Principles of Calibration

In Chapter 1, two characteristic of mathematical problem where discussed. Both of these must be applied when calibrating dispensing machines. It would be a rare occurrence for a dispensing machine to distribute the desired amount of material. In this case perfection is not expected. Therefore levels of acceptability must be established. The amount of error that would be considered acceptable varies for each machine and situation. In some cases guidelines or standards have been established, but in others it is up to the owner/operator to determine how much error will be accepted. Standards of acceptability for each machine will be discussed in more detail for each machine.

All of the methods used to calibrate agricultural machines are based on three principles: (1) all dispensing machines meter (control the rate) the flow of material at a predetermined rate selected by the operator, and (2) calibration occurs by collecting material dispensed by the machine in units of volume, weigh or mass, or number of granules or seeds per unit area. For example, to calibrate a row crop planter it is necessary to determine the seeds per acre or seeds per hectare. To calibrate a sprayer, the application rate is determined in units of gallons per acre or liters per hectare. Seeding rate of certain grasses is in bushels per acre or liters per hectare. Lime is applied in tons per acre or metric tons per hectare. The unit of area used during calibration will usually be an acre or hectare or some fraction of these. (3) There are two methods for collecting the material, stationary or mobile. With the stationary method, the wheel that drives the machine metering unit must be elevated so it doesn't contact the surface and then it is turned for a fixed number of revolutions. In the mobile calibration method, the flow of material is interrupted into a container as the machine is pulled through the field. The advantage of the stationary method is that the machine can be calibrated in off productive time. The mobile method is best for machines that have a large number of metering points. The mobile method may be the only option when methods other than a ground driven wheel are used to drive the metering mechanism. It is important to remember that the larger the area used during calibration, the greater the accuracy of the calibration procedure.

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