Irrigation Data Logger

Monitors and records soil moisture for an entire season

 Granular Matrix 

 Prototype 

 Test Run in Lab 

 Test Results 

 Calibration 

 Field Test 

INTRODUCTION

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. 
 
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HARDWARE 

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. 
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SOFTWARE 

 
Two software programs are included with the irrigation monitor. One, called the firmware is burned into the microprocessor on the circuit board. The second program is on a diskette to be loaded into a Microsoft Windows computer to control the uploading of data. 

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. 
  

User Setup 

 
Before placing the irrigation monitor in the field the user must set the sampling interval. You can set this interval from one minute to 256 hours. Of course, the time between uploads determines the maximum number of samples that can be taken without overfilling the memory . For example, you can see that at one minute intervals the memory will fill in 136.5 hours, or 5 to 6 days. Whereas, an interval of 256 hours will allow just one read in over 10 days.. 

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. 

The user can also adjust the accuracy of the data loggers clock. However, this adjustment is temperature dependent and should be adjusted at the site of operation. 
  

SENSORS

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. 
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Gypsum Blocks

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 disadvantages of the gypsum block are the need to calibrate it, its short life, slow response, useless in course texture soils, and insensitivity at high moisture levels.  You must calibrate the gypsum block to each texture of soil that you plan to monitor. For agricultural purposes you just need calibration points at, field capacity, 1/3 bar (34 kPa), and wilting capacity, 15 bar, (1500 kPa). 
 

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. 
Gypsum blocks have a slow response time. This is okay to measure the effect of irrigation after the fact, but to actually control the irrigation process for you it's much too slow. You would need a sensor that reacted faster to do this. 
The porosity of the gypsum block is real close to a silty soil. Gypsum, therefore, holds water with more tension than a course soil. As a course soil dries, the gypsum retains the water and gives a erroneous indication of the soil moisture. 
Because of the above, gypsum blocks are insensitive at high soil moisture content. Until the soil dries to a point that water suction is greater than the gypsum, the moisture in the block keeps the electrical resistance nearly constant. Gypsum blocks will indicate the wilting point of the soil, though.   Back to page top 

Granular Matrix Sensors 

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. 
Disadvantages of GMS are higher cost than gypsum blocks, measures soil water suction instead of gravimetric amount, and needs to be calibrated. You'll find a comparison of gypsum blocks and GMS below. 
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IRRIGATION MONITOR PROTOTYPE

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. 
The prototype can have up to three soil moisture sensors, and one temperature sensor. However, you need not use all of the sensors. In test on blueberries and corn we found it beneficial to place one sensor near the soil surface, another at the densest root area, and the last below that. This method allows you to see the timing of the wetting front as it enters the soil and travels to the root depth. Any water that percolates passed the roots is shown by the deep sensor, and indicates a potential danger to the ground water from over irrigating. The temperature sensor, placed near the surface, is used to indicate the amount of sunlight on the soil surface. We used the temperature data as a means of determining the water lost to evaporation. 
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Prototype Test Run

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. 

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Test Run Results

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. 

FIELD  TESTING

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. 
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