These are REALLY important questions, so I'll go on for awhile. First on the sprayer, and the nozzle tips on hand sprayers may be one of the most overlooked items in our tool box. They get clogged and worn as time goes on and eventually the spray volume and pattern that you THINK are coming out of them no longer are. I once had a technician complaining that the flea product he used was no good and was simply not controlling fleas. So, I watched him apply it one time and realized that instead of the even fan pattern he "thought" was being applied to the carpet, his clogged and worn spray tip was putting out two widely spaced pin streams. Probably 95% of the carpet wasn't even being treated because he failed to check his equipment and to test it now and then to be certain it was in good working condition.
Spray tips are designed to put out the proper GPM and spray pattern when the pressure in the tank is a specific PSI. When the pressure is not the ideal pressure for that sprayer and its parts you may be fracturing the droplets into too fine a mist or allowing it to dribble out and miss the proper pattern for that application. B&G Equipment Co. has an excellent article on this topic on their website, and I think I'd call it "must" reading for everyone. We can't use spray tips as hammers or pry bars and expect them to perform properly forever. On this article they state that the B&G sprayer and 4-way tip work best at about 20 psi, but human nature is to want to have to pump that darned tank up as few times as possible during a job. And, we get some feeling of satisfaction hearing the spray pounding onto the surface. So, our tendency is to pump it initially until we absolutely cannot get another pump done, and then to spray until the liquid is barely coming out any more. The reality is that we probably start at 100 psi and go until it's below 10 psi.
This is the advantage that a pressure gauge provides - knowing exactly what the pressure in the tank is so you can keep it within the range that allows the equipment to perform as it is designed to, as well as to keep you within the legal requirements on many product Labels. B&G and others have tried over the decades to develop an effective and reasonably priced "electric" pump hand sprayer that maintains the tank pressure at the optimal level. Their Portable Aerosol System does this today, but only for void injection or crack and crevice applications.
A surface application, for fleas for example, is done best when all of the spray droplets land on the surface you are aiming at. When fine droplets ("mist") are created it is likely they will float off to someplace else, perhaps even onto surfaces that should not be treated. When the pressure is optimal and the spray tip is in perfect condition that "fan" pattern should be ejecting equal amounts of liquid from one side of that fan to the other, and you would not want stray droplets going to other places. Keeping the pressure where it is supposed to be helps to eliminate that "bouncing" effect, also known as "off site" application. For exterior uses the same rules really do apply. Pumping up the sprayer to an unreasonably high pressure only defeats the work of the spray nozzle, although it does give us the satisfaction of less pumping and faster wetting of the surface. Here is where it may be more appropriate to switch to one of the backpack sprayers instead of a small hand tank.
For granular applications, just as with liquids and with fogging, it is critical to know exactly how much material you are applying to any given surface area or volume. I'm willing to bet that 95% of technicians in our industry do not calibrate their granular applications. If you don't really know how much you are applying over that surface you easily could be under or over dosing, one that leads to ineffective control and one that leads to a fine for misuse. Perhaps the best way to calibrate is to accept that YOU turn the handle on the spreader at a rate "normal" for you and that you walk at a speed normal for you. So, mark off a 1000 square foot area, place a measured amount of granules in your spreader, and then treat that 1000 square feet in the normal manner for you. Then measure the weight of the material still in the spreader and the difference is the amount that YOU apply over 1000 square feet of surface. This also relies on using a consistent opening on that spreader if you have one that is adjustable.
I know this is always going to be somewhat of an estimate, as we really don't expect anyone to use a measuring tape to mark off 1000 square foot plots around the yard, but you should get a feel for what that area looks like so your application is reasonably within the Label allowances. Using your suggestion would work too, but instead of trying to make that 8 ounces evenly cover that 100 feet, and in so doing perhaps try to change your own normal application style, just find out how much you do use over 100 square feet and then you will know for other uses how much to put in the spreader.
Mr. Pest Control
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Please note, Mr. Pest Control is answering questions supplied by PMP customers across North America. His answers are generated from industry and manufacturer-provided information. The answer may not be specific to the laws and regulations for your State, Province, Territory or Country. In addition, products mentioned may not be registered and or available in all areas. Always check with your local Univar office for specific information to your area. Always read and follow label directions.