Since March this year I have an MB Pro Red running in parallel with a Davis Envoy system. Before switching to solely using MB readings, I wanted to check for differences between the two systems, both using the same wireless signals from ISS and Soil/Leaf station.
Most delta's, if any, are small and usually due to slight changes in timing, rounding etc, but after 3 months of recording the MB Soil Moisture readings appeared to be up to 40% higher compared to the Davis Envoy system, see chart below.
- Soil Moisture MB - Davis.PNG (41.1 KiB) Viewed 1898 times
The plot shows some systematic shifts in data so I plotted the ratio between MB and Davis readings in time.
- Ratio Soil moisture MB - Davis.PNG (93.47 KiB) Viewed 1898 times
The delta is becoming smaller with summer nearing. When adding Soil Temperature to the time plot there an inverse relationship visible with MB/Davis ratio. Which reminded me about a note in the Davis manual that soil temperature is used to correct the soil moisture readings:
"If no soil temperature sensors are installed, the station will use the default temperature of 75ºF (24ºC) to compensate the soil moisture readings. The soil moisture accuracy will be off by about 1% for each 1ºF (0.5ºC) difference between the actual soil temperature and 75ºF (24ºC)."
In this case the temperature in March was down to 6 degr. C, meaning 18 degr. C = 32.4 degr. F below the Davis reference temperature and thus a 32% reduction applied as correction. That's for the MB /Davis ratio then 1/(100-32)*100 = 1.48 and thus rather close to the 40% difference noted in graphs above between MB and Davis readings of same raw wireless signal.
Is the Meteobridge missing such correction or am I overlooking something?
