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Monday, June 26th 2006, 4:14pm

Question about Rotor vs Spray

I am planning out an irrigation system, and I am considering using the Hunter Pro-Spray Nozzles and PGJ Rotors but I have a question.

I live on a cul-de-sac so my yard is not at all rectangular. Curved edges, as well as odd shaped zones abound.

Most of my zones will require spray nozzles but the main area in front of the house is large enough that I could possibly use PGJ rotors. My concern is this. According to Hunters documentation, the fixed spray nozzles are calibrated to roughly the same percipitation rate for a given nozzle radius regardless of the arc angle being used (Quarter, Half or Full) This is a good thing. The problem I see is when using the rotors. According to the calalog specs, the rotors are not calibrated as such. Using a rotor in a quarter circle arc configuration according to Hunter will yield 4 times the percipitation rate as operating in a full circle configuration. Like wise a half circle will yield twice the percipitation rate as a full circle. The way I see it, the corners where the quarter arcs and edges where half circles are used will get substantially more water than the centers where a full circle is used.

I tend to like rotors. I think they look good, and I like the way they operate but I am concerned that the coverage may not be as uniform as what I can get with spray nozzles. I am sure that for a larger area, it would be a no brainer but in a medium sized area where the corner and edge rotors outnumber the center rotors I am a little concerned about uniformity.

I have noticed other lawns in the neighborhood where the center of the lawn is not as green and lush as the corners.

This is my first attempt at installing an irrigation system. Am I being paranoid or is this a legitimate concern?


Active Member

Posts: 7

Location: USA


Wednesday, June 28th 2006, 5:17am

That is why Hunter sends you 12 different nozzles to go in each rotor that you buy. These nozzles all have different percipitation rates. So if you have a full cirle head lets say you use the 4 gpm nozzle. So that means that your half circle heads will get 2 gpm nozzles and your quarters will get 1 gpm. This is called matched percipitation rate! The quarter circle makes 4 cycles every time the full circle makes 1 but it is putting a quarter of the water down so that they will both be the same.Hope this helps.


Supreme Member


Wednesday, June 28th 2006, 12:59pm

the problem arises (which bubblehead may have been aluding too) is that if you try to match the nozzles the distances increase dramitically- making it difficult to adjust the full circle down to the same distance as a quarter nozzle. You'll actually end up adjusting the full circle down so far as to affect the coverage.

The best way to go is to put your quarters and halfs on one zone and your fulls on another separate zone.

The rainbird 5000 series offers mpr nozzles that address this problem.


Wednesday, June 28th 2006, 1:59pm

In theory, Jabbo is correct but Tom knows exactly what I am talking about. You can't always adjust the nozzles to get the proper precipitation rate.

The Hunter manual lists precipitation rates (P.R) for each nozzle at 180 degrees (half arc) however we can calculate what the P.R. will be for any configuration using the following equation:


Using this equation we can calculate the following:

Source code

GPM   Radius(ft) %Adjustment  90 degree arc   180 degree arc   360 degree arc
---   ---------- -----------  -------------   --------------   --------------
.75      16         <font color="red"> 38%           0.5966</font id="red">        0.2983            0.1491
1.0      19          16%           0.7955        0.3977            0.1987
1.5      22           0%           1.1932        <b>0.5966</b>           0.2983
2.0      25          -12%          1.5909        0.7955            0.3977
2.5      28          -21%          1.9886        0.9943            0.4972

As Jabbo said, matching the GPM will effectively match the P.R. The above table shows the standard P.R for the 1.5gpm nozzle in a half arc configuration in bold. If I wanted to match the P.R. in a quarter arc, I would choose the nozzle with half the flow and adjust it to the same radius as the one I am using in a half arc configuration. In this case the .75-16ft nozzle will match the 1.5gpm-22 foot nozzle if stretched to 22 foot. If you look at the P.R in the first row under 90 degree arc, you will see it matches the value bolded under the 180 degree arc for the 1.5 nozzle. The problem is that the .75 gpm nozzle would have to be at 38% its design radius in order to achieve this. According to Hunter, they are adjustable to 25% of their rated radius. I don't know if 38% is even possible.

Placing them in separate zones sounds like a viable alternative, but choosing a different nozzle and adjusting the flow rate I think would be pushing it.

Thanks to you both for your input.

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