Irrigation Data Logger
Monitors and records soil moisture for an entire season
The #1000915 irrigation data logger is a portable device that can measure and record soil moisture tension, unattended, for an entire growing season. You can bury the device below working depth before field work starts in the spring, or after the crop is planted .Three soil moisture sensors and a temperature probe attach to the data-logger. You can place each sensor at different depths to monitor the moisture through the soil profile, or spread them out at the same depth. The latter method is great for monitoring drip irrigation, to determine the width of the subsurface wetting front. All you need to do after the installation is record the date and time you started the data logger,. then go about your regular farming chores. You have two options of retrieving data from the device.
You can opt to view all of the data at the end of the crop season. With this method you don't have to have the upload cable above ground. If you have a forage crop or a row crop, that needs a lot of cultivation, this will probably be the best method to use to avoid damaging the upload cable. At the end of the season you can upload the data to any PC, and compare the soil moisture data with your irrigation schedule. As you study the changes in soil moisture compared to the time you irrigated you can make adjustments to your irrigation schedule for the following year. If you keep daily weather records, you can factor that into your irrigation decisions, too. The data is stored as a text file that is comma delimited, and can can easily be imported into a spreadsheet.
The second method of retrieving data from the data logger is a
few hours after each irrigation. This method is useful in a field
that needs to have very little machine work throughout the growing
season. Fields of grain, row crops that need little cultivation,
and landscapes are ideal for this method. You can use a laptop PC,
or special data retrieving device for this. In addition to viewing
the data in the field, you can store if for later use, too. Using
this method you can make your irrigation changes right away, and
start saving money immediately.
The irrigation monitor is a small battery operated computer
that measures 3.7 X 12.6 cm, (1.5 X 5 inches). The circuit board
is manufactured by AEC
Systems since 1999 in other products. Included are: the
Model 708 environmental chamber monitor, the Model 908 growing
degree day calculator, and the Model 103 greenhouse monitor.
The firmware controls the time interval between sensor reads. Which, the user determines before placing the device in the field. In addition, the firmware reads each sensor four times in a sampling period and averages them. It then, reverses the direction of current to the sensor to minimize the migration in ions in the soil solution to the electrodes Finally, it saves the readings to memory, and watches the time until the next read.
The PC program
allows the irrigation monitor data to be uploaded and formatted in
the form of a spreadsheet. The user can also set the time interval
between samples, and the accuracy of this time with this program.
For irrigation work a sampling time of 2 to 6 hours is adequate. In this range the data logger can store data for 682 to 2048 days. As you can see there is plenty of extra storage space for more detailed readings.
You can use two types of soil moisture sensors. Each type is an
electrical resistance device that measures soil water tension. The
two sensor types are the gypsum block, and the granular matrix
sensor (GMS), made by Watermark. There are both advantages and
disadvantages to each of these sensor types.
The main advantage of the gypsum block is its low cost. You can
purchase gypsum blocks for less than $10 each. At these low prices
you can monitor a lot more locations in the field. Also, they do
not require maintenance, as does a tensiometer, after
installation, and they are unaffected by the salinity of the soil
solution. Gypsum blocks are useable to a higher soil water suction
than most other soil moisture sensors.
The life of the gypsum block is very short. You can expect at
least one season from of a gypsum block, maybe as many of three.
Soil acidity is the major factor that determines their life, more
acidic less life.
Granular matrix sensors, GMS, were patented in 1985 and
have been used to replace tensiometers in many applications. They
are an electrical resistance soil moisture sensor. A pellet of
gypsum in included in the sensor to saturate, with salts,
the soil solution in the sensor. Therefore this sensor is not
effected by the presence of fertilizers. The advantages of GMS
are, low cost, about $33 each, no maintenance after installation,
unaffected by soil salts, longer life than gypsum blocks, better
response time, and good soil moisture range of response.
The prototype of the irrigation monitor is shown in the figure
to the right. The circuit board, battery, and silica jell are
placed in length of PVC pipe. The circuit board is coated with a
waterproof coating. All of the sensor leads, and the upload cable
are brought out of the pipe through a sealed hole in one of the
end cap. Once the device is sealed, and buried, the user can
control and collect data through the upload cable.
We ran two test of the system in the lab. The soil used is Fairbanks silt loam, moderately sloping collected near the Richardson highway, between Delta Junction and Fairbanks. A weighted amount of air dried soil was mixed with a measured amount of water in a pot, until the soil saturated. After soaking the sensors for two days in distilled water, we placed them in soil. Each day we collected the data from the data-logger, and weighed the pot. The resistance of the sensors is plotted against gravimetric soil water content. You can see that the GMS is usable throughout all soil moisture ranges, whereas the gypsum block did not start responding until very close to the wilting point.
The results of the test run are shown graphically below. You can see that the GMS is able to measure soil moisture through most of the range. The gypsum block on the other hand does not change until the soil becomes dry. For best results on a silt loam soil you should use the GMS. The GMS has a very linear response in the mid-range. In agriculture, this region would be of the greatest interest. You can see that the gypsum blocks' output is very steady until the soil nears the wilting point.
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We plan on putting three of these data-loggers in irrigated
fields during the summer of 2002. The objectives of the trial are
to test the reliability and accuracy of the system to field
conditions. Our greatest concern will be the environmental
conditions this device is subject to in the field. Mainly, such
things as damage from moisture condensation while buried, and
electro static discharge, especially from near lightning strikes.
We would also appreciate permission from the owner of the field to
take a few liters of soil for lab testing, and to place rain
gauges at the site.
Copyright © 2002, by Chuck Mancuso, POB 234
Delta Junction, AK 99737