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As the maximum water storage capacity largely depends on the soil type, the sensors come factory calibrated with a so called “air and water calibration”. An Air/Water calibration is a simple calibration method, suitable if the probe is used to monitor trends rather than absolute volumetric values.
If absolute volumetric values are required, Green Shield offers additional calibration methods.
Volumetric water content, θ, is defined mathematically as:
where Vw is the volume of water and VT = Vsoil + Vvoid = Vsoil + Vwater + Vair is the total volume (that is soil volume + water volume + air space).
To simplify, below saturation, we can consider the total volume as constant and equivalent to the value of dry soil. Adding water to dry soil, in most cases, will not change the total volume. Water will fill the voids in between the soil particles. Once all voids are completely filled with water, the soil is saturated.
As an example, 10 dm³ of coarse, dry sand can be mixed with up to 4 litres of water without increasing the total volume. Consequently, the maximum volumetric water content is 0.4 or 40%.
The GBSSPP1 can be operated with different methods of calibration. The utilized calibration mode can be selected using an Extended SDI-12 command.
As the maximum water storage capacity largely depends on the soil type, the Green Shield Probe comes factory calibrated with a so called “air and water calibration”. This means that a measurement value of 0% corresponds with the sensor placed in air and a value of 100% corresponds with the sensor placed in water. This is a very basic calibration method, not taking into account any soil specific properties.
The dielectric constant of dry soil is higher than the dielectric constant of air consequently an air/water calibrated probe will deliver measurement results higher than 0% when placed in dry soil.
As an example:
Dry Sand: 26%
Dry Volcanic Scoria: 22%
Dry Potting Soil: 15%
Nevertheless, for many purposes, when soil moisture monitoring is rather about observing trends than measurement of absolute values, an air/water calibration is sufficient.
The calibration is long term stable, however it can be repeated any time by placing the sensor in air and sending the extended SDI-12 commands (please see calibration sheets) for air calibration and then placing the sensor in water and sending the extended SDI-12 command for water calibration.
The table shows the measurement response of an air/water calibrated probe in sand compared to the true, absolute volumetric value.
How To Use
When you register your sensors we send you an email with a link to a complete manual. If you did not receive a email please sent Item Serial to firstname.lastname@example.org
The Green Shield product range is compatible with any data logger or remote telemetry unit with SDI-12 interface. When bedding the sensor in the soil, it is essential to establish a close bond between soil and sensor surface, avoiding any air gaps.
Drill or dig a hole (30 to 32 mm in diameter) into the soil, down to the required depth. Sieve the removed soil to remove any stones or debris. Place the sensor at the bottom of the hole and cover it with sieved soil. Pour sufficient water into the hole and wait until it seeps away.
Add more soil and water it again. Repeat this procedure until the hole is completely covered.
It is recommended to place the probe in a tilted position rather than in a horizontal or vertical position.
Please refer to the Install video in the Media link.
Click here to view media.
Don´t pull at the cable to remove the soil moisture probe, as it may damage the product and void warranty. Carefully remove the soil with a small shovel until you can access the body of the sensor. Do not damage the cable jacket or the sensor coating.
For more info on how to remove a sensor please contact us on email@example.com or on our online chat service.
In the case the outer enclosure has too much wear our clients can request new parts at a minimal fee. Parts can be order via our chat service on the website.
SDI-12 is a standard for interfacing data recorders with microprocessor-based sensors. SDI-12 stands for serial/digital interface at 1200 baud.
It can connect multiple sensors with a single data recorder on one cable. It supports up to 60 meter cable between a sensor and a data logger. The standard process to carry out a measurement is to send a measurement request upon which the sensor responds with the time that is required to carry out the measurement and the number of data items being returned.
After waiting the time that the sensor requires to carry out the measurement, the data recorder sends a “Read Command” to get the measurement results.
Start Measurement Command 0M1!
Sensor 0 might respond 00012 which means the measurement will take 1 second and deliver 2 values.
After min. 30 seconds, the data recorder can send the “Read Data Command” 0D0! to which Sensor 0 might reply 0+67.75+17.23.
+67.53+17.23 is the two measurement results which may be 67.75% soil moisture level and 17.23°C soil temperature.
|Is||Supply current||Active mode (during measurement)||8||mA|
|Is||Supply current||Sleep mode||80||µA|
|tm||Measurement Time||Time in active mode upon receiving a measurement command||1||s|
|Volumetric soil moisture measurement range||0 – 100||%|
|Volumetric soil moisture measurement accuracy||Soil specific polynomial calibration||± 3||%|
|Soil temperature measurement accuracy||Over complete operating temperature range||± 3||°C|
|Measurement frequency range||Nominal frequency set to 85 MHz||20||85||160||MHz|
|Temperature drift||Tested in sand at 30% volumetric soil moisture value.||+0.2||%/°C|
|TA||Operating Ambient Temperature Range||-20||+65||°C|
|TSTG||Storage Temperature Range||-40||+85||°C|