OWL2c and conductivity

Description: Moisture deposited on a surface bridges across a grid of wires or circuit board traces. A circuit measures the AC ohms resistance through the grid. Warm, wet weather promotes fungus and insect pests in plants. Quick detection of rain can be utilized in controllers for greenhouse windows and similar applications. Grids can also be used to detect moisture in breath, condensation from steam, or spray from industrial processes. Stainless steel electrodes are embedded in a block of plaster of Paris (gypsum) or a proprietary mixture. Watermark™ sensors are "granular matrix" devices that contain a fine sand aggregate along with gypsum crystals, held inside a permeable membrane and stainless steel sleeve. The gypsum buffers the conductance measurement from ions present in uncontrolled amounts in the soil. This device is buried in close contact with the soil and reaches equilibrium with the soil moisture. The AC ohms resistance of the block can be correlated with soil moisture or evidence of plant stress. An assembled "SMX" module is available for reading the resistance of Watermark blocks into the OWL2 or BASIC Stamp. Electrodes touch the water's surface to indicate the attainment of a certain water level, which can initiate an action or alarm or operate a sump pump. An output can be taken as the water moves up and down a pair of electrodes, with the level correlated to resistance values on the scale. The signal depends on both the water level and the specific conductivity of the water. By filling the water into an exact geometrical space, the variable becomes the conductivity of the water. The circuit measures the resistance of the water at a specific temperature, which is measured simultaneously. Multiplying by a geometry constant of the cell gives the conductivity at that temperature. Conductivity is usually reported as the value it would have at a reference temperature, such as 25 degrees Celsius. From this, the concentration of ions can be inferred if the types and percentages of ions present are known in advance. A special circuit is required to measure electrical resistance through a liquid. An AC voltage is applied between metal electrodes, and the signal is the amount of AC current that flows according to Ohm's law. DC currents must not flow through the wet part of the circuit, or else the metal electrodes will corrode. An oxide layer forms within a fraction of a second on the metal cathode surface. The resistance of this oxide layer is nonlinear and dependent on voltage and history. Corrosion products may enter solution or form "cat whiskers" extending from the electrodes, which spoil the reading and can eventually short circuit the electrodes. AC excitation avoids these issues by reversing the polarity of the current many times per second, preventing any net reaction at either electrode. A related issue is galvanic currents. If two devices are immersed in the same environment, such as two moisture sensors in soil, differences in the environment can generate "ground loop" voltages and currents. The coupling effect between the sensors degrades the readings and exacerbates corrosion. It is crucial to avoid DC "sneak" paths between different sensors. This consideration also applies to other objects in the environment, such as sunken ships, metal tanks, or underwater concentration differences of salt, which can contribute to underwater potential differences driving current through the water and sensors. The following circuit provides AC excitation and galvanic isolation. The output signal is a frequency that depends relatively simply on the resistance of the sensor, from which the amount of solution or its conductance can be determined.

The described circuit utilizes a grid of stainless steel electrodes embedded in a moisture-sensitive substrate to measure soil moisture or water levels. The electrodes are arranged to form a bridge, allowing for the measurement of AC ohms resistance across the grid. This resistance measurement is indicative of moisture content, as moisture bridging the electrodes alters the electrical characteristics of the circuit.

The Watermark™ sensor technology employs a granular matrix that enhances the accuracy of moisture measurement by buffering against ionic variations in the soil, which can affect conductivity readings. The embedded gypsum serves to stabilize the environment around the electrodes, ensuring consistent readings by reaching equilibrium with the surrounding soil moisture.

The system is designed to operate under AC excitation to prevent issues associated with DC currents, such as electrode corrosion and the formation of non-linear oxide layers on the cathode surfaces. This AC excitation method reverses the current flow at high frequencies, which minimizes electrochemical reactions that can degrade sensor performance.

In addition, the circuit includes provisions for galvanic isolation to prevent interference from nearby sensors or environmental factors, such as variations in salinity or the presence of metallic objects. This isolation is essential for maintaining the integrity of the readings and ensuring that the sensors function accurately in various conditions.

The output from the circuit is designed to be compatible with microcontroller platforms, such as the OWL2 or BASIC Stamp, enabling easy integration into automated systems for monitoring soil moisture levels or water presence. The frequency output can be directly correlated to resistance values, allowing for straightforward calculations of conductivity and, consequently, the concentration of ions in the measured solution.

Overall, this circuit design provides a robust solution for moisture detection in various applications, including agricultural monitoring and industrial processes, ensuring reliable operation and accurate data collection.Moisture deposited on a surface bridges across a grid of wires or circuit board traces. A circuit measures the AC ohms resistance through the grid. - Warm wet weather promotes fungus and insect pests of plants. - Quick detection of rain can be used in controllers for greenhouse windows and the like. - Grids can be used to detect the moisture in breath, condensation from steam, or spray from industrial or other processes. Stainless steel electrodes are embedded in a block of plaster of paris (gypsum) or in a proprietary mixture. Watermark ™ sensors are "granular matrix" devices, that have a fine sand aggregate along with gypsum crystals, held inside a permeable membrane and stainless steel sleeve.

The purpose of the gypsum is to buffer the conductance measurement from ions that are found in uncontrolled amounts in the soil. This device is buried in close contact with soil, and reaches an equilibrium with the soil moisture. The AC ohms resistance of the block can be correlated with soil moisture or with evidence of plant stress.

We have an assembled " SMX " module available for reading the resistance of Watermark blocks into the OWL2 or BASIC Stamp. Electrodes touch the surface of the water, to indicate attainment of a certain water level. This can initiate an action or alarm, or operate a sump pump. An output can be taken as the water moves up and down a pair of electrodes, and the level is correlated with resistance values on the scale.

The signal depends on both the water level and on the specific conductivity of the water. See Earth Measurements, units 5 and 6. By making the water fill an exact geomerical space, the variable becomes the conductivity of the water. The circuit measures the resistance of the water at a certain temperature, which is measured at the same time.

Multiplying by a geometry constant of the cell gives the conductivity at that temperature. Usually conductivity is reported as the value it would have at a reference temperature, such as 25 degrees C. From this then the concentration of ions can be inferred, if the types and percentages of ions present are known in advance.

A special circuit is needed to measure electrical resistance through a liquid. An AC voltage is applied between metal electrodes, and the signal is then the amount of ac current that flows per ohms law. DC currents must not be allowed to flow through the wet part of the circuit, or else the metal electrodes will corrode.

An oxide layer is formed within a fraction of a second on the metal cathode surface. The resistance of this oxide layer is nonlinear and voltage volttage and history dependent (like a nightmare diode). Also, corrosion products go into solution or form "cat whiskers" out from the electrodes. These spoil the reading and eventually can short circuit the electrodes. 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), any differences in the environment between them can generate "ground loop" voltages and currents. The coupling effect between the two sensors both degrades the reading and exacerbates corrsion. It is important to avoid DC "sneak" paths between different sensors. The same consideration applies to other objects in the environment. Sunken ships. Metal tanks. Underwater concentration diffferences of salt. Any or all of these can contribute to underwater potential differences that are continually driving current through the water, and through sensors immersed in the water.

The following circuit provides AC excitation and galvanic isolation. The output signal is a frequency that depends in a relatively simple manner on the resistance of the sensor, from which the amount of solution or its conductance can be