Campbell Scientific 237-L Leaf Wetness Sensor - Manual (Page 10)

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 ;<<<<<<<<<<<<<<<<<
When compiling this program, the message “Warning: zero is an
invalid input address, Line: xx” will be returned from the
compiler. Ignore the message, so long as “Line: xx” corresponds
to the line number in the program where “WV Loc Option
[__________]” appears. NOTE
5. Plant Pathology Application
Plant diseases are often associated with wet leaves. Duration of wetness and
air temperature during wetness are inputs to many disease models. When
estimating leaf wetness, the sensor emulates a leaf, thereby approximating the
wetness state of surrounding foliage. The sensor does not (and should not!)
come in contact with leaves. 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.
5.1 Sensor Preparation
Campbell Scientific supplies only uncoated sensors since coating preferences
vary between applications.
Campbell Scientific has not researched, nor does it recommend,
paint formulations. The following information regarding paint
formulation is intended only to introduce the concept. NOTE
Preparing the sensor surface with a thin coat of flat latex paint is a generally
accepted practice in plant disease applications. In addition to providing some
protection for the gold plated electrodes, flat latex allows tiny water droplets to
spread and bridge the electrodes. Gillespie and Kidd
1 found that paint color
had significant effects on performance and found off-white worked well. Their
paint was formulated with 1 part black pigment to 1000 parts white paint.
East
2 found that greater precision is obtained using a high quality flat latex
paint. Some researchers and agricultural weather networks do not paint the
sensor.
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.
6