D40101 Connecting The Theta Probe Soil Moisture Probe To The

Data Dolphin Tech Note D40101 optinst.com

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Connecting the ThetaProbe to the Model 400 Introduction This document details the connection and programming information for the connection of the Type ML2x ThetaProbe soil moisture sensor, manufactured Delta-T devices, to Optimum Instruments’ Model 400 datalogger. The ThetaProbe measures volumetric soil moisture (θv) content by sensing changes in the soil’s dielectric constant. These changes, when converted to a DC voltage, are proportional to the soil moisture content over a wide working range.

Figure 1: Delta-T Device's ThetaProbe soil moisture sensor

Wiring and Connections All of the Model 400’s terminals are clearly labeled via adhesive stickers attached to the sides of the Model 400’s back-up battery pack compartment. Electrical connections are made to the Model 400 by simply inserting the bared leads of the ThetaProbe into the screw terminals of the Model 400 and securing the connection with a small slot type screwdriver. Before making any connections, have the green plug-in terminal connector (otherwise known as the power terminal connector) removed from its socket on the Model 400. The leads of the ThetaProbe are connected to the Model 400 (Figure 2) as follows: RED – Connect this lead of the Thetaprobe to the third terminal position, from the left hand side, of the power terminal connector. The connection at this position will correspond to the Model 400’s Switched VRAW output when the power terminal connector is plugged back into its socket on the Model 400. BLUE - Connect this lead of the Thetaprobe to the fourth terminal position, from the left hand side, of the power terminal connector. The connection at this position will correspond to the Model 400’s power ground when the power terminal connector is plugged back into its socket on the Model 400. YELLOW - Connect this lead of the Thetaprobe to the Model 400 input terminal labeled “ALG 11(+)”. This terminal is the positive side of the Model 400’s precision input 11, when the input is programmed as a differential input (see “Differential Setting” on page 4). Optimum Instruments Inc. #201, 3124 Parsons Road, Edmonton, AB, Canada, T6N 1L6 For more info call (780) 450-0591 or FAX (780) 440-2515 Email: [email protected]

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GREEN - Connect this lead of the Thetaprobe to the Model 400 input terminal labeled “ALG 12(-)”. This terminal is the negative side of the Model 400’s precision input 11, when the input is programmed as a differential input (see “Differential Setting” on page 4). BRAID – Connect the braided shielding of the ThetaProbe’s signal cable to the Model 400’s input ground terminal, labeled “Alg. Comm” To power the Model 400 and ThetaProbe, Optimum Instruments recommends using a 12V, 18Ah lead acid battery. Make the following connections with 18 gauge stranded wire: §

Connect a red colored lead from the positive terminal of the 12V battery to the first terminal position of the power terminal connector. Secure the lead to the positive battery terminal by crimping a Thomas & Betts STA-KON type terminal (14RB-10) on to the battery end of the lead.

§

Connect a black colored lead from the positive terminal of the 12V battery to the second terminal position of the power terminal connector. Secure the lead to the negative battery terminal by crimping a Thomas & Betts STA-KON type terminal (14RB-10) on to the battery end of the lead.

Once all power connections are checked and secured, power the Model 400 by plugging the power terminal connector into its socket on the Model 400 motherboard. The Model 400 will begin making its tick-tock sound reminding you that the clock needs to be set. Refer to the Data Dolphin software manual for information on setting the Model 400’s internal clock.

+ -

Ground Switched Vraw

Connect to 12V, 18Ah Lead Acid

ALG 11(+) ALG 12(-) Alg. Comm

ThetaProbe to Model 400 wiring connections Thetaprobe Red Blue Yellow Green Braid

Model 400 Terminal Switched Vraw Ground Alg. 11(+) Alg. 12 (-) Alg. Common

Figure 2: Wiring connection diagram

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Model 400 Programming and Settings After the electrical connections to the ThetaProbe have been successfully made, the Model 400 will have to be programmed to properly operate the ThetaProbe and interpret the 0-1V output of the ThetaProbe as volumetric soil moisture content (θV). This will require having a copy of the Data Dolphin software installed on a Windows based PC, and using the software to establish an active communication link (wireless or direct cable) to the Model 400. The following sections assume that the reader has a working knowledge of the Data Dolphin software and an operating communication link with the Model 400. For more information on use and installation of the Data Dolphin software and establishing a communication link to an Optimum Instruments datalogger, consult the Data Dolphin Software manual. Sensor Stabilization Time While the ThetaProbe can be constantly powered, operating the probe in this way with a limited (battery powered) power source will quickly deplete your power reserves. It is thus suggested that the probe be powered only prior to and during a reading to save power. For complete stability, use a warm up time of 5 seconds; Optimum Instruments recommends using this as the warm-up time. Good repeatability can be achieved using times down to 1 second; the measurement error associated with this warm up time is ±0.0025 m3. m-3 according to the ThetaProbe documentation. Battery power consumed by the probe, for a 1 second reading, is typically: 19mA * 1s = 0.005 mAh. The setting for the sensor stabilization time is found in the Setup window of the Data Dolphin software, under the Rate & Time tab.

