237 Leaf Wetness Sensor Product Manual

1. Introduction

The 237 Leaf Wetness Sensor measures leaf wetness by determining the electrical resistance on the surface of the sensor (a wet surface is less resistant). It is primarily used to determine the percentage of time that a leaf surface is wet, versus the time it is dry.

2. Precautions

• READ AND UNDERSTAND the Safety section at the back of this manual.
• Although the 237 is rugged, it is also a highly precise scientific instrument and should be handled as such.
• The 237 is designed for short duration ac excitation; dc excitation or continuous ac excitation may damage the sensing grid.
• The black outer jacket of the cable is Santoprene® rubber. This compound was chosen for its resistance to temperature extremes, moisture, and UV degradation. However, this jacket will support combustion in air. It is rated as slow burning when tested according to U.L. 94 H.B. and will pass FMVSS302. Local fire codes may preclude its use inside buildings.

3. Initial Inspection

• Upon receipt of the 237, inspect the packaging for any signs of shipping damage and, if found, report the damage to the carrier in accordance with policy. The contents of the package should also be inspected and a claim filed if any shipping related damage is discovered.
• The model number is printed on a label at the connection end of the cable. Check this information against the shipping documents to ensure the correct product is received.

4. QuickStart

Short Cut is an easy way to program your datalogger to measure the 237 and assign datalogger wiring terminals. Short Cut is available as a download on www.campbellsci.com. It is included in installations of LoggerNet, RTDAQ, PC400, or PC200W.

Use the following procedure to get started:

1. Open Short Cut and select to create a new program.
2. Double-click the datalogger model.
3. Under the Available Sensors and Devices list, select the Sensors | Miscellaneous Sensors folder. Double-click 237 Leaf Wetness Sensor to move the selection to the Selected device window. The wet/dry transition point needs to be determined for each application and sensor (Section 8.2, Calibration (p. 8)). Post-processing will be required to determine parameters such as the percent of time wet (Section 8.1, Calculating Wet Time Fraction (p. 8)).
4. After selecting the sensor, click Wiring Diagram to see how the sensor is to be wired to the datalogger. The wiring diagram can be printed now or after more sensors are added.
5. Select any other sensors you have, then finish the remaining Short Cut steps to complete the program. The remaining steps are outlined in Short Cut Help, which is accessed by clicking on Help | Contents | Programming Steps.
6. If LoggerNet, PC400, RTDAQ, or PC200W is running on your PC, and the PC to datalogger connection is active, you can click Finish in Short Cut and you will be prompted to send the program just created to the datalogger.
7. If the sensor is connected to the datalogger, as shown in the wiring diagram in step 4, check the output of the sensor in the datalogger support software data display to make sure it is making reasonable measurements.

5. Overview

The 237 is often used for plant pathology applications because plant diseases are often associated with wet leaves. Duration of wetness and air temperature during wetness are inputs to many disease models.

The sensor emulates a leaf, thereby approximating the wetness state of surrounding foliage. To determine leaf wetness, the 237 uses a circuit board with interlacing gold-plated fingers. Condensation on the sensor lowers the resistance between the fingers, which is measured by the datalogger. Droplets must touch two fingers simultaneously to change the sensor resistance. For this reason, the 237 is typically coated with flat latex paint, which spreads water droplets.

The color and type of paint affect sensor performance. Campbell Scientific supplies the sensor unpainted because individual modifications will vary with the application. The paper listed in Section 10, References (p. 9), describes the effects of paint color and sensor angle on sensors of this type.

The resistance of the sensor at the wet/dry transition point should be determined. A sharp change in resistance occurs in the wet-dry transition on the uncoated sensor; normally the transition is between 50 and 200 kΩ. Coated sensors have a poorly defined transition which normally occurs from 20 kΩ to above 1,000 kΩ. For best results, the leaf wetness sensor should be field calibrated since the transition point will vary for different areas and vegetation.

