In order to properly control nutrient injection in irrigation units, nutrients need to be added to the irrigation water and the the nutrients need to be properly mixed with the irrigation water before the resulting EC and pH are measured. When using Priva's Nutrijet, this mixing is done in the mixing tank that is part of the irrigation unit. When using Priva's NutriFlex or NutriJet, this mixing is done in the mixing chamber that is part of the irrigation unit.
However when using Priva's NutriOne or a third party irrigation unit, a proper mixing chamber may not be included in the design. As a result, you may see fluctuations in the EC & pH measurements. These fluctuations may be constant or they may vary depending on the flow through the irrigation unit. The chart below shows an example of an irrigation unit that is not properly mixing the fertilizer. You can see the EC fluctuate every 5 seconds as the Compass goes through a dosing cycle. This indicates that the fertilizer is not properly mixed before it is measured and the irrigation has areas with higher and lower EC levels.
Since the dosing percentage is also calculated every 5 seconds, it ends up being calculated at the same point of each fluctuation. In the example above, it ends up being recalculated at the trough of each fluctuation.
Because it is measuring at the trough (the lowest point in the fluctuation) it is basing the control on that lowest point and the result of this is that the average EC level is above the desired setpoint. This can be verified by measuring the EC level at the plant. By the time the water has reached the plant, the nutrients will have mixed with the irrigation water and will be stable.
While the best solution would be to modify the installation so that the nutrients have an opportunity to properly mix with the irrigation water before the EC level is measured, it is possible to get a better result by adjusting some settings in the Compass. In the configuration of each recipe, there are settings called EC control measurement cycle and pH control measurement cycle. By default, these are set to 5 seconds. If you set them to 1 second, it will recalculate dosing percentage every second instead of every 5 seconds. The result will look like this.
You will notice that the dosing rate is still regulated by the 5 second cycle that is defined in the dosing channel driver and you are still getting the fluctuation every 5 seconds. The improvement is that since the dosing percentage is calculated every second, the highs and the lows of the fluctuations are included in the calculations and the average EC delivered by the irrigation unit will be much closer to the desired EC level.
Note that if you adjust the measurement cycle, you may also need to adjust the P factor and the I factor of the recipe to get the desired control.
Also note that this article focuses mostly on the EC control for simplicity. The same challenges are present for pH control however there is an added complication that adding pH produces a chemical reaction and there are more variables in the equation.