Figure 3: Sensor stabilization controls (boxed)

Connecting the ThetaProbe to the Model 400

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To set the sensor stabilization time for the ThetaProbe: 1. Load the Data Dolphin software and connect to the Model 400. 2. Open the Setup window by clicking on the Setup button on the Data Dolphin software’s toolbar. 3. Click the “Load from Dolphin” to load your Model 400’s current settings into the Setup window. 4. Click on the “Rate & Time” tab to view the Model 400’s current time settings. 5. Enter the value 5.0 (seconds) into the Time textbox in the section entitled “Sensor Stabilization”. 6. Click the Apply button next to the Time textbox to program the sensor warm-up time into the Model 400. After programming this setting, the Model 400 will power-up the ThetaProbe five seconds prior to taking a reading. After the reading, it will then disable power to the probe to save power. Differential Setting Each of the Model 400’s precision inputs are logically grouped into differential pairs. An odd numbered precision input and its immediately following even numbered input form a differential pair. By default, each of the Model 400’s eight precision inputs (ALG11-ALG18) operate in single-ended mode, meaning that they operate independently of each other and measure a sensor’s output voltages with respect to Analog Common Ground (Alg. Comm). However, the precision inputs can work together as a differential pair, and thereby measure a sensors output voltage as the resulting difference of the two voltages measured by each inputs. As this technote is assuming a connection of the ThetaProbe to precision inputs 11 and 12, the following procedure details how to program the Model 400 to set these inputs as a differential pair. 1. Follow steps 1 – 3 in the above section if you have not already done so. 2. Click on the “Precision Inputs” tab to view the Model 400’s current precision input settings. 3. Enable the controls on the set-up form for Precision Input 11 by clicking in the “Input 11 On” checkbox, if it is not already done so. 4. Click the checkbox labeled “Differential” to the immediate right of the “Input 11 On” checkbox (Figure 4). After you do this, the controls in the section for precision input 12 will disappear and the message, “Input 12 is currently configured as the secondary differential input of input 12” will appear in their place to indicate that precision inputs 11 and 12 will now constitute a differential pair. 5. Click on the “Apply” button to program the change into the Model 400, and continue to the next section, “Input Configuration” to set the scaling of the differential pair to interpret the ThetaProbe’s output voltage as volumetric soil moisture content (θV). Input Configuration In order to convert the ThetaProbe’s 0 – 1V output signal into a meaningful volumetric soil moisture content (θV) measurement, a mathematical scaling is applied by the Model 400. This scaling is applied to the input configuration to the Model 400. Preprogrammed Thetaprobe input configurations are available in all versions of the Data Dolphin software after and including version 1.8.215. Earlier versions will require entering the linear equation scaling by hand.

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As an example, Figure 4 below shows the precision input 11 and 12 configured for use the ThetaProbe.

Figure 4: Precision input configuration for the ThetaProbe (boxed) To set-up the 11-12 differential input pair to scale the ThetaProbe output voltage: 1. Set-up the precision inputs 11 and 12 as a differential pair if you have not already done so (see “Differential Setting” on page 4). 2. Click on the “Range” drop down list, and select the 2.5 V setting. 3. Click on the “Rate” drop down list, and select the 3 Hz setting. 4. Enter a meaningful name for the ThetaProbe input into the “Name” textbox. 5. The units are preset to the percentage symbol (%) as the value read will be displayed as a percentage with 2 decimal places. The units label may be changed as desired and will not affect the value output. 6. Click on the “Scaling” list, and select “Theta Probe (Mineral)” or “Theta Probe (Organic)”, depending on the soil type being analyzed. For more information on mineral and organic soil type definitions, consult the ThetaProbe user manual. If your version of Data Dolphin software is earlier than version 1.8.215, these selections will not be available; in this case, chose “Linear Equation” instead. 7. After all the settings above have been selected, click on the “Apply” button to program the changes into the Model 400.

Connecting the ThetaProbe to the Model 400

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Expected Output If possible, use the Data Dolphin software to connect to the Model 400 / ThetaProbe. Click on the “Status” button on the Data Dolphin toolbar to open the Status window. When it opens, the ThetaProbe input will appear in the “Precision Inputs” section of the window. Click the “Refresh” button to take a reading from the ThetaProbe. The input should read 0% when the ThetaPobe is inserted into air, 15% in very dry soil, and approximately 93% when in water. The sensor becomes more non-linear as moisture content approaches 100%. The Dolphin internally uses a 5th order curve fit to help overcome this, but the readings will still be somewhat inaccurate for the higher percentages. For instance, the Status window will display a 93% reading when the ThetaProbe is inserted in water, a reading that should theoretically be much closer to 100%.

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Connecting the ThetaProbe to the Model 400