Speed calibrations should be carried out under field conditions. Operate the sprayer with three-quarters of a load of water, at the desired operating speed, with booms folded out, the sprayer pump engaged, nozzles running and agitation occurring. If the sprayer has a wheel-speed sensor or radar, calibration can be achieved by ensuring the rate-controller speed is the same as the accurate or GPS speed.

Approach 1: by calculation (an accurate GPS is not fitted to the sprayer or tractor)

Using a calculator:

Speed (km/h) = distance travelled (m) x 3.6 ÷ time taken (seconds)

For example, if the time taken to travel 100 metres was 18 seconds, then:

Speed (km/h) = distance travelled (m) x 3.6 ÷ time taken (seconds)

Speed (km/h) = 100 (m) x 3.6 ÷ 18 (seconds)

Speed (km/h) = 20 km/h

Approach 2: by matching the controller speed to an accurate GPS speed.

Operate the sprayer under field conditions as per your spray plan. Record the rate controller speed and the GPS speed. (This applies to rate controllers that are running a wheel sensor or radar that is external to the GPS).

Use the following formulas to adjust the rate controller speed constant to the match the GPS or actual speed.

Formula 1 - rate controllers where the constant rate is in pulses per unit (PPU):

Using a calculator:

New PPU = Old PPU x Rate controller speed (km/h)  ÷ GPS speed (km/h)

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Formula 2 - rate controllers where the constant is in units per pulse (UPP):

Using a calculator

New UPP = Old UPP x GPS speed (km/h) ÷ Rate controller speed (km/h)

Once this calibration has been effected, record the new speed constant in the ‘Spray Controller Values Table’ of your manual and record the gear and RPM that was used in the calibration process. Record your tyre pressure.

Approach 2 allows the operator to quickly calibrate speed when paddock conditions change, for example, when spraying on worked ground.

Travel speed considerations during spraying

Tractor transmission issues

Before speed can be calibrated, the tractor needs to be driven to ensure that the desired operating speed is not between clutch packs and, preferably, that the minimum and average speeds can be achieved within the same clutch pack. This is an issue with electronic command gearboxes, full power shifts and sprayers that have automatic transmissions. A constant speed means achieving a consistent dose rate.

New sprayers

Speed challenges arise when purchasing a new sprayer. A new sprayer will usually have a larger capacity than the one it replaced and might require the purchase of a new tractor. It may be prudent to delay the purchase of nozzles for the new sprayer until you can fully test the machine to determine appropriate operating speeds in your conditions. This will avoid a situation where nozzles are not operated correctly.