The three basic parameters the weather station reports are temperature, dew-point and Barometric pressure. This sensor reads these parameters. Actually, it reads temperature, humidity and barometric pressure and the software calculates the dew point from it. The humidity is captured by a factory calibrated sensor from Honeywell. It is said to have a total error of <5% over the entire range and a long term stability of <1.2%. Having compared this sensor to the much more expensive and hard to find Sensiron SHT21 I found that it compares favorably, however the Sensiron device does better between 90 and 100% humidity.
The pressure and temperature reading is provided by the BOSCH BMP085 device. It has 16-bit ADCs for both temperature and pressure and has a set of factory calibration parameters stored in the device. When comparing the reading to the calibrated reading from the local airport, these devices are spot on.
The devices are placed on their own PCB to thermally isolate the temperature sensor from any heat sources on the main board, global warming is bad enough as it is, we don’t want to add to it by mistake. Secondly, a sensor might go bad over time and it should be easy to replace without having to solder on the board.
The sensor can be located up to 20 feet away from the main board and should be at least 2″ away in order not to be influenced by the heat of the main PCB.
Also see the newer version of this sensor now has a modular jack instead of the flat ribbon cable. A 6pole phone/Fax cable can now be used and makes the hookup and extension of the connection much easier.
A new sensor with a different hygrometer chip is now also available, see here for details.