Model 237 Leaf Wetness Sensor Revision: 7/10 C o p y r i g h t © 1 9 8 8 - 2 0 1 0 C a m p b e l l S c i e n t i f i c , I n c .
Warranty and Assistance The 237 LEAF WETNESS SENSOR is warranted by Campbell Scientific, Inc. to be free from defects in materials and workmanship under normal use and service for twelve (12) months from date of shipment unless specified otherwise. Batteries have no warranty. Campbell Scientific, Inc.'s obligation under this warranty is limited to repairing or replacing (at Campbell Scientific, Inc.'s option) defective products.
237 Table of Contents PDF viewers note: These page numbers refer to the printed version of this document. Use the Adobe Acrobat® bookmarks tab for links to specific sections. 1. Introduction..................................................................1 1.1 Specifications............................................................................................1 2. Wiring............................................................................1 3. Programming ......................................
Model 237 Leaf Wetness Sensor 1. Introduction The 237 Leaf Wetness Sensor is a simple resistive grid configured in a 3-wire half-bridge. The circuit is completed when water bridges two inter-digitate electrodes. Response is non-linear with a rapid decrease in resistance relative to an increase in wetness. The simplicity of the sensor lends it to various applications, means of preparation, and data interpretations. 1.
Model 237 Leaf Wetness Sensor NOTE 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.
Model 237 Leaf Wetness Sensor 3.3 Interpreting Resistance Values Table 3 lists 237 sensor resistance ranges and their interpretation. TABLE 3.
Model 237 Leaf Wetness Sensor 4. Programming Examples 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. 4.
Model 237 Leaf Wetness Sensor 'Check if Leaf wetness is below 150 kOhms transition and count as time dry 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 4.
Model 237 Leaf Wetness Sensor 7: Histogram (P75) 1: 1 Reps 2: 1 No. of Bins 3: 1 Closed Form 4: 2 Bin Select Value Loc [ Rs_kOhms ] 5: 0000 WV Loc Option [ _________ ] 6: 0 Low Limit 7: 150 High Limit ;<<<<<<<<<<<<<<<<<
Model 237 Leaf Wetness Sensor 5.2 Plant Pathology Application Programming An exact range of measurements is impossible to give since the 237 is field calibrated. The manufacture of the sensor is not precise and the quality of water bridging the electrodes varies. As demonstrated in program examples in Section 4, a common practice is to measures grid resistance in terms of kOhms using a 1 bin histogram to calculate at what fraction of the output interval the sensor is wet.
Model 237 Leaf Wetness Sensor 6. 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. NOTE 8 The citation of researcher does not imply the endorsement of Campbell Scientific products by any researcher or institution.
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