Wednesday, February 13, 2008

Fertigation Controllers

Fertigation is the irrigation of plants with nutrient enriched (and possibly pH corrected) water. A full fertigation controller is thus a combination of a multi-station horticultural irrigation controller and a fertiliser injection system. A modern fertigation controller like the NutriDose II range will normally have a variety of trigger sources to start irrigations including time-of-day programmed irrigations, solar integration triggering possibly with the ability to modify the integration rate to compensate for humidity, temperature and wind speed. The solar integrator (with temperature and humidity modifiers) will automatically increase the frequency of irrigations in sunny, hot dry weather and reduce it in dull, cool, damp weather. A rain override is useful for outdoor crops as it can be set to zero the integration counter if rain exceeds a specified level. These controllers may generate a single irrigation trigger and then sequentially irrigate all stations in turn. The Autogrow controllers can be set to do this or can be put into multi-trigger mode where each station has its own trigger point and when reached only that station will be irrigated. This allows crops with differing watering requirements to be serviced by a common controller. In fact with the NutriDose IIb or NutriDose IIi it would be possible to mix inside with outside crops and crops requiring watering every half hour with those that need watering once per week. Of-course each station can be set to have a different EC and may even be fed with a mixture having a different recipe. It is important that the solar integrator, fertiliser injection and irrigation functions are all within the same controller as this provides the highest level of functionality. For example, if a separate solar integrator was used, the manual triggering of an irrigation will not be “seen” by the solar integrator and so its counter will continue counting. The solar integrator will not have its counter cleared and may trigger a second irrigation immediately after the manual one. In the case of an integrated controller this will automatically be taken care of. Using an electronic controller employing feedback control allows a number of different methods to be used to actually get the fertiliser into the pipe. The first and most obvious method uses variable speed injection pumps to inject direct into the line. This becomes very expensive as soon as larger flow rates are required. The second method uses venturi eductors to entrain the nutrient and pH stock solutions into the flow of water through them. To adjust the rate of nutrient uptake it is common to use simple on/off solenoid valves that continuously pulse on and off. By varying the ratio of the on time to the off time good proportional control can be achieved. This method is usually called “pulse width modulated” (PWM) control. To work reliably the venturis need a significant pressure drop across them and this normally leads to an auxiliary pump being required to develop this. This is a simple robust method but does require high pressure sensors in the line and also the system design must ensure satisfactory mixing occurs before the concentrated stock solutions are allowed to mix with each other. If they meet in concentrated form a chemical reaction can take place which will change the formulation of the fertiliser. In addition, the resulting precipitation can cause solids that can later block drippers and be a general nuisance. The last method uses an in-line turbulent mixing vessel into which the fertiliser stock solutions are dosed. This method provides excellent mixing and has the advantage that the dosing and sensing is done at ambient pressure. Typically a drum of one-to-two hundred litres capacity is used and is filled at the same rate as it is being emptied. The NutriDose IIi controller can be used in all three of the above methods, has a built-in solar integrator counters, irrigation control and interface to a PC to provide a user friendly interface, remote alarms and data and event logging. It can be expanded up to a maximum of thirty irrigation stations which may be a mixture of inside and outdoor growing areas. The NutriDose IIb controller performs a similar function but is a batching controller. Instead of adding the fertiliser on-the-fly during an irrigation, a batching controller mixes a complete batch of fertilised water into a batching tank which is then used for irrigation. This is a simpler process and tends to be a little lower in cost and is also easier to set up to get really high accuracy. In other respects it is the same as the NutriDose IIi and can be used in multiple batch – multiple trigger, single batch – single trigger, single batch – multiple trigger and multiple batch – single trigger modes. The down side of batched irrigation is that time is used in mixing the batches and in a large installation or one where very frequent irrigations are required, it is possible to run out of time and triggers may start to queue up and causing excessive delays in the irrigation of some stations. Link :

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