Onset HOBOnet RXW TEROS-21 User manual

HOBOnet® Wireless Sensor Network
RXW TEROS-21 Soil Water Potential Sensor (RXW-T21-xxx) Manual
24850-B
This sensor measures water potential and temperature in numerous soil types. It is designed to
work with the HOBOnet (HOBO® RX) Wireless Sensor Network in which data is transmitted
wirelessly from the sensor mote across the network to the station and then uploaded to
HOBOlink® web-based software. With HOBOlink, you can monitor sensor readings, view graphs,
set up alarms, download data, and more.
Specifications
Water Potential
Measurement Range
-2,000 to -9 kPa in soils up to 10 dS/m
Accuracy
* ±10% of reading + 2 kPa from -100 to -9 kPa
Resolution
0.1 kPa
Dielectric Measur
ement
Frequency
70 MHz
Temperature
Measurement Range
-40° to 60°C (-40° to 140°F)
Accuracy
±1°C (1.8°F)
Resolution
0.1°C (0.18°F)
Wireless Mote
Operating Temperature
Range
Sensor: -40° to 60°C (-40° to 140°F)
Mote: -25° to 60°C (-13° to 140°F) with rechargeable batteries
-40° to 70°C (-40° to 158°F) with lithium batteries
Radio Power
12.6 mW (+11 dBm) non-adjustable
Transmission Range
Reliable connection to 457.2 m (1,500 ft) line of sight at 1.8 m (6 ft) high
Reliable connection to 609.6 m (2,000 ft) line of sight at 3 m (10 ft) high
Wireless Data Standard
IEEE 802.15.4
Radio
Operating
Frequencies
RXW-T21-900: 904–924 MHz
RXW-T21-868: 866.5 MHz
RXW-T21-921: 921 MHz
RXW-T21-922: 916–924 MHz
Modulation Employed
OQPSK (Offset Quadrature Phase Shift Keying)
Data Rate
Up to 250 kbps, non-adjustable
Duty Cycle
<1%
Maximum
Number of
Motes
Up to 50 wireless sensors or 336 data channels per one HOBO RX station
Logging Rate
1 minute to 18 hours
Number of Data Channels
3
Battery Type/
Power Source
Two AA 1.2V rechargeable NiMH batteries, powered by built-in solar
panel or two AA 1.5 V lithium batteries for operating conditions of -40° to
70°C (-40° to 158°F)
Battery Life
With NiMH batteries: Typical 3–5 years when operated in the temperature
range -20° to 40°C (-4°F to 104°F) and positioned toward the sun (see
Mounting and Positioning the Mote), operation outside this range will
reduce the battery service life
With lithium batteries: 1 year, typical use
Memory
16 MB
Dimensions
Sensor: 9.6 x 3.5 x 1.5 cm (3.8 x 1.4 x 0.6 inches)
Sensor diameter: 3.2 cm (1.3 inches)
Cable length: 5 m (16.4 ft)
Mote: 16.2 x 8.59 x 4.14 cm (6.38 x 3.38 x 1.63 inches)
Weight
RXW-T21-xxx sensor and cable: 103 g (3.65 oz)
Mote: 223 g (7.87 oz)
RXW Soil Water Potential
Sensor
Models:
•RXW-T21-900 (US)
•RXW-T21-868 (Europe)
•RXW-T21-921 (Taiwan)
•RXW-T21-922
(Australia/NZ)
Included Items:
•Cable ties
•Screws
Test Equipment Depot - 800.517.8431 - 5 Commonwealth Ave, MA 01801 - TestEquipmentDepot.com

RXW TEROS-21 Soil Water Potential Sensor (RXW-T21-xxx) Manual
5
•Make sure the mote door is closed, with both latches fully
locked to ensure a watertight seal.
•Consider using a 3/16 inch padlock to restrict access to the
mote. With the mote door closed, hook a padlock through
the eyelet on the right side of the door and lock it.
•Position the mote towards the sun, making sure the solar
panel is oriented so that it receives optimal sunlight
throughout each season. It may be necessary to
periodically adjust the mote position as the path of the
sunlight changes throughout the year or if tree and leaf
growth alters the amount of sunlight reaching the solar
panel.
•Make sure the mote is mounted a minimum of 1.8 m (6 ft)
from the ground or vegetation to help maximize distance
and signal strength.
