BMP180 Barometric Pressure/Temperature/Altitude Sensor- 5V is the best low-cost sensing solution for measuring barometric pressure and temperature. Since pressure changes with altitude, you can also use this sensor as an altimeter. The sensor is soldered onto a PCB with a 3.3V regulator, I2C level shifter and pull-up resistors on the I2C pins.
The BMP180 is the next-generation of sensors from Bosch, and replaces the BMP085, but it is completely identical to the BMP085 in terms of firmware/software/interfacing, so you can use Adafruit’s BMP085 tutorial and any example code/libraries as a drop-in replacement.
BMP180 Barometric Pressure/Temperature/Altitude Sensor- 5V is 5V compliant - a 3.3V regulator and an I2C level shifter circuit is included so you can use this sensor safely with 5V logic and power.
Using BMP180 Barometric Pressure/Temperature/Altitude Sensor- 5V is easy. For example, if you're using an Arduino, simply connect the VIN pin to the 5V voltage pin, GND to ground, SCL to I2C Clock (Analog 5) and SDA to I2C Data (Analog 4). You can download Adafruit’s BMP085/BMP180 Arduino library and example code for temperature, pressure and altitude calculation. Install the library, and load the example sketch. Immediately you'll have precision temperature, pressure and altitude data. Adafruit’s detailed tutorial has all the info you need including links to software and installation instructions.
Adafruit Product Page: https://www.adafruit.com/products/1603
Adafruit Learn Page: https://learn.adafruit.com/bmp085