# Basic Stamp Based Altimeter

Posted on Feb 6, 2014

A method of determining altitude is by the use of barometric pressure; however, it is not done without difficulty. The relationship of pressure vs. altitude is not a linear one, it is actually a fairly complex one, which the army came up with in the 1930`s. This is the equation for calculating altitude: Where P is the pressure at an unknown altitu

de, and P_0 is the pressure at sea level (zero feet). P and P_0 can be expressed in any unit because they are computed as a ratio. As you can see this is a fairly complex formula, but with a little bit of work and some math it is possible to compute with the Stamp. Figure 1 shows the non-linear relationship of pressure vs. altitude. As you can see on the graph as pressure decreases, altitude increases, but the higher the altitude gets the less pressure changes. In other words the higher the altitude gets the stepper the slope of the curve gets. That is what gives the curve its non-linearity. Since the Stamp will be calculating altitude and doesn`t have log or floating point math capabilities a method know as linearization will be implemented in the Stamp code to get an accurate representation of the equation. The theory behind linearization it is that straight lines changing in slope at intervals along the curve can represent a non-linear curve. In other words, one non-linear equation can be represented by multiple linear ones that the Stamp is capable of computing. The heart of the altimeter is the Motorola`s MPX4115 pressure sensor. The sensor is temperature compensated from -40 to 125 C, this means that when fluctuation is temperature occur, it will not effect the sensors pressure reading. The sensor outputs an analog voltage proportional to pressure using this linear equation. The sensor also has on-chip signal...

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