Figure 9 and Figure 10 are examples of water measurement. In the hot water heater to the left, although the temperature of
the water can be high, a long enough tube combined with air column separation provides a buffer for measurement. Water and
Air are good insulators, so temperature variation can be quite drastic in a short distance. This is a design advantage in isolating
the sensor from heat.
In Figure 10 the diagram has a method to improve resolution of the water level. The vertical height measurement is kept
constant, but the volume of water dispensed or left in the bottom is improved. This is done by simply making a conical bottom.
The resolution in vertical height can be used to improve the volume measurement. Additional ADC counts per a volume of water
are achieved, than a shape with a broad based bottom.
AUTO ZERO
A summary of the application note AN1636 is that the offset of the pressure sensor can change during board mount, stress,
or temperature shifts. By sampling the offset pressure (voltage output with no pressure input) and storing this value in the MCU,
the changes in the offset are negated. This improves the overall accuracy of the voltage output of the sensor, since the offset
changes are negated. Normally this would occur in a final product when there is a startup or a cycle when the sensor is not
pressurized.
CALIBRATION
The best calibration would be a two part calibration. A Pressure would be taken at 0 mm of H2O and also when the tubing is
at 40 cm of H2O. The difference in ADC counts divided by 40 cm will give the number of ADC counts per a cm etc. for each sensor.
This is the best calibration, but is not implemented on the DEMOAPEXSENSOR in software rev 1.0.
First Point: 0 Level, Water Surface
Second Point: Bottom of Container
Figure 11. Two Part Calibration
AN3956
Sensors
Freescale Semiconductor
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