I went to the Rainbird site and read the specs. Here's a cut n paste from their site.
* These ranges are based on proper pressure at nozzle. 12-, 15- and 18-VAN have matched precipitation rates with Rain Bird MPR nozzles.
** Rain Bird recommends using
1800 PRS spray heads to maintain optimum nozzle performance in higher pressure situations.
www.sprinklerwarehouse.com/Rain-Bird-1800-Series-Sprinkler-Spray-p/1804-prs.htm
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So basically what they're saying is, if you have more than 30 lbs working pressure they recommend you use the PRS spray head which regulates the pressure at the nozzle to 30 lbs.
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Do you have 60 pounds static or working pressure?
Let me just say if you have 60 lbs of pressure at the nozzle the water will just mist and fly away in the wind.
I myself try to avoid the VANs. I'll use a regular spray nozzle whenever I can. I find the VANs are unpredictable how the ends of the arc spray plus they break over time.