•Place the mote so there is full line of sight with the next
mote. If there is an obstruction between two sensor
motes or between the sensor mote and the manager, then
use a repeater mounted on the obstruction. For example,
if there is a hill between the sensor mote and the
manager, place a repeater at the top of the hill between
the sensor mote and the manager.
•There should not be more than five motes in any direction
at their maximum transmission range from the manager.
Data logged by a wireless sensor must travel or “hop”
across the wireless network from one mote to the next
until it ultimately reaches the manager connected to the
station. To make sure the data can successfully travel
across the network, the mote should not be more than
five hops away from the manager.
•The HOBOnet Wireless Sensor Network can support up to
50 wireless sensors or 336 data channels per one HOBO
RX station.
•Use a #4-40 screw to attach a ground wire to the port on
the back of the mote if you are deploying the mote in a
location where lightning is a concern.
Sensor Accuracy and Calibration
The sensor is calibrated at a saturated state (0 kPa), at a dry
state equivalent to -2,000 kPa, and at four calibration points
between 0 kPa and -100 kPa, resulting in accuracy of ±10% of
reading + 2 kPa over the range of -9 to -100 kPa.
At water potentials drier than -100 kPa, the sensor relies on the
linear relationship between the logarithm of water content and
the logarithm of water potential. Laboratory evaluations have
shown good accuracy to at least -1,500 kPa (plant permanent
wilting point). Independent field evaluations have also shown
low sensor-to-sensor variability down to permanent wilting
point.
Sensor calibration is not affected by soil type because the
sensor only measures the water potential of the ceramic discs
in equilibrium with the soil. The sensor works in any soil type or
other porous media as long as it is installed correctly with
adequate hydraulic contact (to ensure timely water potential
equilibrium between the sensor and the medium of interest). In
most situations, soil undergoes brief periods of wet up followed
by longer dry down periods. The sensor calibrates during the
the drying period after the soil wets up. Therefore, the readings
are most accurate as the soil dries, as that is most important for
irrigation.
The sensor can be used in frozen soils, but it does not
accurately measure water potential in frozen soil conditions.
However, the water potential of the soil under frozen soil
conditions can be estimated by measuring the soil temperature
accurately. For each 1°C (0.8°F) decrease in temperature below
0°C (32°F), the water potential in the soil decreases by ~1,200
kPa. This relationship has been shown to be valid in field soils
for water potentials below about -50 kPa.
Maintenance
The ceramic discs in the sensor are brittle and can chip or crack
under duress. The metal screens afford the discs some amount
of protection, but sharp trauma on the disc edges or massive
impact (such as dropping the sensor onto a hard surface) can
cause the ceramic to break. One or two small chips on the edge
of the disc do not affect the sensor accuracy significantly.
However, a cracked ceramic disc results in a loss of accuracy.
The ceramic discs must readily take up water to accurately
measure water potential. Exposure to oils or other hydrophobic
substances compromises the ability of the discs to take up
water from the soil, which can lead to slow equilibration times
and loss of accuracy. Minimize exposure of the ceramic
material to skin oils, grease, synthetic oils, or other
hydrophobic compounds.
The mote is designed for outdoor use, but should be inspected
periodically. When inspecting the mote, do the following:
•Verify the mote is free of visible damage or cracks.
•Make sure the mote is clean. Wipe off any dust or grime
with a damp cloth.
•Wipe off any water before opening the mote.
•Make sure the interior seal is intact and the latches are
fully locked when the mote door is closed.
Updating Mote Firmware
If a new firmware version is available for the mote, use
HOBOlink to download the file to your computer.
1. In HOBOlink, go to Devices, then RX Devices, and click your
station name.
2. On the station page, click Overview and scroll down to
Device Information.
3. Click the Wireless tab. This icon appears next to the
mote if there is a new version of firmware available.
4. Click the firmware upgrade link. Click Download and
save the firmware .bin file to your computer.
5. Connect the mote to the computer with a USB cable (open
the mote door and use the USB port to the right of the
LCD). The blue LED is illuminated while connected.
6. The mote appears as a new storage device in the
computer’s file storage manager. Copy the downloaded
firmware file to the new storage device (the mote). The
blue LED will blink slowly while the file is copying.
7. After the file is copied to the mote, the LED will stop
blinking and remain a steady blue. Eject the storage device
from the computer and disconnect the cable from the
mote. The firmware installation process will begin