Features:

• Sensor is shipped unpainted so customer can choose appropriate surface finish that best fits their application
• Imitates characteristics of a leaf
• Compatible with Campbell Scientific CRBasic dataloggers: CR200(X) series, CR300 series, CR6 series, CR800 series, CR1000X, CR1000, CR3000, CR5000, and CR9000(X)

6. Specifications

Resistance at Wet/Dry Transition: normally 50 and 200 kΩ (uncoated sensor); normally 20 to 1000 kΩ (coated sensor)

Operational Temperature Range: 0 to 100 °C

Short-Term Survivability Temperature Range: -40 to 150 °C; sensor may crack when temperature drops below –40 °C

Dimensions:

• Width: 7.1 cm (2.75 in)
• Length: 7.6 cm (3.0 in)
• Depth: 0.64 cm (0.25 in)

Weight: 91 g (3 oz) with 10 ft cable

7. Installation

If you are programming your datalogger with Short Cut, skip Section 7.3, Wiring to Datalogger (p. 5), and Section 7.4, Datalogger Programming (p. 6). Short Cut does this work for you. See Section 4, QuickStart (p. 1), for a Short Cut tutorial.

7.1 Sensor Preparation

Water droplets that form at the onset of condensation are often too small to bridge the electrodes and so remain undetected. Droplets can be detected earlier in formation by application of a non-conductive spreader to the surface of the sensing grid. The spreader most commonly employed is flat latex paint. Campbell Scientific supplies only uncoated sensors since coating preferences vary between applications.

Applying a thin coat of flat latex paint on the sensor surface is a generally accepted practice in plant disease and other applications. Besides providing some protection for the gold-plated electrodes, this paint allows tiny water droplets to spread and bridge the electrodes.

Gillespie and Kidd¹ found that paint color had significant effects on performance and found off-white worked well. Their paint was formulated with one part black pigment to 1000 parts white paint. East² found that greater precision is obtained using a high quality flat latex paint. Some researchers and agricultural weather networks do not paint the sensor.

Before painting the sensor, clean it gently with alcohol.

However the surface is prepared, the response of the sensor is, in reality, only an index against which actual leaf wetness can be estimated. While the absence of a spreader will decrease sensitivity and increase the chance of scratching the gold-plated electrodes, bare sensors may grant greater consistency and less maintenance across a network.

7.2 Sensor Deployment

The sensor should not come in contact with leaves. The sensor is not supplied with a mounting bracket. Gillespie and Kidd¹ found that sensor orientation affects performance. As with surface preparation, orientation varies across applications and users. A common practice is to mount the sensor such that it receives minimal direct sunlight at mid-day during the growing season. Gillespie and Kidd favor a 60 degree tilt on a north facing sensor such that water runs away from the cable connection to minimize puddling on the electrodes. FIGURE 7-1 shows a simple-to-construct mounting bracket.

FIGURE 7-1. Example 237 Field Installation

This diagram illustrates a mounting bracket for the 237 Leaf Wetness Sensor. It shows a flange collar connected to the sensor's cable. This assembly is attached to a securing plate, which is then fastened to a tripod leg using wire ties. The diagram provides front and side views, indicating that the sensor should be tilted approximately 60 degrees towards the north to allow water to run away from the cable connection.

7.3 Wiring to Datalogger

FIGURE 7-2 is a circuit schematic of the 237. TABLE 7-1 describes wiring to Campbell Scientific dataloggers.

FIGURE 7-2. 237 Sensor Schematic

This is a circuit schematic for the 237 Leaf Wetness Sensor. It depicts the sensor's internal resistance (Rs) connected in a bridge configuration with resistors R1 (100 kΩ) and R2 (1 kΩ). The input terminals are labeled EX (connected to Black wire), HI (connected to Red wire), PURPLE, and CLEAR. A SHIELD connection is also shown. The schematic illustrates how the sensor's resistance changes with wetness.

¹U channels are automatically configured by the measurement instruction.

