Therefore, it can be difficult to determine the significance of aparticular pressure measurement without additional information. However, once the pressure at a particular location and elevation is determined, the pressure can be calculated at any other altitude. Mathematically, atmospheric pressure is exponentially related to altitude. This particular system is designed to track variations in barometric pressure once it is calibrated to a known pressure reference ata given altitude. For simplification, the standard atmospheric pressure at sea level is assumed to be 29.9 in–Hg. “Standard” barometric pressure is measured at particular altitude at the average weather conditions for that altitude over time. The system described in this text is specified to accurately measure barometric pressure variations up to altitudes of 15,000 ft. This altitude correspondsto a standard pressure of approximately 15.0 in–Hg. As a result of changing weather conditions, the standard pressure at a given altitude can fluctuate approximately ±1 in–Hg. in either direction. Table 1 indicates standard barometric pressures at several altitudes of interest.

In order to measure and display the correct barometric pressure, this system must perform several...
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