OWL2 to Irrometer Tensiometer

(c) 1998 , 2002 EME Systems, Berkeley CA U.S.A.
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Irrometer Tensiometer


Tensiometers consist of a porous ceramic cup attached to the bottom of a sealed tube. The tube is filled with water and is fitted with a pressure sensor that can electrically monitor the water pressure inside the tube. As the soil in contact with the ceramic cup dries out, there is more suction "trying" to pull water out of the tube into the soil. This is the same osmotic pressure, or "tension" that plant roots have to work against to draw water from the soil. Soils vary widely in their characteristics, due to capaillary effects between the grains and due to chemical wetting effects on the surface of the grains. The pressure sensor transmits the tension reading to the monitoring equipment. Older gages simply have a dial gage that a person can read. The instrument is called a tensiometer. Another device for monitoring soil moisture tension is the gypsum block or granular matrix sensor.

Tensiometers from Irrometer Corp convert soil moisture tension in the range 0-93 centibars into a current of 4-20 milliamps. Using a 150 ohm termination reistor, the output voltage to the ADC is 600 millivolts at 4 mA, zero cbar tension, and 3 volts at 20 mA, 93 cbar. When using Irrometers, the OWL2c should be powered by a 12 volt battery. At full scale reading of 0.020 * 150* = 3 volts, that leaves 9 volts of overhead for operation of the pressure transmitter (required!)

                                                             Analog/Digital converter
                        long paired cable                         ---
  tensiometer   |------------------------------o-------->  power supply >= 12 volts
     4-20 mA    |
     xmtr       |
                |------------------------------o-------->  + input  
                                               \    3 volt
                                      150 ohms /   full scale
                                        0.1%   \    @ 20 mA
                                               o-------->  - input
                                               o-------->   power supply common
  Program example:
 mbar   var word
 gosub ADread ' get result, 600-3000 millivolts
 mbar = result - 600 */ 99 ' 0-930 milliibars soil tension ' [930/1600*256=99.2]
 ' mmbar = result - 600 */ 992 ' 0-9300 milliibars*10 soil tension 
 ' [9300/1600*256=1488]
 ' second expression resolves to 0.1cb

Note that Irrometers can also respond to positive pressure heads in saturated soils. This effect is useful in geological studies. The current dips below 4 millamps. The above formula should be modified so that the subtraction comes after the division. The positive pressure heads will come out as a negative number.

 mbar   var word
 gosub ADread ' get result, 400-2000 millivolts
 mbar = result */ 99 - 233 ' -233 to 930 milliibars
 ' mbar = result */ 992 - 2325 ' -232.5 to 930.0 milliibars

Low Tension Irrometers have highly porous cups that are used for measurements in wet soils. These respond only in the range of 0-40 cbar. The span is 4-10.88 milliamps. With a 150* resistor the output voltage is 600 to 1632 millivolts, a range of 1032 millivolts.

Sometimes it is desirable to log the data as a one byte positive value, in order to conserve logging memory. How this is done depends on what range the data will cover. It is best to discard the range that will never be encountered. Here is one solution, which covers the range from -11.2 cbar up to 39.8 cbar with a resolution of 0.2cbar, stored as one byte.

 result = result min 310 max 1630 - 310 */ 99 - 112
 ' 930 mbar / 2400mVfs *256 = 99 for */
 ' input limited to 310mV (-11.2 cbar) to 1630mV (39.8cbar)
 ' 600mV is zero cbar (150 ohms x 4 ma).
 ' low tension irrometer to 40cb (10.88 ma, 1630mV)
 '-112<result<398 mb, span 510 mbar.
 EEdata = result+112/2 ' 0->512 becomes 0->256
 ' compute EEdata*2-112 to get mbar back
 gosub show1 ' show result, -11.2 to +38.8 cbar
 gosub EElog ' store byte 0-256

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