Sentek EnviroSCAN profiles

These soil sensor profiles are inserted into a buried PVC tube to measure soil water content and salinity. The sensor profile assembly consist of a plastic probe, sensors placed at 10cm intervals along the probe, and an SDI-12 interface (circuitboard) at the top of the assembly. Sensors are connected to the SDI interface with a ribbon cable running along the probe. Installation and communication with old dataloggers are both a bit tricky.


Standard auger method

This installation is for uniform, fine textured soils. Details here. In rocky mountain soils it is better to opt for the slurry method below (in my opinion).

Slurry method

This installation is suitable for soils in which augering a uniform hole for the access tube is difficult. This includes rocky, non-uniform soils such as those found in subalpine forests. A hole slightly larger than the diameter of the access tube is augered to depth with a standard bucket auger. Rocks must be broken up and removed with the auger, so if this is not possible with the larger rocks, the hole will have to be relocated. Once a suitable hole is augered, it is filled ~1/3 full with a slurry mixture of grey cement, kaolinite clay powder, and water. A sealed PVC access tube is then inserted into the slurry and pushed down to the desired depth, and the slurry is allowed to set around the tube. The sensor probe is then inserted into the access tube and wired once the slurry is dry.

In theory, the slurry around the access tube equilibrates with the soil water content of surrounding soil and allows good measurements of relative soil water content. This method does not allow accurate measurements of absolute soil moisture unless extensive calibrations are done using the slurry and field soil (this sounds like it would be very difficult in practice).

Datalogger Wiring

Our sensors have the old revision of the SDI interface board (v1.2). At the top of the board are 5 input/output pins labeled from 1 to 5. Most pins have broken off and they are now soldered to wires. The terminals are (in order):

  • +12Vin
  • not used
  • not used
  • Ground
  • SDI-12 data

The +12V and Ground wires must be connected to an appropriate power source (12V/SW12V and G on a Campbell 23x for example) and the SDI-12 data wire must connect to an SDI-12 capable serial port.

Datalogger control

Measurement commands are issued with SDI-12 from a serial port. For example, on a Campbell edlog datalogger, the SDI-12 Recorder instruction is issued using a control port and an SDI address. If there are multiple SDI devices on the same line, they may need separate SDI addresses to work. The sensor then takes measurements at each sensor along the probe and sends back a scaled frequency measurement and a water content that is derived from this scaled frequency (WC alone can also be sent).

Edlog program

media/code/sentek_test_1.csi is a working program for measuring a Sentek probe with 4 sensors using a Campbell cr23x datalogger.