Frost Protection - UTL Repository

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F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S
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and to read the thermometer. This method is less accurate than using a dew-point
hygrometer, but it is often sufficiently accurate for determining start and stop
temperature for sprinklers.
FIGURE 7.11
A simple method to estimate the dew-point temperature
Slowly add ice cubes to the water in a shiny
can to lower the can temperature. Stir the
water with a thermometer while adding the
ice cubes to ensure the same can and water
temperature.
When condensation occurs on the outside of
the can, note the dew point temperature.
In most of the literature on using sprinklers for frost protection, the start and
stop air temperatures are determined relative to the dew-point and wet-bulb
temperatures. In reality, they should be based on the ice point and frost-bulb
temperatures since ice covered plants are more common than water covered
plants at subzero temperatures. However, a table of air temperatures
corresponding to subzero dew-point and wet-bulb temperatures is nearly
identical to a table of air temperatures corresponding to the ice point and frostbulb
temperatures. Therefore, only the dew-point and wet-bulb temperatures
are used in Table 7.5, to avoid confusion with common practice.
To use Table 7.5, locate the wet-bulb (T w ) temperature in the top row that is
greater than or equal to the critical damage (T c ) temperature for the crop. Then
locate the dew-point (T d ) temperature in the left-hand column and find the air
temperature in the table that corresponds. Make sure that the sprinklers are
operating before the air temperature measured upwind from the crop falls to the
selected air temperature. The values in Table 7.5 are for sea level, but they are
reasonably accurate up to about 500 m elevation. For more accuracy at higher
elevations, the SST.xls application program, which is included with this book,
does these calculations and it can be used to determine exact starting and
stopping temperatures for any input elevation.
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