machines required the operator to rely on machine knowledge, frequent inspection and monitoring, and a keen ear to listen for subtle sound changes, newer machines have replaced many of those duties with instrumentation. Shaft monitors Early on, simple magnetic pickups were used to monitor shaft rotation, and issue a warning when they deviated beyond preset limits. Temperature sensors can also give warning when bearings overheat due to lack of lubrication, sometimes leading to combine fires. Loss monitors The job of monitoring how much grain is wasted by the thresher by being discharged with the chaff and straw used to require going behind the machine to check. Yield monitors work like a microphone, registering an electrical impulse caused by grains impacting a plate. A meter in the operator's cab displays the relative amount of grain loss proportional to speed. Yield monitoring Measuring the amount of yield (bushels per acre or tonnes per hectare) has become increasingly important, particularly when real-time measurement can help determine which areas of a field are more or less productive. These variations can often be remediated with variable crop inputs. Yield is determined by measuring the amount of grain harvested in relation to the area covered. Cameras Cameras placed at strategic points on the machine can eliminate some of the guesswork for the operator. Field mapping The advent of GPS and GIS technologies has made it possible to create field maps, which can assist in navigation, and in the preparation of yield maps, which show which parts of the field are more productive. Combine Size Classification While all combines aim to achieve the same result, each machine can be classified based on its general throughput which is based upon the rated horsepower rating of the combine. Currently combine classifications, as defined by Association of Equipment Manufacturers (AEM) are as follows: Class VI - 268 HP - 323 HP Class VII - 324 HP - 375 HP Class VIII - 376 HP - 485 HP Class IX - 486 HP - 674 HP Class X - 675 HP+ While this classification is current, the classes themselves have and will evolve over time. For instance, a class 7 (VII) combine in the year 1980 would only have boasted 270 horsepower and been one of the largest machines available in the world at that time but in the 21st century the same machine would be considered small. Combine fires Grain