Calibration Of Professional Turf Spreaders

Roberto N. Barbosa, Parish, Richard L.  |  11/30/2007 7:17:48 PM

Determination of Effective Swath Width

Before you can calibrate any spreader, you must know the effective swath width. The effective swath width is the greatest distance between the centerlines of successive passes that will yield an acceptable distribution pattern. With some spreaders, more than one width will give an acceptable pattern, but you will get the best operating efficiency if you choose the widest swath width that is acceptable. For broadcast spreaders, this will never be the total swath width.

With a drop spreader, the effective swath width is the width of the hopper (at the bottom). With rotary or pendulum spreaders, determining the effective swath width is not as easy. Even knowing the total throw width is of little value, since some spreaders may have an effective swath width of 70-85% of the total throw width, while other spreaders may require an effective width of 20% (or less) of the total throw width.

The only way to determine the effective swath width is to conduct a pattern test – and this is not something that the typical operator wants to do. If you buy your granular products from a reputable, major supplier of granular turf products and use a quality professional spreader, this step will be done for you. You can use the effective width recommended on the product label for your brand of spreader. If you do not have this information available to you on the label, only a pattern test will tell you what the effective swath will be. Be aware that not all label recommendations are valid. Some have no basis in fact and are merely guesses. If you don’t buy from a quality major granular products supplier and use a quality professional spreader, there is a good chance that the label recommendation will be incorrect.

Pattern Test Procedure

If you do not have access to information on the effective swath width for your spreader and product, you will need to conduct a pattern test. The test should be conducted on a flat site with no wind. Short turf will give you the best result but may be damaged by overapplication during the test, so most people conduct the tests on pavement and then sweep up the granules after the test. Conducting the test on pavement will allow granules to bounce into the trays, thus distorting your results somewhat.

You will need at least 15-20 shallow pans or boxes (no more than 2 inches deep). Ideally, the trays should be subdivided (“egg crate” partitions) to reduce bounce out of the trays. These collection trays need to be fairly large to collect a meaningful sample of granules. One foot square is probably the minimum size, but larger pans can be used. Flat cardboard boxes, cake pans, etc. can be used. The pans should be spaced out on a line perpendicular to the direction of travel of the spreader. For professional walk-behind spreaders, use a 1-foot tray spacing. For tractor spreaders, a 2-foot spacing is adequate. You will need to remove the trays where the spreader or tractor tires will pass.

To conduct the test, make several passes (in the same direction) over the trays at the speed to be used for the actual application (3 mph is standard for professional walk-behind spreaders and 4.5 mph for tractor turf spreaders). Be sure to open the spreader well before reaching the trays and not close it until you are well past. Afterwards, collect the material from each pan and measure it. You can weigh it or measure it volumetrically if you have enough granules.

Once the material in the trays is measured, the data must be analyzed. The easiest way of analyzing the data is to plot a graph of the pattern on graph paper. This allows you to visualize the pattern, and you can readily detect pattern problems such as skewing, peaks in the pattern and light areas in the pattern. Given the information from a graph, you can then adjust the pattern settings on the spreader and make another pattern test, repeating this procedure until you obtain the best pattern possible with your spreader and product.

A graph of the spreader pattern allows a rough estimate of the effective swath width. This method of determining swath width is not as effective as calculating an overlapped swath width but is better than nothing. To estimate the swath width from a graph, you should draw a horizontal line through the graph about where you estimate the average rate (of the center section) to be (see Figure 1). Now add a second horizontal line at half that rate. Draw vertical lines down from the points where the half rate line intersects the pattern edges. The distance between the two vertical lines is the approximate effective swath width. The vertical lines represent the overlap points from adjacent patterns at the selected swath width. If the pattern is skewed, one vertical line may be further from the center than the other.

The LSU AgCenter offers a computer program, SPREADER.EZ, to help you analyze your pattern more accurately.

Rate Calibration

Rate calibration is easy with most spreaders and involves no complicated mathematics. Although there are as many calibration methods as there are people recommending them, the method recommended here is the easiest to use. In the most general format, all the operator has to do is find out how long it takes (in seconds) for the spreader to cover a known area (typically 1,000 square feet), collect and weigh the material discharged for that length of time at a given setting, and this will give the rate in pounds per 1,000 square feet. The operator can then adjust the setting and try again until the desired delivery rate is achieved. Note that this method does not require any calculations of speed, flow rate, etc.

Perhaps the easiest to calibrate is a pendulum spreader. The spout can typically be removed by loosening two bolts. Once this is done, the output can be caught in a bucket, bag or washtub. If using a bucket, it is easier to set the bucket on the ground under the machine and hold a second bucket with a hole in the bottom over the oscillating spreader opening so that the flying granules are caught by the second bucket and funneled down to the main bucket. Some pendulum spreader manufacturers sell bags for calibration. It is essential that the PTO be engaged and the spreader be operating when the test is conducted; the delivery rate will be much less at a given setting if the spreader is not operating. Be very careful working around the spreader with the PTO engaged. The PTO shaft itself is dangerous, and so is the moving spout/yoke mechanism.

Most drop spreaders can be calibrated by hanging a pan or tray under the shut-off bar and then pushing the spreader for a predetermined distance while catching the output. A piece of rain gutter with the ends plugged will work. So will a piece of PVC pipe cut in half lengthwise. When using this method it is very important that the pan not hang on or even touch the shut-off bar because this can not only affect the test results but can permanently change the calibration of the spreader.

With most rotary spreaders, it is more difficult to catch the output. In some cases, the impeller can be removed, allowing the material to drop into a container. In most cases, it is possible to fasten a box, bag or tarp around the impeller to catch the product. If these methods are not practical on a given spreader, it may be necessary to put a weighed amount of product in the spreader, push the spreader for the recommended time (based on width) and spread while making the pass, then weigh the amount remaining. The difference between the two weights divided by the area covered will be the application rate. The spreader must be operating (i.e. PTO-engaged on tractor spreaders) during the test.

Calibration boxes are available from some suppliers (Figure 2). A calibration box can also be made from a cardboard box (Figure 3). When using one of these boxes, the impeller is left in place. You put material in the hopper, make a guess at a setting, push the spreader in a straight line with the ports open for the recommended number of seconds, and weigh the granules caught in the box. The weight in pounds will be the delivery rate in pounds per 1,000 square feet.

The following table will give you the number of seconds required to cover 1,000 square feet at 3.0 mph with a professional walk-behind spreader.

Effective swath width   

Seconds to cover 1,000 sq. ft.

2.0

114

2.5

91

3.0

76

3.5

65

4.0

57

5.0

45

6.0

38

7.0

32

8.0

28

9.0

25

10.0

23

11.0

21

12.0

19

13.0

17

14.0

16

15.0

15


The following table will give you the number of seconds required to cover 10,000 square feet at 4.5 mph for tractor spreaders. Note that in this case, you will have to move the decimal point one digit left (to divide by 10) to get results in pounds per 1,000 square feet.

Effective swath width

Seconds to cover 10,000 sq. ft.

10

152

11

138

12

126

13

117

14

108

15

101

16

95

17

89

18

84

19

80

20

76

21

72

22

69

23

66

24

63

25

61

26

58

27

56

28

54

29

52

30

51

31

49

32

47

33

46

34

45

35

43

36

42

37

41

38

40

39

39

40

38


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