Pressure Sensors Absolute – A Sensor That Measures Input Pressure in Relation to a Zero Pressure. We Will Use the Absolute Pressure Sensor to Calculate Altitude. Differential – A Sensor That Is Designed to Accept Simultaneously Two Independent Pressure Sources. The Output Is Proportional to the Difference Between the Two Sources. We Will Use the Differential Pressure Sensor to Calculate Indicated Airspeed.

Determining Altitude From Absolute Pressure The equation for converting pressure to altitude is: Where pressure is in KPa Altitude is in feet above the zero pressure reference level The zero pressure level in Lexington, KY as of last night was 99.7 Kpa

Determining Indicated Airspeed From Differential Pressure The equation for converting the pressure difference to airspeed is: Velocity in mph Dynamic pressure in Newton / meters ^2 or Pascal's Density in kg / meter ^3 Standard density of kg / M ^3 (*note* -- the air density can change with weather, and altitude)

Pressure Sensor Basics

Classes of Motorola Pressure Sensors Uncompensated Pressure Sensors – These standard, low cost, sensors permit manufacturers to design and add their own external temperature compensation and signal conditioning networks. Compensated Pressure Sensors – These sensors have built in temperature compensation and signal conditioning. Integrated Pressure Sensors – These integrated sensors have built in temperature compensation and signal conditioning just like the Compensated Pressure Sensors. The Integrated Pressure Sensors are designed specifically to use with microcontroller applications and have an easy one wire interface to hook to the A/D port on a microcontroller. For each three classes there are a variety of (A)bsolute and (D)ifferential pressure sensors.

Maximum & Minimum Pressure Parameters For the Absolute Pressure Sensor it is obvious that the pressure range needs to be greater than 115 kPa in order to exceed the Zero Pressure Level of anywhere in the U.S. and smaller than 1kPa in order to exceed 120,000ft. In order to keep the maximum sensitivity an Absolute Pressure Sensor should have a pressure range of kPa which makes the Motorola MPX4115A an obvious first choice. For the Differential Pressure Sensor the three Motorola Differential Pressure Sensors with the highest sensitivity have Maximum Pressure Ratings of 10, 50, and 100 kPa which would have the ability to measure Maximum Indicated Airspeeds of 280, 640, and 900 MPH respectively. Assuming the Indicated Airspeed will not exceed 600 MPH this makes the 50 kPa sensor the obvious choice because it can measure Indicated Airspeeds in excess of 600 MPH and has twice the sensitivity of the 100kPa sensor. This sensor is the Motorola MPX5050D.

The Motorola MPX4115 Absolute Pressure Sensor Features Pressure Range From 0 to 115 kPa 1.5% Maximum Error over 0 to 85 C Ideally suited for Microprocessor or Microcontroller-Based Systems Temperature Compensated Over – 40 to +125 C Maximum Power Rating of 50 mW with a Typical Power Rating of 35 mW High Sensitivity of 46 mv/kPa

The Motorola MPX5050D Differential Pressure Sensor Features Pressure Range From 0 to 50 kPa 2.5% Maximum Error over 0 to 85 C Ideally suited for Microprocessor or Microcontroller-Based Systems Temperature Compensated Over – 40 to +125 C Maximum Power Rating of 50 mW with a Typical Power Rating of 35 mW High Sensitivity of 90 mv/kPa that would be able to measure velocity from MPH

Additional Reference Sites for Motorola Pressure Sensors

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