In the trials a specially prepared liquid was sprayed through the 11005 fan jets at 2.5 bar and a 75% duty cycle, applying a rate of 136 litres/ha at 12 km/hr.
when it was turned off. Without turn compensa- tion, as expected, there was a huge difference in measurements across the boom – with deposi- tions at the 2 m position 270% greater than at the centre of the boom. Nozzles on the outer section, at 34 m, were underdosing by 30%. Turn compensation delivers an enormous improvement. The measured differences were less than +/- 10% across the 36 m width of the boom. In the more flowing, 55 m radius turn, the dif- ferences are less, but still considerable. With PWM turned on there was a negligible differ- ence in depositions between the 2 m and 34 m positions. With PWM turned off, however, the nozzles in the 2 m position applied 50% more than those in the 34 m position.
Variable rates The potential problems with using PWM to vary rates include inaccurate dosing and gaps or misses caused by the nozzles being turned off for too long. A 25% dose requires a four times turn-down ratio. Trial results show that, when it comes to variable rate applications, PWM probably ‘needs improvement’, receiving the lowest score in the trial (3.5 stars). Tests looked at the differences between using the traditional method and PWM to vary the dose between 100%, 75%, 50% and 25%. “We tested its performance against the theo- retical mean – the deposition we would expect to see,” says Tom. “PWM didn’t exactly match
these, but was pretty close – it didn’t do too badly. Within +/- 10% at 75% and 50% doses and within +/- 20% at the extremes. “An in- triguing observation was how the frequency of PWM increased – you couldn’t see this, but you could easily hear that it was working faster. I compare this to a light bulb – which works at 50Hz – but you won’t see it turning on or off,” he notes. Crucially, the increased frequency filled in any gaps, and the targets were uniformly covered.
Operation “There’s no doubt that the more one under-
stands about the principles and limitations of PWM, the better the results,” says Tom. “ Crucially, with turn compensation, reducing the flow to the inner nozzles is relatively sim- ple, but the flow to the outer nozzle can never exceed 100%. This sets the maximum speed for any given radius of turn. The tighter the turn, the lower the maximum sprayer speed has to be. An audible warning as one approaches 100% output at the outside nozzle could prevent the operator underdosing the outside of a turn,” he suggests.
Spray deposits were collected on strips of ten 50 mm discs laid in the direction of travel – placed lengthways directly under a nozzle and 25cm away, in between two nozzles.
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