Acclima TDR Soil Moisture Sensors

Acclima’s TDR Sensors are Integrated Time Domain Reflectometers that contain an

ultra-fast step

function generator, an ultra-fast waveform digitizer and a precision time base. The

step function

generator and waveform digitizer are connected directly to a waveguide of length

5 cm, 10 cm or 15 cm

without an intervening coaxial cable. The key to their operation is a patented

waveform digitizing

hardware and firmware set that has an effective digitizing rate of 200 billion samples

per second. The

step function generator launches a voltage step on the waveguide with a 20%/80%

rise time of 150

trillionths of a second. The digitizer acquires a digital image of the incident wave and

its returning

reflections with 5 trillionths of a second resolution. The image is then analyzed to

determine the roundtrip propagation time between the incident wave and the first

reflected wave. Using the propagation

equation that governs the speed of light through a medium the permittivity of the

medium is then

calculated as:

                        E = t2/4l2

where t is the round trip propagation time, l is the length of the waveguide and E is

the relative

permittivity of the medium though which the wave has passed. The fact that

propagation time is

independent of soil electrical conductivity leads to the fact that the calculated

permittivity is also

independent of soil electrical conductivity. This is the main advantage of digitized

waveform time

domain sensors over all non-time domain electronic sensors. Volumetric Water

Content can be derived

from permittivity using the Topp equation or a suitable dielectric mixing model.

Electrical Conductivity in

soils is affected by compaction and hence non-TDR sensors tend to be very sensitive

to both installation

compaction and follow-on soil settling. TDR sensors report the true volumetric water

content

independent of soil electrical conductivity, compaction and settling.

An 80 MHz floating point processor inside the sensor is used to process the

waveform image. The total

time required to send a measurement command to the sensor, acquire the digitized

waveform image,

process the image and calculate the soil permittivity, water content and conductivity

is 0.25 seconds.

The sensors require a power supply from 3.5 volts to 15 volts. The current draw

during a reading ranges

from 88 mA at 3.5 volts to 32 mA at 15 volts. The idle current is less than 10 uA.

Because of the very fast

operation time the total energy consumed from a battery is only 0.1 Joule per

reading. A typical goodquality 18650 Li-Ion battery stores about 40,000 Joules.