7.4 Datalogger Programming

Short Cut is the best source for up-to-date datalogger programming code. If your data acquisition requirements are simple, you can probably create and maintain a datalogger program exclusively with Short Cut. If your data acquisition needs are more complex, the files that Short Cut creates are a great source for programming code to start a new program or add to an existing custom program.

A Short Cut tutorial is available in Section 4, QuickStart (p. 1). If you wish to import Short Cut code into CRBasic Editor to create or add to a customized program, follow the procedure in Appendix A, Importing Short Cut Code Into CRBasic Editor (p. A-1). Programming basics for CRBasic dataloggers are in the following section. Complete program examples for select CRBasic dataloggers can be found in Appendix B, Example Programs (p. B-1). Programming basics and programming examples for Edlog dataloggers are provided at www.campbellsci.com\old-manuals.

The datalogger program measures Vs/Vx and then uses this measurement to calculate resistance. Information about calculating the wet/time fraction is provided in Section 8, Operation (p. 7). Refer to Appendix B, Example Programs (p. B-1), for suggested implementation of measurement and processing concepts.

7.4.1 Measurement of Vs / Vx

The base measurement of the 237 sensor is Vs/Vx, where Vs is the voltage measured and Vx is the excitation voltage supplied by the datalogger. Vs/Vx is measured by the datalogger with the instructions and parameters listed in TABLE 7-2.

¹ The mV200C, mV25C, and mV50C input range codes check for an open connection indicating that no sensor is connected or a cable has been cleanly cut. If an open connection occurs, NAN will be returned. A floating voltage can occur that falsely indicates the presence of a missing sensor when the mV200C, mV25C, and mV50C input range codes are not used.

7.4.2 Calculating Sensor Resistance

The CRBasic program should include an expression that calculates sensor resistance. With reference to FIGURE 7-2, sensor resistance (Rs), expressed in kΩ, is calculated as follows:

Rs = R2 / (Vs/Vx) – R2 – R1

Therefore,

Rs (kΩ) = 1/(Vs/Vx) – 101

Except for the CR200(X), Campbell Scientific also suggests including the following after the resistance calculation:

If Rs > 10000 Then
    Rs = 10000
EndIf

The high resolution of Campbell Scientific dataloggers can pick up very small signals, which can result in an erratic signal when the sensor is dry. The wet/dry transition is less than 1000 kΩ, and therefore, 10,000 kΩ gives plenty of range outside of what one would consider to be dry.

8. Operation

An exact range of measurements is impossible to give since the 237 is field calibrated. The sensor is not precise and the quality of water bridging the electrodes varies. As demonstrated in Appendix B, Example Programs (p. B-1), a common practice is to measure grid resistance in kΩ using a one bin histogram to calculate at what fraction of the output interval the sensor is wet (Section 8.1, Calculating Wet Time Fraction (p. 8)). If resistance meets the threshold (≤150 kΩ in the example program), the grid is considered wet. Since the output interval is 60 minutes, if the histogram fraction equals 0.33, the leaf was wet for 20 minutes during that hour.

8.1 Calculating Wet Time Fraction

Fraction of time wet are common data derived from 237 measurements. Calculating time fraction requires a wetness threshold. Refer to Section 8.2, Calibration (p. 8), for more information on determining the threshold.

Fraction of time wet is calculated in all current Campbell Scientific dataloggers, except the CR200(X), by using the Histogram() instruction with a single bin and closed form. The bin select value for the histogram is the Variable containing sensor resistance (Rs). The lower limit of the histogram is zero, and the upper limit is the wet / dry threshold. This will give the fraction of the output interval that the sensor is wet. A fraction of time wet of 0.33 when the output interval is one hour means that the sensor was wet for 20 minutes during that hour.

Refer to Appendix B, Example Programs (p. B-1), for information on calculating fraction of time wet with the CR200(X).

