Density altitude is the altitude your air-dependent machine _thinks_ it is at.
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.