Two sensor technologies. Very different behaviour in the field. Here's the accuracy comparison that actually separates them — and why it matters for your harvest data.
See the Green Growth Optical KitAn impact plate sensor is mounted on the clean grain elevator. As harvested grain flows up and strikes a hinged or fixed metal deflector plate, the force of impact is measured — typically by a load cell or strain gauge behind the plate. More grain hitting harder means a higher flow rate.
Impact plate technology has been the dominant method in factory-installed yield monitoring systems for decades. John Deere, CLAAS, New Holland, and Case IH all use variations of this approach in their OEM equipment.
The physics are straightforward — but the measurement is inherently sensitive to anything that changes how grain hits the plate. That creates a calibration challenge.
An optical yield monitor places a sensor across the path of grain flow in the elevator. An infrared or visible light emitter projects a beam, and a receiver on the opposite side measures how much of that beam is being interrupted by passing grain. Greater interruption means higher flow.
Because the measurement is contactless — the sensor never physically touches the grain — there is no plate to wear, no mechanical drift, and no sensitivity to how hard or softly grain hits a surface.
The Green Growth yield monitor uses optical sensor technology. It mounts directly on the elevator housing and begins measuring right away; calibration is handled remotely in software, with no in-field load-wagon runs.
Measures force of grain striking a deflector plate.
Measures volume of grain interrupting a light beam.
| Feature | Impact Plate | Optical (Green Growth) |
|---|---|---|
| Measurement principle | Force of grain impact | Light beam interruption |
| Affected by grain moisture | Yes — requires recalibration | No |
| Mechanical wear over time | Yes — plate surface degrades | No moving parts |
| Calibration during harvest | Required — load wagon runs | Remote — no load wagon runs |
| Consistent across crop conditions | Varies with conditions | Yes |
| Compatible with any combine brand | Typically OEM-specific | Any brand, any model |
| Works on older machines | Rarely available as retrofit | Yes — retrofit kit |
| Installation | Factory fitted | ~2 hours, DIY |
| Data ownership | OEM platform, often locked | Farmer owns data, full export |
The core practical issue with impact plate systems is that their accuracy depends on calibration being current. A calibration done at the start of the day may not reflect conditions by the afternoon if the crop has dried significantly, or at a different point in the field with different variety or moisture.
Calibration requires a load wagon run: harvesting a known area and weighing the output, then adjusting the sensor offset. Doing this properly at the intervals needed to maintain accuracy means stopping the machine — which has a real cost during harvest windows.
In practice, many operators calibrate once at the start of a run and accept the drift that follows. The result is yield maps that may be broadly correct in their spatial patterns, but carry systematic measurement error that makes year-on-year comparison unreliable.
For many applications — simply knowing which zones of a field are consistently high or low — the relative spatial patterns from either sensor type are adequate. An impact plate system, even with some drift, will still show you that your north-east corner yields less than the centre.
Where the difference becomes significant:
An impact plate yield monitor measures grain flow by detecting the force of grain striking a metal deflector plate on the clean grain elevator. It is the technology used in most factory-installed yield monitoring systems from John Deere, CLAAS, New Holland, and Case IH.
An optical yield monitor uses light sensors to measure the quantity of grain passing through the clean grain elevator. No physical contact with the grain is required, so there is no plate wear and no sensitivity to grain moisture.
Optical yield monitors require significantly less calibration than impact plate systems. Green Growth's optical yield monitor needs no load-wagon weigh runs in the field — any recalibration is done remotely in software, and because the measurement is based on light, not physical force, it is not affected by grain moisture or mechanical wear.
Impact plate readings are sensitive to grain moisture (wetter grain hits differently), plate wear (surface changes over time), and flow angle. Without recalibration, the yield readings drift from true value. Calibration requires load wagon runs — harvesting a known area and weighing it to set the sensor offset.
Both can achieve good accuracy when properly calibrated and maintained. The practical advantage of optical is consistency: the sensor holds its calibration state throughout harvest without manual intervention, while impact plate systems drift between calibration events. In real-world conditions, this typically means more reliable year-on-year data from optical systems.
Yes. Green Growth's optical yield monitor is a retrofit kit that installs on the clean grain elevator of any combine harvester — regardless of brand, age, or model. The only requirements are a clean grain elevator and a 12V power supply. Installation takes approximately 2 hours.
University extension work backs up how much calibration discipline matters for impact-plate systems. The University of Nebraska–Lincoln's CropWatch guidance calls the mass-flow sensor "the most critical component of the yield monitoring system," its calibration "absolutely vital for accurate yield measurements," and notes that because readings "may be affected by crop type, moisture content, and test weight," operators should consider performing separate calibrations for each of those circumstances — exactly the repeated-recalibration burden optical sensing avoids. Source: UNL CropWatch: best practices for collecting accurate yield data.
Green Growth's optical sensor kit installs in ~2 hours and works on any combine — CLAAS, John Deere, New Holland, Case IH, and older machines.
See the Kit