According to the FAA, density altitude is formally defined as “pressure altitude corrected for nonstandard temperature variations.” Why is it so important? Because the published performance criteria in the Pilot’s Operating Handbook (POH) are generally based on standard atmospheric conditions at sea level (that is, 59°F or 15°C. and 29.92 inches of mercury). Density altitude is one way to predict the performance of an aircraft in the real world.
The most common method today for finding density altitude involves calculation using either a circular slide rule called an E6B, or an electronic flight computer. A pilot enters their pressure altitude and temperature, and density altitude is given. Simple enough. Then why are there still horror stories about density altitude?
This project will take GPS altitude and integrate the inputs from three sensors (pressure, temperature, and humidity) to produce a real time density altitude display.
There are plenty of detailed explanations of density altitude on the internet, but you have to scratch the surface a bit to find out what it really means. This is what it really means:
As I was casting about looking for the parts to make up this device, I discovered that Bosch had come out with the BME280, a new environmental sensor that combined pressure, humidity, and temperature and could communicate by either I2C or SPI. It was exactly the three measurements I needed, and in a package that is 3mm square! However, at the time I couldn't find anyone who had developed a breakout board. So, I made my own. I laid out the sensor footprint in Eagle, put it and two 0402 bypass capacitors onto a two layer PCB that is 0.4" square (only because I wanted room for all 8 pins), and sent it off to OSHPark. A week and $0.80 later (yes, that is eighty American cents, with shipping included!), I had three bare breakout boards. I have checked the layout, and everything seems to work, so I'm sharing the design through OSHPark. You can order your own here.
Since then, I found that Embedded Adventures sells the MOD-1022, a breakout board with the sensor mounted and configured for an I2C interface at their website. Right now, I am using their open source driver with the Teensy LC I won a few weeks ago, along with an Adafruit color OLED I had lying around, to test out both the sensor and some performance enhancements to the SSD1331 driver for the Teensy.
A couple of weeks ago I received my order from SparkFun with the HTU21, BMP180, and a small OLED display. I've gotten everything cobbled together, and a picture will be coming shortly.
I've also been experimenting with running everything at 1.8 volts. Everything seems to be working fine, but I still have the OLED and '328 running at 3.3 volts. This has also spun off a couple of other projects for supplying 1.8 volts from various sources. More on that in the near future.
Last week, I discovered that Bosch now has a single sensor that measures all three environmental parameters that I need. Nobody sells a breakout board for the BME280 yet, so I threw one together in Eagle and sent it off to OSHPark, and also ordered a few of the sensors from Mouser. I'll release the Eagle files once I've gotten the hardware in my hand and validate that it is correct.