Harv:
One of my winter projects is to build a boom sprayer like you are looking for. I looked at the engine driven and electric pump types like TSC and Northern sell but decided against them. It cost us about $250 to have 3 acres sprayed for weeds this spring. That was the lowest bid of three. Being a glutton for punishment, I decided that I would try to build a sprayer that I could also spray our orchard with. I estimate that I can build it for $300 to $350 using the best (most expensive) pump I found. More on that later. The most expensive components are the pump and the tank. I managed to scrounge a pair of 35 gal plastic drums with threaded bungs, one for herbicide, the other for insecticide. Should be able to do about an acre at a fill. I plan to mount the drum on a platform that rests on the top of the 3ph lift links, and strap it in. The nozzles and tubing are readily available from TSC. These will probably end up mounted to a piece of 2x4 lumber. Not elegant, but functional. Haven't figured out yet how to mount the 2x4, but that's part of the fun.
Now, about pumps. These are PTO mounted roller pumps. TSC sells them, but I got my prices out of the Grainger catalog. A roller pump is a positive displacement pump, which means that you can't shut off the flow or you will blow out the weakest thing in the pressurized side of the system, or break the pump. There must always be a flow through the pump. This can be used to advantage as it provides a simple way to keep the solution in the tank moving should you be spraying something that requires agitation. Since these pumps can make 300 PSI at 4+ gallons per minute, it should be useable as a light duty pressure washer. There seem to be three grades of pump material. Plain cast iron ($105), NiCast alloy ($191), and SilverCast ($245). The claim is that you need at least the NiCast to withstand an acidic chemical like Roundup. That will be a concern weather you build or buy a PTO sprayer. Does anyone out there have an opinion on the merits of the pump materials for Harv and I? Does it matter for the amount of use that most of us will put on a pump?
One more detail. The nozzles on these sprayers are calibrated for pressure and speed. You can pretty simply adjust the pressure with a regulator, but if your tractor is hydro, the speed thing becomes complicated. I will either use a magnetic pickup bicycle speedometer (about $20) adapted to the tractor, create something electronic that reads the axle rotation, or use the TLAR (that looks about right) method.
Craig
Hershey, PA