Ok here's the rundown of the differences of the three Decagon sensor that can measure EC.

First generation: ECH2O-TE (Discontinued)- Measured VWC with 70 MHZ capacitance and a two point calibration, and measured EC using a four electrode wenner array. This worked beautifully except for the fact that the gold would sometimes scratch down to the copper underneath, which would then oxidize, flaking off the gold.

To replace the ECH2O-TE, we developed the 5TE. The 5TE measures VWC exactly the same way, but with a 5 point dielectric calibration, which substantially decreases sensor to sensor variability for VWC. The 5TE uses a two electrode array to measure EC using those two stainless steel screws, which we have found to have some trouble with contact resistance (see below).

A little about contact resistance: seems that contamination on the screws (from your hands, grease, etc) can be a huge source of error with the two electrode system, and appears to have the strongest effect at higher EC values.As long as you clean off the screws before using them, we don't see the contact resistance issue.

Anyway, the 5TE is great in soils, but it appears that soilless substrates start to pull away from those screws when the substrates are drying out and lose contact. When the sensors lose contact, you get a reading of "0" for EC.

The 5TE also can communicate using SDI-12, if this is an important component of your study.

So, to fix that issue with the soilless substrate and the screws, we created the greenhouse sensor (to be released Spring 2011), which has three stainless steel needles to make the measurements. The sensor is still a capacitance sensor using 70 MHz with a five point calibration. So, the cool thing is we went from a tiny surface area (two small screws) to a very large surface area for the EC measurements in soilless substrates. However, there is still the potential for contact resistance so the need to clean the prongs before use is still there.

The greenhouse sensor can also communicate in SDI-12.