• slide

Separating H-SELECT and Pulse Width Modulation

The two nozzle control technologies couldn't be more different

Productivity is always front of mind for farmers and the rise of big self-propelled sprayers and their even bigger booms certainly helps you get more hectares sprayed per hour.

However large booms must be able to compensate for the speed changes in each spray run, as well as along the boom during turns.

Without this accurate nozzle control, speed variations can cause patches of over- or under-dosage which will impact the crop yield in those areas.

Two of the most advanced nozzle control technologies currently available to Australian operators are Pulse Width Modulation (PWM) and HARDI H-SELECT.

It’s a fairly common misunderstanding that H-SELECT is just HARDI’s PWM offering. But the two systems are actually very different.

A quick definition

Pulse Width Modulation works by switching nozzles on and off several times 

“PWM works by switching nozzles on and off, while H-SELECT works by switching between nozzles”

per second. This reduces the amount of fluid leaving the nozzle, which maintains the target Set Rate when the boom is moving slowly – on the inside of a turn, for example.

On the other hand, H-SELECT is a Multi-Step technology, which manages the application rate by switching between nozzles while keeping the fluid flow constant.

The deeper dive on PWM

PWM systems are defined by two factors: Pulsing frequency and duty cycle. The pulses are driven by an electric solenoid with a fixed frequency. If the controller opens the nozzle for 80% of those pulses to meet the set rate, that’s a duty cycle of 80%.

It stands to reason that the maximum speed a PWM system can manage equals its 100% duty cycle – that is, when the nozzles are open the whole time.

Because speeds across the boom vary whenever the sprayer makes a turn – the outer tip of a 48.5m boom moves roughly twice as fast as the inner tip when turning – best practice is to set the spraying speed for about an 80% duty cycle.

On a 15Hz system an 80% duty cycle equates to 12 ‘opens’ per second (15 x 0.8 = 12) and, at 20 km/h, a nozzle will travel 370mm per 1/12th of a second. So on an 80%, 15Hz duty cycle it will spray for 300mm and shut off for the other 70mm.

One way to close that gap between pulses (and extend the working speed range) is a ‘blended’ system. This is basically two PWM installations with the nozzles physically offset and electronically out of phase. The two cycles then overlap or ‘blend’ the nozzle output in terms of both duty cycle and ground coverage.

How is H-SELECT different?

Multi-Step systems like H-SELECT combine up to four nozzles per nozzle body to maintain a set rate as the boom speed changes. The nozzles on each nozzle body will be selected in whatever order and combination best serves the forward speed of the boom at that moment.

This selection is constantly being compared to the ground speed and angular speed of the boom, and adjusted instantaneously with micro-second blending to avoid coverage gaps.

Up to 16 possible combinations make four nozzle systems accurate over a very broad speed range – from starting out to long, fast runs in broadacre paddocks. You could think of them as ‘tiered spraying on steroids’ with up 16 tiers instead of the usual three.

In fact, H-SELECT will work with virtually any combination of speed and boom width. It’s already working successfully on 54 metre HARDI PARAGON Aluminium booms.

Changing nozzles rather than pressure or flow rates preserves the quality of the spray application. It also means pressure adjustment can be used to switch between Very Fine, Fine, Medium, Coarse and Very Coarse droplets while on the move.

Not all Multi-Step systems use four nozzles per nozzle body. Operators may also use their four nozzle system as a three (with 12 possible combinations), saving the fourth position for a special purpose nozzle such as a fertilizer jet.

That said, four nozzles appears to give the best speed range and overall rate accuracy.

Making the choice

PWM’s on-off switching makes it unreliable with very coarse droplets and unsuitable for air induction nozzles. A continual flow Multi-Step system doesn’t have either of those problems.

The sheer number off possible configurations does mean a Multi-Step system needs careful planning – something HARDI addresses with the Nozzle Watch Windows® PC application. This software can recommend an optimum nozzle selection for any speed range or calculate performance for a specific nozzle set – and predict rate accuracy for the whole spray job.

Whether you prefer PWM or a Multi-Step system like H-SELECT can depend on your sprayer setup, boom width and spraying speeds.

But at least now you can tell them apart!


1. Gordon, Bill. Spray Application Manual for Grain Growers, Module 16, Overview of the spraying systems available, GRDC, Undated.

2. Campbell, Bill & Gordon, Bill.  Pulse Width Sprayers: What we have learnt, correct operation and looking ahead, GRDC, April 2019.