SMX: soil moisture transmitter

Electrical Interface for Watermark™ or Gypsum Block Sensors.

Motivation

Watermark Granular Matrix sensors are available from Irrometer Company, Riverside CA 951/689-1701 <http://www.irrometer.com/sensors.html#wm>. These "granular matrix" devices consist of a fine aggregate mixed with a gypsum buffer, held inside a permeable membrane and a perforated stainless steel sleeve. This device is buried in intimate contact with soil, and reaches an equilibrium with the soil moisture. Electrodes are embedded in the granular matrix, and the the electrical conductance between them is the parameter measured. Conductance increases with increasing soil moisture (and also depends on temperature,which can be compensated). The purpose of the gypsum is to buffer the measurement from ions that are found in uncontrolled amounts in the soil. Solid gypsum block sensors are similar, electrodes embedded in a gypsum block with proprietary additives and controlled manufacturing processes. The Watermark formulation claims to have a wider range of response and a longer field life and is especially suited for the range of soil moisture typically encountered in agriculture. The gypsum block sensor may offer extended range at the dry end of the scale.

The bottom line is that the sensor may be used as a tool to optimize irrigation and to warn of plant stress. The sensor reading is resistance in Ohms, but that may be translated by calculation or by a table to a value in kilo-Pascal (kPa, same as centi-bar). For best results, the calibration should be carried out in the soil type to be monitored.

A special circuit is needed to measure the electrical resistance of the Watermark or gypsum block sensors. Continuous DC currents must not be allowed to flow through the wet part of the circuit, or else irreversible reactions occur on the metal surface that spoil the readings. AC excitation avoids these problems, by reversing the polarity of the current many times per second, so that no net reaction takes place at either electrode.

A related issue is galvanic currents. If two devices are immersed in the same environment (say two moisture sensors near each other in the soil), or a ground connection to a data logger can generate "ground loop" voltages and currents. The coupling effect between the two sensors or any other conductive path both degrades the reading and exacerbates corrosion. It is important to avoid DC "sneak" paths between different sensors. The same consideration applies to other objects in the environment: Metal tanks, ground rods, salinity gradients. Any or all of these can contribute to underwater potential differences that are continually driving current through the wet medium, and through sensors immersed in the medium.

The SMX module from EME Systems

The SMX circuit provides the AC excitation and galvanic isolation required. The output signal is a frequency, or alternatively a current or a voltage, that depends in a relatively simple manner on the resistance of the sensor.  Two wires connect to the watermark sensor, and the other wires produce the voltage, current or frequency signal that is routed to a data logger or controller.  In addition to its intended use with the Watermark sensor, the SMX has found more unusual applications from airplane wing deicer bags to baby diaper monitors, using specially designed sensor electrodes.

Specifications: Supply Voltage: 4 to 15 DC Frequency output 50 hz@ open circuit,12 khz @ 0 ohms, open collector square wave (needs pullup resistor to read out frequency.) Current output (also supply current) 0.2 mA dry to 1.0 mA wet (as high as 1.5 ma with sensor short circuit) Voltage output 0.2 volts to 1.0 volts typical (as high as 1.5 V with sensor short circuit) less than 0.01% per Volt supply variation. Operating Temperature: -0°C to +70°C no meaningful signal below 0°C PRICE:  encapsulated module, $55 ea. core SMX PCB, $22 ea.

left: encapsulated module ---------------- right: core PCB The SMX is a potted module (0.825" square x 0.25" thick) for placement in the field near the sensor. There are two wires for connection to the Watermark or gypsum block sensor, and four wires for connection to the data logger or other equipment.  The SMX is also available in the form of a core circuit board, without the wires or potting, and this form is suitable for experiments and for incorporation in custom systems. connections (see diagram, below) red: + 5 to 15 volts DC green: signal frequency, needs pull-up to +V white: voltage signal black: common or current signal blue wires: soil moisture block PDF data sheet

Wiring the SMX to the data logger:

Attach the Watermark sensor to the pair of blue wires. The polarity does not matter. (Please consult EME Systems if you are located in an area where there is lightning. The soil moisture sensor is intrinsically grounded at the point where it is buried. If there is a long cable between the sensor and your equipment, or if the equipment is itself connected to a different ground point, you must add a zap protection circuit to the SMX where it connects to the probe, and make careful consideration of the ground loop.)

Figure 1 shows how to connect the sensor for digital frequency output. The resistor (10kohms, value not critical) can pull up to any voltage from 3 to 7 volts dc. The output signal then is a square wave, and its frequency varies from 50 hz when the sensor is bone dry, up to 10000+ hertz when the sensor is soaking wet. This output can be measured using a COUNT or PERIOD function on the data logger. Note that the white and the black wires are connected together. 

 Figure 1: connection for 50hz to 10,000 hz frequency output

Figure 2 shows how to connect the sensor for voltage output. The green wire is not used and should be connected to the white wire (not the black wire!). The output signal is a voltage that varies from <0.2 volts when the sensor is bone dry, up to over 1 volt when the sensor is soaking wet.

Figure 2: connection for voltage

Figure 3 shows how to connect the sensor for two wire current output. The black wire should be connected to the white wire. The output signal is a current that varies from 0.2 milliamp when the sensor is bone dry, up to over 1 milliamp when the sensor is soaking wet. The current on the two wire circuit may be converted to a voltage at the input of the data logger. A 1 k½ resistor will convert the 0.2 to 1.0 millamp current into a 0.2  to 1 volt signal. The power supply voltage must be a high enough voltage to maintain the 4 volts required by the SMX module itself.

Figure 3: connection for current

SMX response curves:

Resources:

The Irrometer web site <http://www.irrometer.com/agcat.htm#watermark> offers links to a number of agricultural engineering publications that evaluate the Watermark sensor. Here are additional links:

Sowacs

The main clearinghouse for all aspects and techniques of soil moisture measurement

 

Sowacs mail archive

The mail list covers a wide range of questions and answers. Moderated by Bruce Metelerkamp.

 

Resurrecting the Gypsum Block for Soil Moisture Measurement

Sowacs-mea-gypsum block email

Measurement Engineering Australia - Environmental Monitoring Systems

Articles by Andrew Skinner of Measurement Engineering Australia (MEA), and a link to the company web site.

 

Irrometer

Manufacturer of the Watermark sensor, corporate web site

 

GMS Malheur OSU - Clinton Shock

Malheur experiment station at Oregon State University, bibliography of experimental research

 

Watermark calibration

Soil Water Management

Watermark to HOBO data logger

Articles from Rick Allen at the University of Idaho

 

UArizona/GLOBE Soil Moisture Project

University of Arizona educational project on parameters of global environmental importance

 

Delmhorst Instrument Company

Soil Moisture Corporation

Manufacturers of soil moisture sensors and equipment

Remote Measurement Systems - Sensors and Techniques

Another conductance-->voltage converter.

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