Any calibration procedure requires the identification of your constants. When you have a back pack sprayer there are hardly anything constant about them. The pressure is usually different as you are applying, and the speed at which your hand moves is rarely exact. So calibration is rough at best but you can still get pretty close with the following methods.
I have marked an area out in my maintenance yard for calibration purposes. I tried to find a big enough area that resembled 1000 square feet. I marked an area that had measurements of 50’X20’. I use this area to calibrate all of my hand equipment. It has come in handy over the years.
The first step for calibration is to make sure the equipment is clean of all chemicals and it is operating properly. A small rock in the nozzle can throw you off. Now that everything is clean and ready I use a measuring cup to fill the sprayer to an exact quantity. In my case I add the full capacity of three gallons. I don’t trust the line on the tank because sometimes they can be off by quite a bit.
Now that I have filled the sprayer I am ready to go over to the area that I have marked off to measure how much I will apply over 1000 square feet. The tough part is to make sure you try to make each pass as though you would in the field. It is also important to keep the pressure up in the tank so that you are not getting a poor application. Using the water you can see your pattern as you go and simply fill in the entire square. Once you have finished the square take the remaining water and measure it back out.
The amount that you measure is then subtracted from the beginning volume you measured into the tank to get you application rate over 1000 square feet. You can then do the simple math to see how much area a tank will cover. Once you know this number you can easily follow the label and add the proper amount of chemical to the tank.
Calibration is highly recommended especially when you are going to apply chemicals to shorter cut grass. I have seen many times when a “glub” will do, and they have polka dots on their tees from an improper rate of herbicide.