I dug through my "junk box" to see what sensors I have acquired over the last couple of years. I anticipated building enhancements to the existing consumer weather stations, but found that closed software and proprietary protocols were not friendly to the process. As I mentioned that , plus the failure of my third consumer station (2 Davis & 1 Peet Bros.), motivated me to just build the complete thing.

Here is a list of what I found in my project box (-10 to 50 °C typical accuracies listed):

• 6ea - DS1620 digital temperature sensor, ± 2.0 °C
• 6ea - DS18B20 digital temperature sensor, ± 0.5 °C (2ea waterproof probes)
• 1 ea - HTU210F digital temperature + humidity sensor, ± 0.3 °C, ± 2%RH
  
• 4ea - SHT11 digital temperature + humidity sensor, ± 0.4 °C, ± 3%RH
  
• 2ea - MCP9808 digital temperature sensor, ± 0.25 °C
  
• 1ea - HM55B digital compass sensor, ± 9 μT
• 1ea - MPL3115A2 digital pressure sensor w/altimetry, ± 0.4 kPa
• 1ea - LSM303DLHC digital accelerometer + magnetometer, ± 60 mg, ± 1.3 μT
• 1ea - PR300A? (Fairchild 5154-20) selenium photocell, >10Meg dark, approx 500 ohm light
• 1ea - AS3935 Franklin Lightning Sensor, TBD, greater than noise floor (can set from 28-2000 μVrms)
• 3ea - RT100 platinum temperature sensor, ± 0.5 °C
• 1ea - CMA-4544PF-W microphone + MAX4466, combined calculated at ±3.5 dBV
• 4ea - various light sensors (more on these later)
• 2ea - Anemometers + wind direction, need rebuilding due to wear
• 2ea - Tipping bucket rain gauge sensors, 8 inch, 0.01 inch accuracy
• 3ea - Watermark soil moisture sensors, accuracy depends on soil - must be calibrated
• 2ea - Unknown type GPS modules, likely USGlobalsat with SiRF StarIII chipset

As I mentioned all of these were acquired over the last two years and ranged from a few cents at hamfests/swapmeets to about $7-12 USD for those on breakout boards. Nearly all of the above (or their improved successors) are available from Adafruit.com currently. I highly recommend them as a source for any DIY sensors and/or breakout boards for prototyping.

One of the stated objectives of this project is to have high inherent accuracy to eliminate, or at least simplify, the calibration of the final station.

In the discussion a suggestion was made to make this unit wireless. That is a nice goal for "Phase 2", along with solar power. However, to keep things simple and prove the concept I will make the prototype and initial station as wired with PoE. That will allow it to run on a single Cat5e direct burial cable and will allow for actual power consumption evaluation in preparation for solar power.

My research on the Stevenson Screen also turned up several papers and in-depth measurement comparisons for natural measurements versus aspirated (fan driven) measurements. Surprisingly the aspirated "gold standard" was invented and has been in use since the late 1800's. A LOT of data is, therefore, available.

From all this, I am leaning toward having both types of sensors in the initial proof of concept system. My conclusion from the papers is the natural aspiration tends to be good at providing stable, consistent measurements where the aspirated ones are superior in measuring rapid changes and layering.