Deploying The Hobonode Soil Moisture Sensor - Onset HOBOnode User Manual

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Deploying the HOBOnode Soil Moisture Sensor

Operating Environment

The HOBOnode Soil Moisture Sensor provides accurate readings for soil between 0 and +50°C (+32° and +122°F). The sensor

will not be damaged by temperatures as low as ‐40°C (‐40°F); it is safe to leave the sensor in the ground year‐round for

permanent installation. The sensor adapter electronics (housed in the gray tube on the sensor cable) are rated to +75°C

(+167°F) and are mounted outside the logger enclosure and secured to the mounting pole. The cable and sensor adapter

are weatherproof.

Operation

The HOBOnode Soil Moisture Sensor measures the dielectric constant of soil in order to determine its volumetric water

content. The dielectric constant of water is much higher than that of air or soil minerals, which makes it a sensitive measure

of the water content. During operation, values of 0 to 0.5 m

are physically possible.

A value of 0 to 0.1 m

indicates oven‐dry to dry soil respectively. A value of 0.3 or higher normally indicates a wet to

saturated soil. Values outside the operating range may be a sign that the sensor is not properly installed (poor soil contact

or foreign objects are adjacent to the sensor) or that a soil‐specific calibration is required.

Note that sudden changes in value typically indicate that the soil has settled or shifted, which are signs that the sensor may

not be installed properly or that it has been altered or adjusted during deployment.

Maintenance

The HOBOnode Soil Moisture Sensor does not require any regular maintenance. If cleaning, rinse the sensor with mild soap

and fresh water.

Calibration

The HOBOnode Soil Moisture Sensor comes pre‐calibrated for most soil types. If, however, your soil type has high sand or

salt content, the standard calibration will not be accurate. In such cases, you will need to convert the data provided by the

probe with a specific calibration for your individual soil type. To determine the soil specific calibration formula, refer to the

Calibrating ECH

O Soil Moisture Probes application note, available at:

http://www.onsetcomp.com/support/faq/sensor‐notes‐soil‐moisture‐smart‐sensor

HOBOnode Outdoor Wireless System User's Guide

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