Pulse width modulation (PWM) technique in precision anhydrous ammonia application and its valve durability

Pulse-width-modulation (PWM) offers the potential to improve anhydrous ammonia application accuracy and control. A multi-point PWM metering system with stacked manifold for variable rate ammonia application was evaluated. The manifold had one valve per knife and was tested for lateral distribution and flow characteristics. It ensured that liquid ammonia was fed to the valves for correct metering. The distribution test was conducted by determining the flow rate from each shank using a dynamic water absorption system. The flow was tested for valve duty cycle from 10% to 90% and recorded an average of 15.3 kg/hr/knife at 10% to 112 kg/hr/knife at 90% duty cycle. The lateral coefficient of variation was in the range of 5.5 to 9.3%. The valve was modified with larger orifice and back-drilled to enhance the flow rate to an average of 50 kg/hr/knife at 20% duty cycle to 215 kg/hr/knife at 90% duty cycle. Ammonia flow was a function of valve duty cycle and pressure drop across the valve.; The solenoid valve used in the manifold is central to the PWM metered anhydrous ammonia applicator. Valve durability plays a major role in acceptance by the farmers and provision of warranty by the manufacturer. This study described a test program to determine the reliability of PWM valves through detection of failure by changes in flow and pressure. The Success Run Theorem was applied to estimate the reliability of the valves at 3.2 million cycles to be 84%, with a 90% confidence level.; A single point PWM metering system with two valves and single outlet feeding to a vertical dam manifold was developed and tested for lateral distribution and flow characteristics. The single point system was designed to lower the cost of PWM metering for applicators with narrow knife spacing, as are common for wheat. The system ensured that liquid ammonia was fed to the valves for correct metering. The distribution test was conducted using the same procedure used for multi-point PWM manifold. The flow was tested for valve duty cycle from 10% to 90% and recorded an average of 39 kg/hr at 10% to 112 kg/hr at 90% duty cycle. The lateral coefficient of variation was in the range of 5 to 8%.