Delta T and Spraying

Why is it important to consider Delta T when spraying?

This parameter defines the evaporation rate of the water. A high Delta T means fast evaporation of water. Most of the active product applications are made on water and are dissolved in the order of 90 to 99%. This is why evaporation plays a crucial role in the droplet size and concentration of the active ingredient.

Delta T is defined by the ratio between relative humidity and dry bulb temperature. At low humidity the atmosphere has a high demand for water and at high temperature evaporation takes place rapidly.

General recommendations indicate that application should be made with a Delta T of 2 to 8, as these are the ideal conditions. The recommended action is in the yellow band of the graph. If the droplets are too large or the solutes are oil-based, the limit can be increased.

If Delta T is too high:

Water evaporates quickly.

Droplets reduce their size by increasing drift with small gusts.

The drops dry quickly, reducing the absorption of the active ingredient.

There are mini-weather stations that are placed on the tractor so that the climatic conditions of the application can be known at any time.

If Delta T is too low:

The water does not evaporate, keeping the smaller droplets available for drifting.

In night conditions, the potential danger of inversion increases.

Leaf survival will be very long and herbicide uptake will be reduced by drift or rain.

The application windows are mainly closed during the summer time of the year, when high temperatures limit the application range.

Wind is also key in determining a quality application. Do not apply in winds above 15 km/h as the drift will start to become dangerous. Nearby crops may be affected if the wind speed is higher.

In cases where spraying cannot be delayed until better weather conditions prevail, spraying should be resorted to:

Apply at low tractor speeds.

Use nozzles to generate coarse or very coarse droplets.

Increase water rates to compensate for speed.