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20
Calibration Deviations
Deviations Between the Calibration Test and the Actual Seed Rate
The most frequent cause for changes between the calibration test and the seed rate lies in the flowing
properties of seed during sowing. These changes in properties generally result from reactions of the
dressing agents to temperature, humidity or abrasion. These changes will become even more obvious
when the bottom flaps are incorrectly set. If the setting of these flaps leaves too large a gap an
uncontrollable additional flow of seed can occur during seeding; especially when assisted by the drill
bouncing, a condition not simulated while conducting the calibration tests. For this reason the basic setting
of the bottom flaps should be checked at regular intervals.
Residues from the seed dressing on the bottom flaps and metering wheels can also influence the flowing
properties of the seed and thus the seed rate. In such cases a balance will occur only after a period of
time and it is recommended to repeat the calibration test to confirm the seed rate after 2-3 seedbox fillings,
nominally when the seed box is half empty.
Only then will a balance occur and the seed rate will stabilise.
Use of Seed Rate Calculator
Determining the gear box scale setting
using the calculator.
Usually the first calibration test yields a
different seed rate. However with the
value determined from the first test it is
possible to determine the correct
gearbox setting with the aid of the
enclosed disc calculator, Fig 8. The
disc calculator consists of 3 scales.
An outer white scale (1) for all seed
rates above 30 kg/ha and an inner
scale (2) for all seed rates below
30 kg/ha. On the middle coloured
scale (3) are all the gearbox setting
numbers to a maximum value of
100.
Setting Example (Desired
seed Rate 125kg/ha)
1 From the calibration procedure at a
gearbox lever setting of ‘70’, a seed rate of
175 kg/ha is obtained.
2 Turn the inner disc until the measured seed rate of
175kg/ha (A) is in line with the related actual gearbox setting
of ‘70’ (B) (Fig 8).
3 Read off from the disc rule the necessary gearbox setting for the required seed rate of 125kg/ha (C).
In this example the correct setting is ‘50’ (D) (Fig 8).
4 To be on the safe side the new gearbox setting can be checked by another calibration test.
Fig 8