AN1979

Sensors

Freescale Semiconductor

6

a location in FLASH. To exit the calibration mode, press the

SEL (PB1) button.

The second calibration is done for the altimeter. The

Altimeter requires a one-point calibration where a known

altitude is entered with a known pressure. This ensures that

changes in atmospheric pressure are due to increases or

decreases in altitude and not changes in barometric pressure.

By returning to the main menu, and selecting the “Set

Elevation”, the user can select an elevation by pressing the

SEL button to cycle through the Elevation options from 0 to

12000 feet in 100-foot increments. Once the selection has

been made the elevation is flashed into the microcontroller

and the user is brought to the Altimeter/Barometer function.

Calibration is required for each use of the altimeter module.

Figure 6. Analog Output to Pressure

CONVERTING ANALOG OUTPUT TO PRESSURE

Freescale pressure sensors have an extremely linear

analog voltage output that is proportional to the pressure

input. Since the sensor output is linear, the pressure can be

calculated by using the equation of a line, y = mx + b, where y

is the output voltage, the slope, m, is the Sensitivity, and the y

intercept, b, is the Offset:

With algebraic manipulation, pressure can be determined by:

Below is an example of determining the pressure from the

analog output of 9.5 mV using the Sensitivity and Offset of the

MPX2102a sensor specified in the datasheet:

= (9.5 mV – 0.5 mV) / 0.1 mV/kPa

= (9.0)/0.1 mV/kPa

= 90 kPa

where 0.5 mV is the typical offset for the MPX2102 and

0.1 mV/kPa is the sensitivity with a 5.0 V supply

This system uses additional amplifiers and an A/D

converter that all add additional offset and gain errors;

however, the translation function was corrected with the two-

point calibration. The known pressure values that are used for

calibration are the maximum and minimum pressures for the

system, 105 kPa and 64 kPa respectively. The A/D values for

these known pressures are saved in the flash memory of the

microcontroller.

By algebraic manipulation, the following equation is

reached to find the barometric pressure:

Converting Pressure to Altitude

The method of determining altitude for this reference

design is measuring the changes in barometric pressure. The

relationship of pressure vs. altitude is not linear. As pressure

decreases, altitude increases, but the higher the altitude gets

the less pressure changes. The equation that was used for

this reference design is:

P = (P0) e^[-(g/(RT)) x (Z — Z0),

where P = pressure at an unknown altitude,

P0 = pressure at a known altitude,

e = a constant,

g = gravitational constant 9.8 (m/s^2),

R = dry air constant 287 J/(kg x K),

T = temperature at unknown elevation in Kelvin,

Z = unknown altitude in meters,

and Z0 = known altitude also in meters.

This equation originates from the hydrostatic equation:

dP = -ρgdZ

in conjunction with the ideal gas law:

P = ρRT

After some algebraic manipulation, plugging in constant

values and converting meters to feet, the following equation

was generated:

Z = Z0 — 27,887 in (P/P0),

where Z = unknown altitude in feet,

Z0 = known altitude also in feet,

P = known pressure at unknown altitude,

and P0 = known pressure at known altitude.

For this system to calculate an altitude, Z, at a known

pressure P, the user must enter a known pressure, P0, and its

corresponding altitude, Z0. To accommodate for changes in

barometric conditions, the known pressure and altitude data

must be re-entered during each use to ensure accuracy.

Simple Weather Prediction

Atmospheric pressure at the Earth’s surface is one of the

measurements used to make weather predictions. Air in a

high-pressure area compresses and warms as it descends.

The warming air inhibits the formation of clouds. Therefore,

the sky is normally sunny in high-pressure areas with a small

chance of haze or fog. However, in an area of low atmospheric

pressure, the air rises and cools. With enough humidity in the

40

35

30

25

20

15

10

5

0

–5

kPa

0

25

50

75

100

3.62

7.25

10.87

14.5

PSI

SPAN

RANGE

(TYP)

OFFSET

(TYP)

P1 > P2

MIN

TYP

MAX