Calibrating Grain Drills

The traditional end wheel grain drill includes a hopper, a metering unit, seed tube and furrow opener for each row, and the metering unit drive train, Figure 8.4. Grain

Metering units - -Hopper

Metering units - -Hopper

Row spacing

FIGURE 8.4. End wheel grain drill.

Row spacing

FIGURE 8.4. End wheel grain drill.

drills also use bulk or volume metering. The rows are usually spaced 6-10 inches (15-25 cm) apart. Combining the row spacing and number of metering units is the traditional method for indicating the width of the drill. A drill identified as 13-6 would have 13 metering units spaced 6 inches apart or a width of 18 x6/12 = 9 ft. The calibration of grain drills is more critical than the calibration of fertilizer spreaders because drills dispense seeds. A small error in seeding rate can have a greater impact on the yield than an error in fertilizer application. In addition, it is more important that the seeds are planted uniformly. Grain drills can be calibrated stationary or mobile. The units cancellation method can be used in either situation. For both the stationary and mobile methods a container is attached to each metering unit, the drill is driven a measured distance (mobile), or the drill is jacked up and the drive wheel is turned (stationary) for a selected number of revolutions.

Problem: An 18-6 (18 metering units spaced 6 inches apart) end wheel drill is set to apply 1.0 bu/ac of wheat. The quantity of seeds collected during calibration is shown in Table 8.2. The diameter of the drive wheel is 26.0 inches, and the drive wheel was turned 25 revolutions. Is the drill planting the correct amount of seed (bu/ac)? (Note: Because each wheel of an end wheel grain drill powers half of the metering units, only nine units are shown.)

Solution: The first step is to determine the total weight of seed collected.

Wt = 0.10 + 0.10 + 0.12 + 0.10 + 0.11 + 0.11 + 0.11 + 0.15 + 0.13 = 1.03 lb

The next step is to determine the bushels per acre. To complete this step, the problem can be broken down into several steps or the units cancellation can be used in one continuous equation. A previous section explained that calibration is determining a volume of material for an area. Using this concept, the drill can be calibrated by determining the volume (bu) and the area (ac).

It is common to use a volume measure, bushels, to express the desirable seeding rate for a grain drill. This means that when the seeds are measured in pounds, a conversion from weight to volume must be made. The relationship between weight and volume is the specific weight (y) where:

where y = specific weight (weight/volume); W = weight (lb); V = volume (bu).

Table 8.2. Pounds of wheat collected from nine grain drill metering units.

Unit lb

123456789 0.10 0.10 0.12 0.10 0.11 0.11 0.11 0.15 0.13

60 lb

The standard specific weight of wheat is-. Therefore:

1 bu

1.03 lb

The area is:

43,560 ft2 V rev 12 in

6 in 1 ft x 9 unit x —- x unit 12 in 1 ac x 170 ft x 4.5 ft = 0.017561983 ac

43,560 ft2

The seeding rate in bushels per acre is:

/bu\ 0.0171667 bu bu

Vac/ 0.017561983 ac ac

In this example, breaking the problem down into parts requires less math ability than trying to determine the answer by using one equation. The basic equation is relatively simple:

/bu\ Vol (bu) nr (rev) R — =-:—- x ac / nr (rev) A (ac)

But when the values are included to arrive at bu/rev and rev/ac the math is more complicated. The first variable is determined by:

x nr (rev) y (lb) nr (rev) the second variable is determined by:

nr rev 1 rev

A (ac) / 1 ft \ / width 1 ft \ 1 ac n x dia x ——— x nu x ■ •

12 i^^ V" uni^ 12 i^ 43,560 ft2 where nu = number of metering units. Putting the two parts together produces: ' bu\ W (lb collected)

1 rev

The solution for the sample problem is: R ' bu \ 1.03 lb ac

1 rev x

1 ft \ / 6.0 in 1 ft \ 1 ac n x 26 x —— x 9 x -— x i-2

0.000687 bu 1 rev bu

rev 0.000703 ac ac

The desired planting rate is 1 bu/ac resulting in an error of 0.02 bu/ac. Is this acceptable for planting wheat? This is an example when perfection is not expected, therefore the level of acceptability must be established. Some drill manufacturers publish acceptable seeding rates for their products, or this information may be found in an extension bulletin. A standard has been established for grain drills. The seeding rate should be within plus or minus of 5% of the desired rate. When this method is used acceptable limits are set on each side of the desired rate. If the actual rate falls within this limit, it is acceptable. In this example the upper limit (Lu) is:

ac and the lower limit (Ll) is:

Using this standard, the accuracy of the grain drill is acceptable (1.05 > 0.98 > 0.95).

The calibration of the drill is not completed until the uniformity of distribution is also checked. This is accomplished by using the same rule and setting the limits around the mean amount of seeds collected from the metering units. This will give:

Wt 1.03 lb

The distribution upper limit is:

DLu = 0.114... + (0.114... x 0.05) = 0.1197 or 0.12 —

The distribution lower limit is:

DLi = 0.114... - (0.114... x 0.05) = 0.1083 or 0.108 —

A comparison of these limits to the calibration results shown in Table 8.2 indicates that although the grain drill seeding rate is acceptable, the distribution is not. The rate for metering unit #8 and #9 is above the upper limit. Both metering units should be repaired before the drill is used.

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