8.2 Calibration

A wet / dry threshold of 150 kΩ is used in Appendix B, Example Programs (p. B-1). While this threshold may work well, refining the threshold for a specific sensor and installation is recommended. A sharp change in resistance occurs at the threshold on uncoated sensors. A less defined threshold occurs with coated sensors. The threshold of uncoated sensors is normally between 50 and 200 kΩ. The threshold of the coated sensor is normally between 20 and 1,000 kΩ.

The transition point varies for different areas, vegetation, and water quality. Therefore, for best results, the sensor should be field calibrated using the following procedure:

1. Clean the sensor gently with alcohol. Contaminants change the sensitivity of the sensor.
2. Place the sensor in the vegetation where the wetness is to be monitored.
3. When the vegetation is at the desired wetness, measure the resistance and use that resistance as the threshold.

9. Troubleshooting

TABLE 9-1 lists the causes of unusual resistance readings.

10. References

1. Gillespie, T.J. and Kidd, G.E. 1978. Sensing duration of leaf moisture retention using electrical impedance grids. Can. J. Plant Sci. 58:179-187.
2. East, David (Ohio State University). 1994. Field Testing of Phone Accessible Multi-Channel Datalogger for Tomato IPM Programs. Unpublished.

Appendices

A. Importing Short Cut Code Into CRBasic Editor

Short Cut creates a .DEF file that contains wiring information and a program file that can be imported into the CRBasic Editor. By default, these files reside in the C:\campbellsci\SCWin folder.

Import Short Cut program file and wiring information into CRBasic Editor:

1. Create the Short Cut program. After saving the Short Cut program, click the Advanced tab then the CRBasic Editor button. A program file with a generic name will open in CRBasic. Provide a meaningful name and save the CRBasic program. This program can now be edited for additional refinement.
2. To add the Short Cut wiring information into the new CRBasic program, open the .DEF file located in the C:\campbellsci\SCWin folder, and copy the wiring information, which is at the beginning of the .DEF file.
3. Go into the CRBasic program and paste the wiring information into it.
4. In the CRBasic program, highlight the wiring information, right-click, and select Comment Block. This adds an apostrophe (') to the beginning of each of the highlighted lines, which instructs the data logger compiler to ignore those lines when compiling. The Comment Block feature is demonstrated at about 5:10 in the CRBasic | Features video.

B. Example Programs

Each example program measures leaf wetness and outputs a sample resistance and a time fraction the sensor is wet. In these examples, the output interval is set to 60 minutes, so a time fraction wet of .33 is equivalent to 20 minutes during that hour. Wetness threshold is set at 150 kΩ.

CRBasic Example B-1. CR1000 Program Example

Public Vs_Vx
Public Rs_kOhms

DataTable(Wetness,true,-1)
    OpenInterval
    DataInterval(0,60,Min,10)
    Sample(1, Rs_kOhms, FP2)
    Histogram(Rs_kOhms, FP2, 0, 1, 001, 1 , 0, 150) 'Enter threshold in 8th parameter
EndTable

BeginProg
    Scan(60,Sec, 3, 0)
        BRHalf(Vs_Vx, 1, mV25, 1, VX1, 1, 2500, True, 0, 250, 1, 0)
        Rs_kOhms = (1 / Vs_Vx) - 101
        If Rs_kOhms > 10000 Then
            Rs_kOhms = 10000
        EndIf
        CallTable Wetness
    NextScan
EndProg

CRBasic Example B-2. CR200(X) Program Example

'CR200(X) Series Datalogger
Public Vs_Vx
Public Rs_kOhm
Public ScanIntervalWet
Public ScanIntervalSum
Public TimeFractionWet

DataTable (Wetness,1,-1)
    DataInterval (0,60,min) 'Interval must match IfTime interval (below)
    Sample (1,Rs_kohm)
    Sample (1,TimeFractionWet)
EndTable

