AN1979

Sensors

7

Freescale Semiconductor

air, the rising air will cool, the air will condense forming clouds

and precipitation in the form of rain or snow.

This reference design saves the current pressure reading

and compares it to past pressure measurements. It

determines if there was a pressure drop or a pressure

increase. Using this information, it makes a simple weather

prediction by sending a message of ‘sunny’ for a pressure

increase, ‘rainy for a pressure drop, and ‘steady’ for no

CONCLUSION

The Altimeter is one of many applications for the

MPXM2102AS pressure sensor. This reference design can be

used as a reference for developing more integrated

barometer applications such as hand-held weather stations,

altimeter features for camera or GPS systems, as well as

barometric pressure monitoring systems for industrial

systems. The MPXM2102AS is an excellent pressure sensor

for this application since it is calibrated and temperature

compensated. By having these features available on-chip,

there is a large savings in PCB real estate in addition to

savings in cost for external components

.

1. This information was found from the USA Today Weather Book from USAToday.com.

Table 4. Elevation Pressure and Temperature Changes

Altitude Above Sea Level

Temperature

Barometer

Atmospheric Pressure

Feet

Meters

F

C

mm * Hg

psi

kPa

-1000

-305

63

17

787.9

15.23

105.0

-500

-153

61

16

773.9

14.96

103.1

0

0

59

15

760.0

14.69

101.33

500

153

57

14

746.3

14.43

99.49

1000

305

55

13

733.0

14.16

97.63

1500

458

54

12

719.6

13.91

95.91

2000

610

52

11

706.6

13.66

94.19

2500

763

50

10

693.9

13.41

92.46

3000

915

48

9

681.2

13.17

90.81

3500

1068

47

8

668.8

12.93

89.15

4000

1220

45

7

656.3

12.69

87.49

4500

1373

43

6

644.4

12.46

85.91

5000

1526

41

5

632.5

12.23

84.33

6000

1831

38

3

609.3

11.78

81.22

7000

2136

34

1

586.7

11.34

78.19

9000

2441

31

-1

564.6

10.91

75.22

9000

2746

27

-3

543.3

10.5

72.40

10,000

3050

23

-5

522.7

10.1

69.64

15,000

4577

6

-14

429.0

8.29

57.16