AN3956

Sensors

Freescale Semiconductor

18

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

This is the best calibration, but is not implemented on the DEMOAPEXSENSOR in software rev 1.0.

Figure 11. Two Part Calibration

First Point: 0 Level, Water Surface

Second Point: Bottom of Container