Calibrating Yield Monitors Important

By Jane Fyksen Crops Editor
Thursday, September 11, 2008 6:23 AM CDT

Grain yield monitors provide valuable information to growers on the fly, not to mention offer a “visual diversion from the boredom of harvest,” says Purdue University agronomist Bob Nielsen. However, it’s “garbage in, garbage out” if they’re not properly calibrated, he states.

Monitors provide historical yield records potentially more detailed than elevator weigh tickets. They’re an alternative to weigh wagons or farm scales for measuring yields in on-farm research trials, too. When connected to a DGPS receiver, they generate a source of geo-referenced yield data that lets growers see yield variability within fields, he details.

Common uses for yield monitor data include: comparing one field to another, one specific spot in a field to another, one hybrid's performance to another, early versus late harvest season, and experimental treatments in on-farm field trials.

Most yield monitors operate similarly. Typically, a grain flow impact sensor is at the top of the clean grain elevator. Grain hits the impact sensor on its way to the loading auger. The impact of the grain flow is translated to electrical signals by the sensor. The electrical signal data are translated to estimates of grain flow rate by the yield monitor's internal software. If equipped with a DGPS receiver, the yield monitor matches the individual yield estimate data points to geographic locations in the field, he explains.

Yield estimates on a whole field or individual load basis by a well-calibrated yield monitor are accurate and very closely match yield estimates calculated from weigh wagons or commercial weigh scales. However, Nielsen says they must be "trained" to correctly interpret the electrical signals generated by the impact sensor into estimates of grain flow rate. He encourages growers to “faithfully and regularly” calibrate their yield monitors.

Calibrating a yield monitor simply requires harvesting individual “loads” of grain that represent a range of grain flow rates (or a range of yield levels) expected in the fields you’re going to harvest. Each calibration “load” requires 3,000 to 6,000 pounds (50 to 100 bushels) of grain depending on manufacturer recommendations for the specific model and make of monitor. The grain weight of each “load” is estimated by the yield monitor as the grain is harvested. Grain for that specific "load" is then offloaded from the combine hopper and weighed on weigh wagon or commercial scale. The actual weight is then entered into the yield monitor console, and the yield monitor makes adjustments to curve.

“Conceptually, the calibration process is about fitting a response curve between grain flow rate and flow sensor signal strength in order to estimate low, medium and high yields. Makes of monitors appear to differ in the nature of the calibration curve that is determined,” Nielsen explains.

Some manufacturers suggest only one grain load is necessary for calibration, which he says implies calibration response curve is a straight-line or near-linear relationship between grain flow rates and flow sensor signals. “While the standard recommendation is for only one grain load, the ‘fine print’ in the owners' manual suggests that additional calibration loads may be added to fine-tune the accuracy when necessary,” he cautions.

Other manufacturers recommend three to six grain loads, suggesting the calibration response curve for their yield monitors isn’t a straight-line, but rather some sort of non-linear response curve that requires a number of calibration points to best "train" the monitor how to interpret the flow sensor signals.

The goal, says Nielsen, is to "capture" the full range of grain flow rates (yield levels) you expect to encounter during harvest. “Capturing a range of grain flow rates can be a nuisance because it typically requires harvesting individual full-header-width ‘loads’ at different speeds or partial-header-width ‘loads’ at a constant speed. This headache plus the time it takes to off-load and weigh the individual grain loads are among the most common reasons why growers do not faithfully calibrate their yield monitors,” he admits.

“Yield monitor accuracy can be excellent if well-calibrated. Yield estimates by calibrated yield monitors that I use in my field-scale research trials are typically within 1 percent of the actual grain weight measured with a weigh wagon or farm scales,” he discusses.

“Conversely, yield estimates can be very poor if yield monitors are not well-calibrated,” he states. “The error in accuracy can be as much as 100 percent if the yield monitor is taken ‘off the shelf’ and put into service without any calibration.”

“Errors in accuracy can easily range as high as 7 to 10 percent late in the harvest season if the yield monitor was calibrated only at the beginning of the harvest season,” Nielsen continues, adding that errors in yield estimates are especially likely if the full anticipated range of harvested grain flow rates aren’t included in the calibration of the yield monitor, too.

The accuracy of your calibration can be influenced by yield levels outside the range of grain flow rates used for calibration, by seasonal changes in temperature, by grain moisture content early versus late in the season, by hybrids in terms of their differences for grain weight, grain shape, and grain moisture, and by field topography.

“Calibrating your yield monitor once a season will typically not be satisfactory,” Nielsen stresses. Check the accuracy of the calibration occasionally by harvesting and weighing additional calibration loads.

And don’t forget to: calibrate your combine’s grain moisture sensor; calibrate for the zero-flow combine vibration; re-read the yield monitor operations manual prior to harvest; create a pre-season and in-season yield monitor checklist of all adjustments and settings; and go through the yield monitor checklist every morning before beginning the day’s harvest.

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