Missouri | Some variability is bound to happen, but what you described sounds excessive. When I run a combine, I know how many bushels my tank holds when the tank full buzzer sounds. I periodically compare the number of bushels counted in a binful. It's not precise, but I can tell if it gets off by more than 5%. Heavily loaded swaths have a tendency to under indicate. Low flow (that long last two row swath with six row head) usually tends to over indicate. Intermittent flow (lots of short rows in a terrace point or corner of field) usually under indicate, but in some situations I've seen them over indicate.
[excuse this paragraph. It's a mess, but I've written a long reply, and have lost interest in revising]
A yield monitor is not, and was never intended to be, a weigh scale replacement. It is convenient to look at the monitor total tab, and with a good calibration, that should be within a few percent of scale tickets, but if the total bushels is what you want to know, it is not a substitute for a scale. They don't weigh grain, they measure grain flow, and from that grain flow calculate an *estimated* yield volume. A yield monitor does not weigh or measure grain. It measures the force applied to the impact plate from the grain thrown at it as the paddles cross over the top of the clean grain elevator. From that force, it calculates a grain flow. From that grain flow, it further calculates an estimated yield volume. When you consider how all of this is done, it is really a wonder that they are as accurate as they are. If I can send you an email, I have two small, 6mb videos that illustrate this very well, and show some things you can look for when troubleshooting an issue like this.
I have been calibrating yield monitors for 10 years, and I learn something new every year. Achieving the perfect calibration is really an art form. Conditions will change, machine parts will wear, and eventually it will be necessary to perform a recalibration to restore accuracy to the changed conditions. The machine is always the most accurate in the field in which it was calibrated, and in the hybrid in which it was calibrated. So many things can affect it. Kernel/seed size, test weight, moisture, all affect the way the grain hits the impact plate, which affects the force measured by it, which in turn affects the grain flow calculated from that force, which in turn affects the yield volume calculated from that grain flow. I've seen a corn combine get set to thresh too tight, and get too much cob in the bin. Excess cob cushions the grain flow at the sensor, lowering the indicated grain flow. Readjusting the separator brings it back. The same thing can happen in beans with lots of pods in the grain flow. Mechanical issues can also have effects. As the elevator chain wears and stretches, that affects the way it throws the grain at the top of the elevator too. That's not the only issue. We had a straw chopper that was so badly out of balance you could feel the vibration in the cab. The year in which we repaired that issue, the yield monitor had half as much error as the year before. Even the way the machine vibrates affects the way the flow meter measures grain flow.
The real value in a yield monitor is not the total bushes on the summary screen. It is the map that shows you, proportionately, how many bushels came from where. Having an accurate calibration is important, but if it is off a few %, that does not substantially reduce the value of that data. Say you were wanting to divide your field into three zones, to identify the high, average, and low yielding thirds of the field. If the calibration is off a few percent, the numbers listed on the breakpoints for the legend of that map will be off, but the overall map will be almost unchanged. At least, that would be the case with a single machine harvesting a field. Having multiple combines in the same field does somewhat complicate that, and having as accurate a calibration as possible becomes more important in this case.
dpilot83 - 2/20/2013 15:05
So what's the problem? Are John Deere yield monitors not as accurate as Ag Leader monitors still even after they have Ag Leader sensors and an Ag Leader style calibration process? Is our harvest crew doing something wrong? Is it something else? Any thoughts would be much appreciated. Thanks.
I don't think the issue breaks down along brand lines. I have seen that kind of difference from one Ag Leader equipped JD combine to another. My Dad's combine has a two year old yield system on a 60 series Deere. With older systems, we got into a habit of calibrating annually, in each crop. In the second year, I calibrated his corn, and using the previous years calibration, it was only off about 2.5%. He never called me to calibrate beans. I asked him about it after the season, and he said he had compared the monitor to scale tickets, and it was too close to recalibrate.
Another customer, with another 60 series deere, had an issue we never figured out. On the day we calibrated, it was amazingly accurate. Day by day, as the season went on, it was under indicating more every day. He sold that combine before I ever got the chance to find the problem. I uninstalled his yield kit, and reinstalled it on a 70 series deere, and he never had a complaint on that combine. I'll never know what caused that issue.
Most of them work really well. There's always the exception that performs flawlessly, and there's always the exception that performs unsatisfactorily. The hard part is getting a customer with an unsatisfactory system to let go of his combine during harvest long enough for you to work on it.
I don't know how the deere machines manage their calibration, but with an Ag Leader system, you can always add in additional calibration loads to further dial in a calibration, at any time. It's a good idea to cut a test load anytime an operator has reason to suspect the accuracy is beginning to diminish.
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Reliability/Accuracy of yield monitors in S-series JD combines
