Acclima TDR Sensor User Manual
For Sensor Models:
TDR305H
TDR310H
TDR315H
Product Description:
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 round-
trip 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:
𝜀 =
𝑡
2
4𝑙
2
where
t
is the round trip propagation time,
l
is the length of the waveguide and
𝜺
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 good-
quality 18650 Li-Ion battery stores about 40,000 Joules.
