Calibrating Turfgrass Chemical Application Equipment

Bulletin 817-00

Checking the Delivery Rate

"Delivery rate" refers to the average amount of product applied over a known area, and it usually is expressed in pounds per 1,000 square feet. The easiest way for an operator to check the actual output rate is to collect and weigh the granules actually applied to a known area. You can do this by following one of the two methods explained on pages 19-22. Both methods are acceptable, but the first one produces a more precise calibration.

Fig. 13. A tarp is used to determine the spreader application rate.

1. Spread out a plastic tarp of known size on the ground (Fig. 13).
   
   
2. Operate the spreader at a known speed across the tarp.
   
   
3. Determine the area covered. If the swath width is wider than the tarp (Fig. 14), the area covered is equal to the area of the tarp. If the swath is narrower than the tarp (Fig. 15), multiply the swath width by the tarp length to determine the area covered. It is highly recommended you keep the tarp width wider than the swath width for a more accurate measurement of the delivery rate.
   
   
4. Place all the granules on the tarp into a container and weigh them in ounces or pounds.
   
   
5. Divide the weight of granules collected by the area covered to determine the actual delivery rate in ounces per square feet or pounds per square feet.

   You may have to do some calculations to express the delivery rate in terms of lb/1,000 sq ft or lb/Acre. Use following equations when converting units.

   oz/sq ft	= lb/sq ft
   16

   lb/sq ft x 1,000 = lb/1,000 sq ft

   lb/1,000 sq ft x 43.565 = lb/Acre

lb/sq ft x 43,565 = lb/Acre

6. Compare the measured (actual) delivery rate with the rate recommended on the label. Make proper adjustments and recalibrate the spreader until you reach the desired accuracy (a variation within ± 5 percent of the label recommendation is acceptable).

Fig. 14. Area covered = tarp width x tarp length.		Fig. 15. Area covered = swath width x tarp length.

Example:

You are calibrating a drop-type spreader with a swath width of 2 feet. You spread a 3-foot-by-6-foot piece of tarp on the ground. Then you filled the spreader hopper with granular fertilizer and traveled through the entire 6-foot length of the tarp at your normal application speed. Next, you collected all the granules on the tarp and weighed them. They weighed 2 ounces. What is the actual delivery rate of the spreader?

Fig. 16. Determining the spreader delivery rate.

1. Determine area covered

   Area covered = Swath width x travel distance marked

   Swath width = 2 ft
   Travel distance = 6 ft
   Area covered = 2 ft x 6 ft = 12 square feet

2. Determine delivery rate in ounces per square feet
   
   

   Delivery rate	=	weight of granules
   		area covered
   	=	2 oz
   		12 sq ft
   	=	0.166 oz/sq ft

   
   
3. Determine the delivery rate in lb/1,000 sq ft
   
   

   0.166 oz	=	1 lb	x 1,000 = 10.4 lb/1,000 sq ft
   sq ft		16 oz

   
   
4. Determine the delivery rate in lb/Acre
   
   

   10.4 lb	x	43,560 sq ft	= 453 lb/Acre
   1,000 sq ft		Acre

   In this example, the spreader is broadcasting 10.4 pounds of fertilizer per 1,000 square feet or 453 pounds per acre. Let’s assume the label on the 40-pound bag specifies a total coverage of 5,000 square feet. What is the application error?

   Rate recommended	=	40 lb/5,000 sq ft
   	=	8 lb/1,000 sq ft
   Actual rate	=	10.4 lb/1,000 sq ft
   Percent error	=	10.4 - 8	x 100 = 30%
   		8

   This spreader is overapplying fertilizer by 30 percent. Reduce the size of openings at the bottom of the hopper and recalibrate the spreader until your error is less than 5 percent.

Method II

The second method is not as precise as the first one, but it is preferred by many operators because it does not require a tarp. However, the principle is the same. You are trying to determine the amount of product applied over a known area. For calibrating, this area should be at least 500 sq ft for drop spreaders and 1,000 sq ft for rotary spreaders. The following table shows distance you need to travel to cover an area equal to 500 or 1,000 square feet for different swath widths.

Once you set up the known area that will be covered, follow the steps below to complete calibration.

1. Fill the spreader to a known level marked on the inside of the hopper (Fig. 17).
2. Operate the spreader over the known area already set up.
3. Weigh the amount of material needed to refill the spreader to the mark (Fig. 18). Record this weight in pounds.
4. Pound of product actually applied per square feet can be calculated as follows:

   Amount needed to refill (lb)	= Product applied (lb/sq ft)
   Area covered (sq ft)

If you need to convert this calculation to other units, use similar calculations explained in the previous method.