Thursday, May 24, 2018

Safe Spraying - What is Delta T?

Delta T is used by the agricultural industry. It is an important indicator for acceptable spraying conditions. It is indicative of evaporation rate and droplet lifetime. Delta T is calculated by subtracting the wet bulb temperature from the dry bulb temperature. When applying pesticides, Delta T should ideally be between 2 and 8, and not greater than 10. Select the right Delta T to determine the best weather conditions for spraying(PDF 593 Kb).

The diagram below relates air temperature and relative humidity to values of Delta T.

Graph of Delta T

This detail comes from the Australian Bureau of Meteorology, and Delta T values are generally available on many site pages, especially where regular automatic data collection is occurring.

Considered a very useful tool if you are spraying agrochemicals, with a guide being that if in the range of 3-7, it is okay to be spraying [ a 2-8 range is about the safe limit as above hence a small safety margin for security].

You can check for previous conditions as a check for likely conditions the next day in many areas where weather is reasonably stable.

In the Top End, evening and early morning are generally safest, with the Delta -T value within a suitable range, but......not always.

If you can, plan ahead and check just before spraying.  Warm temperatures will drive it up, but there is often a guide for many chemicals to use below 30C, sometimes a bit difficult to achieve, so less effectiveness might be expected.

It may be possible to adjust spray parameters - pressure, droplet size and spray direction for example to compensate for poor conditions if spray events are critical, but often it may be prudent to wait a little time for better conditions eg early am or evening. 

In the dry, but cooler dry season in the north, it is quite common for the Delta T to be well above 10 in the afternoon, when both above 30C is common and RH is very low [less than 30%] - obviously avoid this time for spraying!!

There are commercial services available from a number of agribusiness firms, some of which do add some additional options.  However, with the Delta T values and graph available, you can quickly check yourself too for no real cost, except your time to check.  It is simple, but a desirable check before spraying.

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