BeginProg
    Scan (1,Min)
    'Measure Wetness
    ExDelSE(Vs_Vx,1,1,1,mV2500,500,.0004,0)
    'Zero measurement when measurement < 0
    If Vs_Vx < 0 Then Vs_Vx = 0
    Rs_kOhm = (1 / Vs_Vx) - 101
    'Sum Scan Intervals
    ScanIntervalSum = ScanIntervalSum + 1
    'Check if Leaf wetness is below 150 kOhms transition and count as time wet
    If Rs_kohm < 150 AND Rs_kohm > 0 Then
        ScanIntervalWet = ScanIntervalWet + 1
    EndIf
    'Calculate Time Fraction Wet at top of each hour
    If IfTime (0,60,Min) Then 'Interval must match data table interval
        TimeFractionWet = ScanIntervalWet / ScanIntervalSum
        ScanIntervalWet = 0
        ScanIntervalSum = 0
    EndIf
    CallTable (Wetness)
    NextScan
EndProg

Limited Warranty

Products manufactured by Campbell Scientific are warranted by Campbell Scientific to be free from defects in materials and workmanship under normal use and service for twelve months from the date of shipment unless otherwise specified on the corresponding product webpage. See Product Details on the Ordering Information pages at www.campbellsci.com. Other manufacturer's products, that are resold by Campbell Scientific, are warranted only to the limits extended by the original manufacturer.

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Safety

DANGER - MANY HAZARDS ARE ASSOCIATED WITH INSTALLING, USING, MAINTAINING, AND WORKING ON OR AROUND TRIPODS, TOWERS, AND ANY ATTACHMENTS TO TRIPODS AND TOWERS SUCH AS SENSORS, CROSSARMS, ENCLOSURES, ANTENNAS, ETC. FAILURE TO PROPERLY AND COMPLETELY ASSEMBLE, INSTALL, OPERATE, USE, AND MAINTAIN TRIPODS, TOWERS, AND ATTACHMENTS, AND FAILURE TO HEED WARNINGS, INCREASES THE RISK OF DEATH, ACCIDENT, SERIOUS INJURY, PROPERTY DAMAGE, AND PRODUCT FAILURE. TAKE ALL REASONABLE PRECAUTIONS TO AVOID THESE HAZARDS. CHECK WITH YOUR ORGANIZATION'S SAFETY COORDINATOR (OR POLICY) FOR PROCEDURES AND REQUIRED PROTECTIVE EQUIPMENT PRIOR TO PERFORMING ANY WORK.

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General

• Protect from over-voltage.
• Protect electrical equipment from water.
• Protect from electrostatic discharge (ESD).
• Protect from lightning.
• Prior to performing site or installation work, obtain required approvals and permits. Comply with all governing structure-height regulations.
• Use only qualified personnel for installation, use, and maintenance of tripods and towers, and any attachments to tripods and towers. The use of licensed and qualified contractors is highly recommended.
• Read all applicable instructions carefully and understand procedures thoroughly before beginning work.
• Wear a hardhat and eye protection, and take other appropriate safety precautions while working on or around tripods and towers.
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• Use only manufacturer recommended parts, materials, and tools.

Utility and Electrical

• You can be killed or sustain serious bodily injury if the tripod, tower, or attachments you are installing, constructing, using, or maintaining, or a tool, stake, or anchor, come in contact with overhead or underground utility lines.
• Maintain a distance of at least one-and-one-half times structure height, 6 meters (20 feet), or the distance required by applicable law, whichever is greater, between overhead utility lines and the structure (tripod, tower, attachments, or tools).
• Prior to performing site or installation work, inform all utility companies and have all underground utilities marked.
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Elevated Work and Weather

• Exercise extreme caution when performing elevated work.
• Use appropriate equipment and safety practices.
• During installation and maintenance, keep tower and tripod sites clear of un-trained or non-essential personnel. Take precautions to prevent elevated tools and objects from dropping.
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Maintenance

• Periodically (at least yearly) check for wear and damage, including corrosion, stress cracks, frayed cables, loose cable clamps, cable tightness, etc. and take necessary corrective actions.
• Periodically (at least yearly) check electrical ground connections.

Internal Battery

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