Spirometer Demo with Freescale Microcontrollers

Software architecture
Figure 20. State Sending results flow chart
4.6 Calculating parameters
This section provides a summary of the formulas used to calculate the spirometry parameters. These formulas are embedded
in the MCU software.
With each new sample taken by the ADC, the next calculations are performed:
If ADC Value > Signal offset:
If ADC Value < Signal offset:
If (FlowX >4.7), then flowfactor = 0
If (FlowX ≤4.7), then flowfactor = (4.7―FlowX) / 2.7
Instant flow = FlowX +(FlowX*flowfactor)
Instant volume = Instant flow * sampling period
Volume = Volume + instant volume
• ADC Value—The value read from the ADC.
• Conversion factor—This is a factor that converts the pressure measured with the sensor to flow units. It depends on the
mouthpiece used. Using the sensor mentioned in Spirometry, this factor is 0.0136.
• Signal offset—The baseline of the measurement. This value corresponds to an air flow of 0 (no differential pressure on
the sensor).
• Sampling period—The time between consecutive samples (seconds).
• Instant flow—The corresponding instantaneous flow of the current sample (liters/second).
Spirometer Demo with Freescale Microcontrollers, Rev. 1, 12/2012
20 Freescale Semiconductor, Inc.