1. Web links to sensors
2. A brief summary
Web Links to sensor types
There are a large variety of sensor types available. The folowing
links have been compiled from the web however this does not represent any
endorsement of any product. This is not a comphrehensive listing.
Electronic
Electronic - portable or low cost
Mechanical
Alternatives to sensors
Electronic
1.
Sutron leaf wetness sensor
2. Campbell
Scientific
3. Luft
4. Climate
data services
5. The Davis
weather station
6. Texas weather
instruments
7.
Terra temperature, RH, leaf wetness combisensor
8. Micros
9. Skye
SWD sensor
10. Global water
Electronic - portable or low cost
1. Spectrum Scientific
2. ICRISAT low
cost sensor
Mechanical
1. Belfort
leaf wetness recorder
A brief summary
The design of sensors began with mechanical sensors (Hirst 1954), which
were later replaced by electronic ones. More recently, types based on emerging
technologies have been suggested as replacements. Mechanical sensors measure
a change in sensor length, size or weight caused by moisture deposition.
There are numerous examples including the Wallin-Polhemus dew recorder
(Wallin 1963) and the De Witt leaf wetness recorder (Post 1959). Disadvantages
of mechanical sensors are that they are not readily compatible with modern,
electronic digital weather station technology. They also are reported to
give inconsistent results (Lomas and Shashoua 1970).
The most common method of measurement of SWD is with an electronic grid. Electronic sensors measure surface water deposition by a change in sensor resistance or capacitance. A sensor consists of an electrode, ranging in size from crude wires to fine grid networks etched on a printed circuit board (Huband and Butler 1984, Weiss and Lukens 1981). Some workers have investigated the direct measurement of SWD by placing electrodes directly on the leaf (Weiss et al. 1988). These sensors have not gained widespread adoption, probably because daily checks are required to ensure that they have good contact with the leaf surface (Sutton et al. 1984). Another sensor design is a network of wires covered by a piece of cotton cloth (Weiss and Lukens 1981). They found this sensor outperformed an etched sensor in predicting SWD, as did Weiss and Hagen (1983). In contrast, Armstrong et al. (1993) found that a Campbell (grid) and a cloth sensor both underestimated SWD, but the cloth sensor did not respond as well as the Campbell sensor in heavy dew fall situations.
Another important consideration is that the size and shape of the SWD sensor should be similar to that of the leaf or organ (Sutton et al. 1984). In onions, a cylindrical shaped sensor has been found to be useful (Gillespie and Duan 1987), while Huband and Butler (1984) used a flexible sensor to simulate the moving of wheat leaves in the wind. Other workers have also attempted to mimic fruit shape in sensor construction (Sutton et al. 1984).
Griffioen et al. (1992) presented what they consider to be a novel design of a SWD sensor based upon change in optical reflection at a glass-air interface. However, the performance of the optical sensor was compromised by dirt deposits on the mirror surface. Armstrong et al. (1993) found a beta-ray gauge gave reasonable agreement with visual observations both in the field and in the laboratory. The beta-ray gauge has a detection limit of a water layer of 0.0012mm thickness with 95% probability (Bunnenburg and Kuhn 1977). It also has the advantage of using the leaf surface rather than an artificial surface to measure SWD. The sensor however is unlikely to see routine adoption for disease management due to high cost. Although the detector is cheap, the rate-meter electronics for the detector are comparable to the costs of a datalogger.
Researchers have also tested radar (Gillespie et al. 1990) and
microwave transmission (Bouten et al. 1991) as tools for SWD estimation.
The main problem with these techniques is the internal plant and surface
moisture both influence the transmitted signal. Another recent development
with considerable promise is the use of infra-red thermometry to estimate
SWD (Deshpande et al. 1995, Sadler 1996). The technique estimates
SWD from the time period during which the dew point or wet bulb temperature
is greater than the canopy temperature.
The greatest problem with the design of most SWD sensors is that they provide an indirect measurement of SWD. The SWD measured by an instrument is not equivalent to the SWD on a particular plant. The exceptions are the beta-ray gauge and the infra-red thermometer, which directly measure SWD. However, these sensors are unlikely to be adopted for widespread use due to their high cost. It appears at this time that a SWD measurement by instrument is unlikely to provide a standard.