We found a soil moisture sensor from vegetronix http://www.vegetronix.com/Products/Soil-Moisture-Sensor-Probes.phtml and thought the DrDAQ would be perfect to interface this to. Since the output of the 'VG400-LV' sensor is 0-1.8V we were set to go pretty much straight into the DrDAQ's 0-2.5V range on the sensor sockets without too much work. On the DrDAQ there is a 100K pullup to 2V5 so output impedance of any 'voltage-output' sensor is important if we're going to get an accurate reading unfortunately the vegetonix sensor has 100K output impedance . So we got in contact with Vegetronix to see if this was going to be a problem (after all we can fix non linearity with a DrDAQ scaling file) but they said we should use a buffer, so we ordered a sensor and got to work.
Since we already know the sensor gives the right output voltage a simple voltage follower will do the job to lower the output impedance. To save a little time we found a Rail to Rail amplifier we had so we could go straight from the single 5V supply provided by the DrDAQ and constructed a schematic, see Schematic.Gif.
You could use stripboard and make a nice small board since it's only a few components but I thought it was time to break out the PCB router and make a 'lump' in the cable. We made the board, assembled and covered it with some heatshrink and a couple of cable ties (Piccies attached). To save the need for a crimping tool for 4/4 sockets we got an EL032 cable (http://www.drdaq.com/sensors.html) and cut it in half and stripped the cores back (we'll save the other half for another sensor!). We used a 10K resistor for autosense here ready for the DDS file.
So we had the hardware, now for the scaling file, there are some graphs provided by vegetronix for Voltage Vs volumetric water content and water level in a 2L bottle, http://www.vegetronix.com/Curves/VG400-LV-RevD/VG400-LV-RevD-Curves.phtml, these would be easy to get into a DDS file. I grabbed a pint glass of water and plugged in the sensor, I measured the voltage at the 0mm level and the 90mm level and just put 2 values into the lookup table, DDS file attached. We put the DDS file in the Picoscope directory and restarted. I lowered the sensor in some steps, just lowering and stopping and created the attached Picoscope capture, looking good.